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Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd M. A. Willard*1, L. K. Kurihara1, E. E. Carpenter2, S. Calvin3 and V. G. Harris4 Nanotechnology has spurred efforts to design and produce nanoscale components for incorporation into devices. Magnetic nanoparticles are an important class of functional materials, possessing unique magnetic properties due to their reduced size (below 100 nm) with potential for use in devices with reduced dimensions. Recent advances in processing by chemical synthesis and the characterisation of magnetic nanoparticles are the focus of this review. Emphasis has been placed on the various solution chemistry techniques used to synthesise particles, including: precipitation, borohydride reduction, hydrothermal, reverse micelles, polyol, sol–gel, thermolysis, photolysis, sonolysis, multisynthesis processing and electrochemical techniques. The challenges and methods for examining the structural, morphological, and magnetic properties of these materials are described. Keywords: Chemical synthesis, Magnetic nanoparticles, Superparamagnetism, Reverse micelles, Hydrothermal processing, Precipitation reaction, Thermolysis, Sonochemistry, Polyol chemistry, Electrodeposition, Hydride reduction, Magnetic characterisation, Ferrofluids IMR/420 Introduction Interest in nanoparticle science and technology has experienced resurgence internationally over the past 10 years largely spurred by the Presidential Nanoscience Initiative sponsored by both the Clinton and Bush administrations and their increased utility in a broad range of application. Nanoparticles are typically defined as solids less than 100 nm in all three dimensions. Most often they are particles made to be spherical having diameters on the order of 10 nm or less. At these length scales, a large fraction of the atoms of the particle are at or near the surface providing them with unique properties. In the case of magnetic nanoparticles, crystal symmetry breaking at the surface has profound ramifications. For example, in metallic alloys the surface atoms oxidise quickly forming oxides that are typically ferrimagnetic or, in some cases, antiferromagnetic. If the metallic alloy is not prone to oxidation, then there typically exists a magnetic ‘dead’ layer at the surface of the particle. Atoms in the dead layer do not have enough magnetic neighbours in their first and second coordination sphere to support long-range ferromagnetism. Alternatively, the surfaces of magnetic oxide particles often experience bond bending through surface relaxation. The magnetism in oxide systems stems from double-exchange and/or superexchange interactions, both of which are very sensitive to the bond angles that form between cation– anion–cation arrangements. Surface bond bending in these cases leads to profound changes in the magnetic 1 Naval Research Laboratory, Materials Science and Technology Division, Washington, DC 20375, USA Virginia Commonwealth University, Chemistry Department, Richmond, VA 23284, USA 3 Sarah Lawrence College, Physics Department, Bronxville, NY 10708, USA 4 Northeastern University, Department of Electrical and Computer Engineering, Boston, MA 02115, USA 2 *Corresponding author, email [email protected] ß 2004 IoM Communications Ltd and ASM International Published by Maney for the Institute of Materials, Minerals and Mining and ASM International DOI 10.1179/095066004225021882 interactions, in some instances changing the behaviour from ferromagnetism to antiferromagnetism or to a spin-glass like behaviour. This surface relaxation has been linked to surface spin canting and spin disorder1 that cause anomalously large magnetic anisotropy that requires very large fields to attain magnetic saturation. In order to design magnetic nanoparticles for specific applications, one is challenged by the need to understand the atomic structure of the particle, surface structure and its magnetic structure or spin dynamics. Specifically, for metallic nanoparticles this includes understanding atomic symmetry and chemistry of the interior atoms and surface atoms, as well as the dynamics of both interior and surface spins. In addition, for the case of oxide nanoparticles the surface energy and synthesis methodology may also lead to the stabilisation of defects. All of these issues become more difficult to ascertain as the particle diameter is reduced, whereupon traditional characterisation tools such as X-ray diffraction and electron microscopy have limited utility. The goal of the present work is to provide a broad review of the magnetic nanoparticle research that has been carried out during the past decade. The focus is on chemical synthesis methods for producing magnetic nanoparticles, methods of effectively characterising said particles, and finally a review of the magnetic nature of these nanoparticles. Applications Magnetic nanoparticles have a wide range of uses in many diverse applications. These applications make use of magnetic nanoparticles in a variety of forms, e.g. in solution as ferrofluids for audio speakers; as surface functionalised particles for biosensing applications;2 as particle arrays in magnetic storage media;3 as powder compacts for power generation, conditioning and conversion; in medical applications including magnetic targeted drug delivery; contrasting agents in magnetic International Materials Reviews 2004 VOL 49 NO 3–4 125 Willard et al. Chemically prepared magnetic nanoparticles resonance imaging; and alternatives to radioactive materials as tracers. Ferrofluids Published by Maney Publishing (c) IOM Communications Ltd A ferrofluid consists of surfactant coated magnetic nanoparticles in a liquid host that forms a stable colloidal suspension. A typical ferrofluid has only 5 vol.-% magnetic particles, 10 vol.-% surfactant, and the balance is the carrier liquid. The particles are coated with a surfactant that disperses the particles and prevents agglomeration by overcoming the local magnetic fields and van der Waals forces that exists between particles. As a result, when the ferrofluid is not in the presence of an external magnetic field it has no net magnetisation. However, when a magnetic field is applied to the solution, the particles spontaneously orient with respect to and along the magnetic flux lines. When the field is removed, the particles again disperse randomising their orientation and establishing a state of no net magnetisation. Since the ferrofluid is sensitive to external fields, the particles can be positioned and controlled by fields with the forces holding the particles in place proportional to the applied field strength and the magnetisation of the particles. These unique properties make ferrofluids useful in seals, bearings, dampers, stepping motors, loudspeakers, and sensors.4 Magnetic recording media It has recently been shown that surfactant coated magnetic nanoparticles can self-assemble into periodic arrays. The equal spacing of the particles gives them potential for the next generation magnetic storage materials as individually addressable bits. One example of this new material is the array of FePt nanoparticles created by researchers at IBM.3 In this case, FePt monodispersed nanoparticles were processed by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilisers. The particles self-assemble into a hexagonal self-assembled monolayer with chemical ordering of the particles created by annealing. The crucial properties for media applications is that the particles be monodispersed with a very tight size distribution of ,10%, a high magnetocrystalline anisotropy, and small interparticle interactions. IBM is considering this form of media that may hold 100 times more data than today’s products.5 Power generation, conversion and conditioning High frequency (i.e. f .1 MHz) electronic components, including switch mode power supplies, filters for power conditioning, and power converters, typically use ferrite cores. Ferrites are desirable since they are insulating magnetic oxides with high electrical resistance that minimises conduction related losses. These cores are made by compaction of ferrite particles. Typically, these particles have diameters on the order of micrometres and are processed using chemical reduction methods followed by mixing, firing, compaction, and sintering. Recently, nanoparticle ferrites have been considered for these applications. Benefits of using nanoparticles include lower heat treatment temperatures, fewer processing steps, and, in some instances, improved performance. Difficulties in the use of nanoparticle ferrites include understanding cation disorder and the 126 International Materials Reviews 2004 VOL 49 NO 3–4 role of processing and surfaces. In addition, the compaction of nanoparticles to near their theoretical densities is very difficult and remains one of the major obstacles limiting the wide scale acceptance of nanoparticle ferrites. Targeted drug delivery It is well known that chemotherapy is an effective treatment to fight cancer cells, but since it is delivered as a full body dose, the side effects of its toxicity are often severe and devastating to the patient. One proposed use of magnetic nanoparticles that is particularly exciting is for targeted drug delivery. In this application, the magnetic particle is coated with activated carbon and serves to deliver pharmaceuticals to specific sites. In practice, the administered drug is absorbed to the particle and is localised to a specific site in the body by an external magnet field. The physical force created by the external magnetic field acts to transport the particles through the vascular wall, thus positioning and retaining the drugs in close proximity to the cancer cells. This allows more concentrated doses of the anticancer drugs to be delivered to the cancer and keep them on site for longer periods of time. This targeted drug therapy was in phase I and II trials as of June 2000.6 Magnetic nanoparticles for targeted drug delivery must be tailor-made for in vivo applications. In order to prevent dangerous agglomeration of the particles in the blood stream, the particles must be of a small size relative to the dimensions of the capillaries, spherical in shape, and have a size distribution (i.e. standard deviation) of less than 15%. In addition, the particles must have a high magnetic moment and switch their magnetisation quickly and at low fields.7 In the following sections, the chemical synthesis methods used in processing monodispersed magnetic nanoparticles, the characterisation tools that allow one to determine the structure and phase of the nanoparticles, and finally the magnetic nature of fine particles are discussed. The scope is mainly limited to from 1990 to the present, but reference is made to other reviews predating this period where appropriate. Chemical syntheses Magnetic nanoparticles, like other types of inorganic materials, are developed with the expectation of product uniformity, reliable reproducibility, and property control based on manipulation of processing parameters. Due to the reduced dimensions of nanoparticles, however, each of these ordinary expectations is at risk of misinterpretation and/or inaccurate measurement. Ultimately, as technological advances require ever smaller device architectures, individual magnetic nanoparticles will be necessary for incorporation. Presently, the technological goals and the state of the art processing for magnetic nanoparticles are converging on a solution to these issues. For technological incorporation of magnetic nanoparticles into magneto-bio-electronic devices, the chemical processing control of composition, microstructure, morphology, and phase of the particles is necessary. This includes control of particle size, size distribution, and reduction of agglomeration as product uniformity issues. Knowledge of the most sensitive processing parameters to the formation of the preferred phase and conditions to reduce agglomeration is Published by Maney Publishing (c) IOM Communications Ltd Willard et al. necessary for reliable reproducibility. Finally, control of the processing conditions is needed for manipulation of the structural characteristics of the particles allowing control of the intrinsic magnetic properties. In an ideal sense, the chemical synthesis is controllable for all of these conditions without compromise. In practice, however, the design requires compromises to achieve the best possible set of characteristics. This is a difficult task requiring both careful process control with subsequent well thought out and thorough experimentation to characterise the particles. Chemical synthesis techniques show great promise for producing the high quality nanoparticles needed for future applications, as will be described in the following sections. The three most common approaches used to produce magnetic nanoparticles are physical vapour deposition, mechanical attrition, and chemical routes from solution. In both the vapour phase and solution routes, the particles are assembled from individual atoms to form nanoparticles. Alternatively, mechanical attrition involves the fracturing of larger coarse-grained materials to form nanostructures. In this section, an overview of the chemical synthesis and processing of nanostructured particles is presented. Solution chemical routes often provide the best method for production of nanoparticles due to enhanced homogeneity from the molecular level design of the materials and, in many cases, cost effective bulk quantity production. Solution routes also allow control of particle size and size distribution, morphology, and agglomerate size through the individual manipulation of the parameters that determine nucleation, growth, and coalescence. Surface modification of the particles during synthesis or post-synthesis is easily accomplished, providing additional functionality to the nanoparticles. The synthesis of particles in a solution occurs by chemical reactions forming stable nuclei with subsequent particle growth. This phenomenon of precipitation of solids in solution has been well studied.8,9 Upon the addition of precipitating, reducing, or oxidising reagents to the solution containing the reactants, chemical reactions occur and the solution becomes supersaturated. Supersaturation drives the chemical system far past the minimum free energy configuration for the precipitating species in solution. The thermodynamically equilibrated state is restored by condensation of nuclei of the reaction product and controlled by the kinetics of the nucleation and growth. Kinetic factors control the dynamics of approach to the thermodynamic equilibrium of the system in the growth process.10,11 These factors include reaction rates, transport rates of reactants, and the removal and redistribution of matter. The reaction and transport rates are affected by the temperature, pH and mixing of the solution, as well as the concentration of reactants, and the order in which the reagents are added to the solution. Reaction rates and impurities can influence the structure and crystallinity of the particle. The particle morphology is influenced by factors such as supersaturation, colloidal stability, nucleation and growth rates, recrystallisation, and aging times. Generally, supersaturation has a dominant role in determining the morphology of precipitates. At low supersaturation, the particles are small, with shape depending on crystal structure, composition, and surface energies. As the level Chemically prepared magnetic nanoparticles of supersaturation is increased, larger, dendritic particles form. Finally, at high supersaturation levels, smaller but compacted, agglomerated particles form.10 When the nuclei form at nearly the same time in a supersaturated solution, subsequent growth of these nuclei results in the formation of particles with a very narrow size distribution.12 This narrow size distribution can be maintained as long as agglomeration, Ostwald ripening, and continued nuclei formation do not occur. The growth in solution is interface controlled when the particle is small, after reaching a critical size, it becomes diffusion controlled.12 The formation of stable colloids and dispersion of agglomerated particles have been extensively investigated (e.g. see Ref. 13). As a result of attractive van der Waals forces, and the tendency of the system to minimise the total surface or interfacial energy, nanostructured particles often form agglomerates. Agglomeration of particles can occur during any of the following stages: synthesis, drying, handling and/or post-processing. For applications where dispersed particles or stabilised dispersions are required, agglomeration must be prevented at each processing step. To produce monodispersed particles without agglomeration, surfactants can be used to control the dispersion during chemical synthesis. Surfactants can also be used to disperse as-synthesised agglomerated fine particles. A surfactant (an acronym for ‘surface-active agent’) is any substance that affects the surface or interfacial tension of the medium in which it is dissolved. As such, surfactants need not be completely soluble and may decrease or increase surface tension by spreading over a surface or interface. Surfactants are used during nanoparticle synthesis in order to reduce interparticle interaction through an increase in repulsive forces. They are used to control particle size and distribution in most chemical synthesis routes. Capping or stabilising agents may also be used to help control the particle size and shape. For some applications, consolidation of nanoparticles into bulk solid forms is necessary. Annealing, in many cases, causes an increase in crystallinity, which in turn greatly affects the magnetic and electronic properties.14 However, the heat treatment can cause grain growth if the temperatures are too high or the times are too long. Excessive grain growth adversely affects the magnetic and electronic properties (e.g. magnetic domain formation). Microwave heating typically increases crystallinity with insignificant grain growth. The annealing temperature at which significant grain growth occurs has been shown to be half the melting point of the material.15 It is important to note that nanoparticles generally possess reduced melting temperatures, primarily due to the large surface/volume ratio. Figure 1 shows the chemical synthesis techniques used for the formation of magnetic nanoparticles. Some techniques are frequently used together or in a series of reactions, as noted by the cross-hatching between fields in the figure. For clarity, not all of the combinations found in the literature are shown in the figure. However, the most frequently used combinations are delineated. The dotted line indicates the demarcation between syntheses using organic and those using aqueous solutions. Generally, sol–gel and hydrothermal reactions produce oxide nanoparticles due to their International Materials Reviews 2004 VOL 49 NO 3–4 127 128 International Materials Reviews 2004 VOL 49 NO 3–4 355 356 357 358 359 360 361 362 363 319, 339 254 327 25 26 362 364 320 TEM, XRD, ICP, SQUID TEM, XRD, VSM TEM, XRD, FTIR, VSM, ACSus XRD, BET, MS TEM, XRD, FTIR SEM, XRD, SAXS, SQUID, TEM, XRD, EXAFS, BET, ICP TEM, XRD, ICP, FTIR TEM, XRD, FTIR XRD, ND, ICP, MS, SQUID TEM, XRD, ND, EELS TEM, XRD, BET, DSC, SQUID TEM, XRD, magnetic fluxmeter TEM, XRD, VSM TEM, XRD, ICP, FTIR TEM, XRD, SQUID, ACSus TEM, XRD, SQUID, ACSus Agglomerates Needles Spheres … Needles Needles Needles Needles Platelets … Spheres? Spheres Spheres Spheres Spheres Spheres Spheres PPC PPC and M PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC ,200 30–180 20–200 7–48 20–200 10–150 10–450 ,400 10–100 5–20 y40 4–15 7–25 5–15 ,80 ,10 ,10 Characterisation{ Morphology Fe a-Fe (b-FeOOH) Fe (Fe3O4) a-FeOOH a-FeOOH a-FeOOH b-FeOOH c-FeOOH d-FeOOH MgFe2O4 MnFe2O4 MnFe2O4 MnFe2O4 MnFe2O4 Fe3O4 Fe3O4 Fe3O4/surfactant One of the oldest techniques for the synthesis of nanoparticles is the precipitation of products from solutions. In precipitation reactions, the metal precursors are dissolved in a common solvent (such as water) and a precipitating agent is added to form an insoluble solid. In most cases, a further reduction step is required, either in solution after synthesis (such as borohydride reduction) or to the collected precipitate (i.e. heat treatment in hydrogen gas). Many magnetic nanoparticles can be synthesised using these classical aqueous precipitation reactions to yield nanoparticles that have broad size distribution and irregular morphology. These reactions can generate a wide range of magnetic materials including spinel ferrites, perovskites, metals, and alloys. The major advantage of precipitation reactions is that large quantities of particles can be synthesised. However, it is difficult to tailor the particle size as only kinetic factors are available to control growth. Chelating agents (i.e. ligands with multiple binding sites) may be used to help control the particle size, decomposing as the precursor is heated. The chelate modified precipitation yields magnetic nanoparticles that often have very well Size, nm Precipitation Method{ production in an aqueous solution without a source of reduction. In the following subsections, selected examples of various chemical routes used to prepare magnetic nanoparticles are given, highlighting some aspects of each route. Recently, Hyeon has written an overview focusing on the chemical synthesis of various magnetic nanoparticles.16 An extensive overview of the types of nanoparticles, synthesis techniques used for production, size of particles, and characterisation techniques is given in Table 1; which also includes, as footnote, explanations of the acronyms used throughout this review. Compound* Published by Maney Publishing (c) IOM Communications Ltd PPC: precipitation; H: hydrothermal; HR: hydride reduction; M: micellar or microemulsion; T(OM): thermolosis – organometallic decomposition; T(CO): thermolosis – carbonyl decomposition; UV: photolysis; S: sonolysis; SG: sol–gel; P: polyol; EC: electrochemical; ED: electrodeposition; MSP: multisynthesis processing 1 Schematic diagram showing chemical synthesis techniques for magnetic nanoparticles: crosshatched regions indicate common combinations of synthesis techniques (for clarity, not all synthesis combinations that have been examined appear in diagram ) Ref. Chemically prepared magnetic nanoparticles Table 1 Survey of nanoparticle materials produced by various chemical synthesis techniques illustrating wide variety of types of nanoparticles that can be formed by such techniques: data are arranged by synthesis method used to produce particles and provide composition, size, and morphology information along with techniques used to examine particles, as found in cited references (see table footnote for acronyms in full) Willard et al. Method{ PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC PPC HR HR HR HR HR HR HR HR HR HR HR Compound* Fe3O4, CoFe2O4 CoFe2O4 CoFe2O4 CoFe2O4 CoFe2O4 Co12xNixFe2O4 Co12xMxFe2O4 (M5Gd, Pr) NiFe2O4 NiFe2O4 NiFe2O4 NiFe2O4 NiFe2O4 ZnFe2O4 Mn12xZnxFe2O4 Mn0.66Zn0.34Fe2O4 Ni12xZnxFe2O4 Ni12xZnxFe2O4/a-Fe/a-Fe2O3 Ni0.8Zn0.2Fe2O4 Ni0.5Zn0.5Fe2O4 a-Fe2O3/c-Fe2O3 c-Fe2O3 c-Fe2O3 c-Fe2O3 c-Fe2O3 c-Fe2O3 LiFe5O8 SrFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 (Fe,Ni,Cu) Fe, c-Fe2O3 Co Co Co CoB2, CoFe2O4, Fe3O4, c-Fe2O3 Ni Ni CoPt (L10) (Fe,Zr,B) (Fe3B) (ZrO2) (M,B) (M5Fe, Co, Ni, Mn) 3–15 7–12 5–20 5–130 600–1000 ,30 6–87 4–15 50–200 4–6 3–5 700–900 ,100 3–20 y9 10–20 y20 14–1000 y9 20–50 4–12 2–9 6–12 2–15 ,100 y10 35–40 250–1000 100–500 100–3000 10–50 10–500 400–3000 10–3000 10–200 y40 20–100 ,40 3–8 2–5 25–30 2–5 20–25 5–15 7–12 Size, nm Spheres, self-assembled (SA) Spheres Spheres Spheres, needles Spheres … … … Spheres … Spheres Spheres Spheres … … Spheres Spheres Spheres, agglomerates … Spheres … Spheres Spheres Spheres Platelets, rods … Spheres … Hexagonal platelets, agglomerates Hexagonal platelets, agglomerates … Hexagonal platelets, agglomerates Hexagonal platelets Platelets Spheres Wires Spheres Agglomerates Spheres, SA Spheres … Spheres Spheres … … Morphology TEM, XRD, XPS, SQUID TEM, VSM TEM, XRD, magnetic fluxmeter TEM, XRD, VSM TEM, XRD, XPS, FTIR, AA, magnetic susceptibility TEM, XRD, VSM XRD, TGA, VSM XRD, VSM, MS SEM, XRD, TGA, BET XRD, SQUID TEM, XRD, magnetic birefringence TEM, UVvis, Zeta potential, VSM? SEM, XRD, TGA TEM, XRD, TGA, FTIR, ESR, SANS, VSM TEM, XRD, AA, FMR, VSM TEM, XRD, AA, FTIR, VSM TEM, SEM, XRD, MS, VSM SEM, XRD, VSM TEM, XRD, AA, FMR, VSM SEM, XRD XRD, ND, SANS, FMR TEM, XRD TEM, VSM TEM, XRD, AA, DLS, SQUID TEM, XRD, XPS, MS XRD, FTIR, VSM TEM, XRD, TGA, DTA, SQUID SEM, XRD, TGA, DTA, XPS SEM, XRD, MS SEM, XRD, DTA, VSM TEM, XRD, MS, VSM HRTEM, TEM, XRD, TGA, DTA, Raman, VSM SEM, XRD, TGA, DTA, VSM TEM, XRD, TGA, VSM TEM, XRD, TMA TEM, HRTEM, SEM, XRD, VSM, MS TEM, XRD, ICP TEM, XRD, ICP, DSC, BET, SQUID TEM, XRD, SAXS, SQUID TEM, XANES, SAXS XRD, ICP TEM, XRD, UVVis TEM, XRD, ICP, EDX, VSM, SQUID SEM, XRD, EDX, EXAFS, AA, FTIR, DSC, MS, SQUID XRD, MS, ACSus Characterisation{ Published by Maney Publishing (c) IOM Communications Ltd 365 24 25 26 366 367 368 369 370 371 372 373 374 28, 375 348 29 376 377 348 378 350 379 24 380 381 382 383 384 351 385 20 386 387 388 38 389 390 32 391 56 34 392 393 394 35 Ref. continued… Willard et al. Chemically prepared magnetic nanoparticles International Materials Reviews 2004 VOL 49 NO 3–4 129 130 Method{ HR HR HR HR HR HR HR HR HR H H H H H H H H H H H H H H H H H H H H H H H H H H H H M M M M M M M Compound* (Fe,Ni,B) (Fe,B) amorphous Fe802xCrxB20 Fe802xCrxB20 amorphous (Co,B) amorphous e-Co e-Co e-Co e-Co MnFe2O4 MnFe2O4 Fe3O4 Fe3O4 Fe3O4 Fe3O4 CoFe2O4 CoFe2O4 NiFe2O4 NiFe2O4 MFe2O4 (M5Cu, Ni, Zn) ZnFe2O4 ZnFe2O4 Mn12xZnxFe2O4 Mn12xZnxFe2O4 Mn12xZnxFe2O4/a-Fe2O3 Ni12xZnxFe2O4 SrFe12O19 SrFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19 BaFe12O19/Ba5Fe8O17 REFeO3/RE3Fe5O12 (RE5Er–Lu) a-Fe Fe Fe Fe a-Fe a-Fe a-Fe (b-FeOOH) ,50 50–150 20–200 3–200 y40 y5 y11 7–10 y5 y25 y540 y340 30–200 50–150 12–59 y400 5–25 20–100 y680 3–10 y300 20–80 5–200 10–17 ,12 40–70 50–2000 8–30 100–1900 50–800 200–1000 150–1500 150–10000 40–700 100–200 ,1000 ,80 10–100 2–15 5–150 ,5 50–150 20–1000 10–200 Size, nm Agglomerates Spheres Spheres Spheres, agglomerates Spheres Spheres, SA Spheres, SA Spheres, SA Spheres, SA … Spheres, polyhedra Polyhedra Polyhedra Spheres … Spheres, polyhedra Spheres Polyhedra Polyhedra … Polyhedral Polyhedra Spheres, polyhedra … Spheres, large a polyhedra Spheres Hexagonal platelets Spheres Hexagonal platelets Hexagonal platelets Hexagonal platelets Hexagonal platelets Hexagonal platelets, needles Hexagonal platelets Hexagonal platelets Polyhedra, spheres Spheres Spheres Spheres Needles Spheres Spheres Spheres, needles Polyhedra, spheres, needles Morphology Table 1 Survey of nanoparticle materials produced by various chemical synthesis techniques (continued) TEM, MS TEM, DSC, BET, ICP, MS TEM, XRD, TGA, MS, SQUID TEM, XRD, MS, SQUID TEM, XRD, XPS, ICP, MS SEM, TEM, XRD, SQUID HRTEM, XRD HRTEM, XRD, SQUID TEM, XRD XRD, FB TEM, MS, SQUID TEM, MS, SQUID TEM, XRD, AA SEM, XRD, BET TEM, XRD, FTIR, TGA, VSM TEM, MS, SQUID TEM, XRD, TGA, EDAX, SQUID TEM, XRD, AA TEM, MS, SQUID XRD, VSM SEM, XRD, EDAX, VSM TEM, XRD, AA TEM, SEM, XRD, AA, DTA, TGA, AES, FB XRD, ICP TEM, XRD, VSM SEM, XRD, VSM SEM, XRD, AA, VSM TEM, XRD, DTA, EPR SEM, XRD SEM, XRD, VSM SEM, XRD SEM, XRD, BET, VSM TEM, SEM, XRD, DLS, DTA, TGA, VSM TEM, XRD TEM, XRD TEM, XRD, VSM? TEM, XRD TEM, XRD, EELS, AGM, MS HRTEM, SAD TEM, XRD, SAD, VSM TEM, XRD, optical absorption, VSM, SQUID TEM, XRD, VSM? TEM, XRD, VSM TEM Characterisation{ Published by Maney Publishing (c) IOM Communications Ltd 36 395 396 397, 398 399 40 41 345 39 400 401 401 402 46 403 401 404 402 401 405 47 402 48 406 407 408 49 409 410 51 411 412 50 413–415 52 44 45 62 63 355 416 417 418, 419 420 Ref. Willard et al. Chemically prepared magnetic nanoparticles International Materials Reviews 2004 VOL 49 NO 3–4 Method{ M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M M Compound* Fe/FeB Co Co Co b-Co Co b-Co Co/Au (core/shell) Ni (Fe,Cu) FePt3 (Fe,Cu,B) Co2B FeOOH MgFe2O4, CoFe2O4 MnFe2O4 MnFe2O4 Fe3O4 Fe3O4 Fe3O4 Fe3O4 Fe3O4 Fe3O4 (a-FeOOH) CoFe2O4 CoFe2O4 CoFe2O4 CoFe2O4 CoFe2O4 CoFe2O4 CoFe2O4 CoFe2O4 CoxFe32xO4 Co12xRExFe2O4 (RE5Ce–Er) Zn12xFe2zxO4 Zn12xFe2zxO4 Mn12xZnxFe2O4 Mn12xZnxFe2O4 Co12xZnxFe2O4 Ni12xZnxFe2O4 c-Fe2O3 c-Fe2O3 c-Fe2O3/Fe3O4 SrFe12O19 SrFe12O19 SrFe12O19 ,3 ,10 5–110 5–10 2–10 5–12 ,5 5–25 2–16 5–50 8–10 5–30 2–20 ,80 10–30 5–10 3–10 3–15 3–10 3–12 3–12 5–20 8–100 ,15 3–10 10–18 2–8 2–15 2–8 5–35 5–20 6–11 17–23 2–6 2–50 25–130 3–1000 ,7 20–150 3–12 4–20 2–14 9–120 65–1000 3–100 Size, nm … Spheres Spheres Spheres, SA Spheres Spheres, SA Spheres Spheres Spheres Spheres Spheres Spheres Spheres Needles Spheres Spheres Spheres Spheres Spheres Spheres Spheres … Spheres, needles … Spheres Spheres Spheres Spheres … Spheres Spheres Spheres … Spheres Spheres Spheres Agglomerates Spheres Spheres Spheres Spheres Spheres Hexagonal platelets Hexagonal platelets, agglomerates … Morphology TEM, XPS, BET, MS TEM, XRD, SQUID TEM, Conductivity, AGM SEM, SQUID TEM, STM, XRD, SAXS, SQUID TEM, SEM, SQUID TEM, XRD, SQUID TEM, AFM, UVvis TEM, XRD, SQUID TEM, XRD, EDS, Conductivity, EELS, AGM, MS, SQUID TEM, XRD, SQUID TEM, SEM, XRD, EDS, EELS, MS, AGM, SQUID TEM, SAD, UVvis, AGM SEM, XRD, UVvis, Conductivity TEM, ND, MS, SQUID TEM, XRD, ND, ICP, MS, SQUID TEM, XRD, SQUID TEM, DSC, TGA, SANS, SQUID TEM, XRD, SQUID TEM, XRD, AGM, MS, VSM TEM, XRD, VSM TEM, XRD, AA, DLS, SQUID TEM, XRD, BET, DSC, VSM TEM, XRD, ND, ICP, SQUID TEM, XRD, SQUID TEM, XRD, EXAFS, SQUID TEM, XRD, XANES, SAXS, Conductivity, MS, AGM, SQUID TEM, XRD, EDS, MS, SQUID TEM, XRD, SAXS, EDS, SQUID TEM, XRD, ND, ICP, MS, SQUID TEM, XRD, VSM TEM, XRD, SAD, MS XRD, SQUID TEM, EDS, MS, SQUID TEM, XRD, DLS, BET, ICP, SQUID TEM, XRD, BET, PCS SEM, XRD, BET, DTA, TGA TEM, SAD, EDS, AGM, FMR, SQUID TEM, XRD, BET, PCS TEM, XRD, AGM, MS, VSM TEM, SEM, XRD, MS, SQUID TEM, XRD, VSM TEM, XRD, BET, DTA, TGA, VSM TEM, XRD, BET, DTA, TGA, FTIR, MS, VSM TEM, XRD, DTA, TGA, SQUID Characterisation{ Published by Maney Publishing (c) IOM Communications Ltd International Materials Reviews 2004 VOL 49 NO continued… 421 34, 422 64 423, 424 425–427 428 317 429 249 430 431 432, 433 434 60 71 435 318 435 318 436 437 438 439 70 318 440 352, 441–443 444, 445 446 253, 447 448 449 450 451 452 65 453 454, 455 65 436 456 457 21 73 458 Ref. Willard et al. Chemically prepared magnetic nanoparticles 3–4 131 132 International Materials Reviews 2004 VOL 49 NO 3–4 S S S S S S T(OM) T(OM) T(OM) T(OM) T(OM) T(OM) T(OM) T(OM) T(OM) T(OM) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) T(CO) UV S S M M M M M T(OM) T(OM) BaFe12O19 BaFe12O19 Co[Fe(CN)5NO] Cr3[Cr(CN)6]2 H2O Co4[Fe(CN)6]4 Co/PVP Co Ni/PVP Ni/PVP Ni Ni CoPt, CoPt3, Co/Pt (core/shell) CoxPt12x/PVP CoO, Co3O4/PVP c-Fe2O3 CoFe2O4 SrFe122xO1921.5x Fe Fe/TOPO Fe/polymer (core/shell) Fe, c-Fe2O3 Co, CoO Co Co Co Co, e-Co Co, e-Co Co/PS Co/HPS e-Co/TOPO e-Co/TOPO MnO b-Fe2O3, c-FeOOH Fe Fe/PPO Fe/SiO2 (Fe,Co) Fe/Thiol Fe/PVP Fe amorphous Fe, Co, (Fe,Co) Co Co Method{ Compound* 3–15 5–100 22–31 15–200 12–22 ,2 3–5 17610 ,5 ,5 4615 10–15 ,10 1–5 5–12 6–7 4–9 200–500 5–8 2–11 ,100 10–15 4–5 50–100 5–12 y12 3–17 8–12 5–30 2–15 4–25 15–25 5–10 y30 2–15 1–12 3–8 10–20 3–25 3–8 ,30 2–20 5–10 30–200 Size, nm SA monolayers Spheres Spheres Spheres Spheres Platelets … Spheres Polyhedra Polyhedra Polyhedra Spheres Spheres, SA rods Spheres, SA Spheres, SA Rods Rods Spheres Spheres Spheres Spheres Spheres, SA … Spheres Needles Spheres Spheres Spheres Spheres Spheres Spheres Spheres, rods, cubes, SA Spheres, discs Agglomerates Spheres Spheres, rods, SA Spheres, polyhedra Spheres Spheres Chains Spheres Agglomerates Morphology Table 1 Survey of nanoparticle materials produced by various chemical synthesis techniques (continued) AES, XRD, XANES, XPS, DSC, TGA, FTIR, MS TEM, SQUID TEM, XRD, TGA, DSC, SQUID TEM, XRD, DSC TEM, FTIR, SQUID TEM, SAD, Lorentz microscopy TEM, EELS, FTIR, EDS HRTEM, WAXS, SQUID TEM, SQUID TEM, SQUID TEM, SQUID TEM, FTIR, WAXS, SQUID TEM, HRTEM, WAXS, SQUID TEM, XRD TEM, XRD, SQUID XRD, DTA, TGA, FTIR, FB TEM, SQUID TEM, SQUID TEM, DSC, ICP, FTIR, NMR, SQUID TEM, XRD TEM, EELS TEM, SQUID TEM TEM, SQUID, FMR TEM, XRD, EELS, HRTEM TEM, XRD, HRTEM, SQUID TEM, FTIR TEM, XRF, FMR TEM, XRD TEM, XRD TEM, XRD, SQUID TEM, XRD, XPS, FTIR STM, ESR HRTEM, SQUID TEM, XRD, DSC, TPD, TPR TEM, XRD, VSM TEM, XRD, DTA, TGA, VSM TEM, FTIR TEM, FTIR TEM, FTIR TEM, HRTEM, SQUID HRTEM, WAXS, SQUID Characterisation{ Published by Maney Publishing (c) IOM Communications Ltd 105, 106, 111 101 99, 467 100 468 108 460 85 92 93 96 88 75 91 98 461 95 82 322 97 80 94 462 463 78, 79 89 83 81 77 76 464 465 198 466 102 66 459 66 66 66 74 93 Ref. Willard et al. Chemically prepared magnetic nanoparticles SG SG SG SG SG SG SG SG SG SG SG SG SG SG SG P P P P P 5–20 5–30 40–50 100–200 20–60 50–3300 9–50 100–250 10–100 80–400 10–25 80–85 5–500 30–60 30–4520 5–50 5–250 y1200 25–150 1–500 10–90 6–30 SG SG SG SG SG SG SG 5–20 10–50 ,10 ,25 ,10 12–20 ,20 20–80 7–11617–21 5–30 ,25 5–25 2–10 ,10 1–5 5–15 c 2–100 s 10–50 10–50 5–20 10–80 11–30 S S S S S S S S S S S S SG SG SG SG Ni Ni/SiO2 (Fe,Co) (Fe,Ni) (Co,Ni) Fe3O4/PVA Fe3O4 Fe3O4 Fe3O4, c-Fe2O3 Co3O4, Fe3O4 NiFe2O4 Fe2O3/UDA/OPA/DSA/OTS M12xPx (M5Fe,Co,Ni) Fe/SiO2 (Ni/Pd)/PVP Fe2O3/SiO2 Fe3O4/SiO2 CoFe2O4 Co0.9Mn0.1Fe2O4 Co12xCrxFe2O4 CoBi0.1Fe1.9O4 CoGd0.1Fe1.9O4 CoNd0.1Fe1.9O4 Co12xZnxFe2O4 CoY0.1Fe1.9O4 CoLa0.1Fe1.9O4 NiFe2O4/SiO2 NiFe2O4 Ni0.25Cu0.25Zn0.5Fe2O4 Ni0.5Zn0.5Fe2O4/Mg2SiO4 a-Fe2O3 a-Fe2O3 Fe2O3/SiO2 BaFe12O19 Ba4Co2Fe36O60 BaFe12O19 BaFe12O19 Ba12xSrxFe12O19 BaZnCoFe16O27 La0.67Ca0.33MnO3 Y3Fe3O12 Fe, Ni, Co, (Fe,Co), (Fe,Cu), (Co,Cu) Fe, (Fe,Co), (Fe,Ni), (Co,Ni,Fe) Co Co Co/Mica Size, nm Method{ Compound* Agglomerates Spheres? Agglomerates Agglomerates Spindles, spheres Spheres, platelets Agglomerates Polyhedra Spheres, cubes Hexagonal platelets Platelets Hexagonal platelets Spheres Agglomerates Agglomerates Spheres Spheres Agglomerates … Agglomerates Agglomerates … … … Spheres … … Spheres Spheres Spheres Agglomerates SA monolayers Spheres Spheres Rods Needles Polyhedra Agglomerates Spheres Hexagonal platelets Agglomerates Spheres Spheres Morphology XRD, TGA, FTIR, ESR, MS, VSM TEM, XRD, DTA, BET, SQUID TEM, XRD, DTA, FTIR, ACPerm SEM, XRD, ACPerm TEM, XRD, SAXS TEM, SEM, XRD, ICP, EDX, FTIR, UVvis TEM, ESR, BET TEM, XRD, VSM TEM, XRD, XPS, BET, VSM TEM, XRD, VSM TEM, XRD, TGA, DTA, SQUID, VSM SEM, XRD, MS, VSM TEM, XRD, VSM TEM, XRD, TGA, XPS, FTIR, VSM TEM, SEM, XRD, VSM, SQUID TEM, SEM, XRD SEM, XRD, VSM SEM, XRD, FMR TEM, XRD, CHN, SQUID SEM, XRD, UVvis, IR SEM, XRD, MS, VSM XRD, MS, VSM XRD, MS, VSM TEM, XRD, MS, VSM TEM, XRD, ICP, VSM AFM, XRD, RBS, TGA, MS, VSM XRD, MS, VSM TEM, XRD, TGA, DSC, SQUID TEM, AFM TEM, SANS, DSC TEM, XRD, TGA, DSC, BET, MS, SQUID TEM, SEM, XRD, TGA, BET, VSM TEM, AFM, XRD, DSC, TGA, MS, SQUID TEM, XRD, TGA, MS, SQUID TEM, XRD, TGA, CHN, MS, VSM TEM, SEM, XRD, MS, VSM TEM, XRD, DRS, BET TEM, XRD, BET, ESR, TGA, DSC TEM, XRD, FTIR, EPR, TGA, MS, SQUID, VSM TEM, XRD TEM, XRD, MS, VSM HRTEM, XRD, XPS, VSM TEM, Fluorescent microscopy Characterisation{ Published by Maney Publishing (c) IOM Communications Ltd International Materials Reviews 2004 VOL 49 NO continued… 136 122 471 472 473 474 135 475 131 476 477 129 130 133 132 149, 158 141 166 171 163 128 126 121 123 124 125 127 109 107 103 104 110 114 116 119 118 115 112 113, 117 469 134 108 470 Ref. Willard et al. Chemically prepared magnetic nanoparticles 3–4 133 134 Method{ P P P P P P P P P P P P P P P P P P P P P P P P P P P P ED ED ED ED ED ED ED ED ED ED ED ED ED ED ED ED ED Compound* Co, Pt, CoPt, CoPt3 Co/Ni/LDH Co, Ni, (Co,Ni)/PVP Co, (Co,Ni), (Co,Ni,Fe) Ni, Co Ni/montmorillonite Ni/PVP (Fe,Co) (Ni,Co) (Ni,Co) (Ni,Co) (Ni,Co) (Ni,Co) (Ni,Co) (Ni,Co) (Fe,Ni) (Ni,Co), (Fe,Ni) (Ni,Co), (Fe,Ni,Co) (Ni,Co), (Fe,Ni,Co) (Ni,Co), (Fe,Ni,Co) (Ni,Co), (Fe,Ni,Co) (Ni,Pd)/PVP (Co,Cu) Co2FeO4 a-Fe2O3 c-Fe2O3 c-Fe2O3, a-Fe2O3 Fe2O3, CoO Co Co Co Co, (Co,Fe) Ni Ni Ni Ni Ni, Co Ni, Co Ni Ni Ni, (Pd,Fe) Fe26Ni74 Cu/Co, Fe20Ni80, Co CoPt, FePt CoPt International Materials Reviews 2004 VOL 49 200–500 25–2000 20–40 25–250 25–3000 35–65 20–30 4–8 50–100 2–8 80–100 30–300 3–5 y200 40–50 18–78 y6 1–20 25–30 50–100 18–500 35–500 ,70 20–600 ,40 y18 5–10 25–100 40–60 1–3 5–20 100–600 30–1000 20–500 8–45 5–2000 y20 200–700 200–2000 150–700 210–260 60–500 25–600 200–500 Size, nm Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Templated Templated Nanowires Templated Templated Wires Wires Spheres Templated Templated Templated Spheres … Templated Wires Wires Wires NO 3–4 nanowires nanowires nanowires nanowires nanowires nanowires nanowires nanowires Spheres Turbostratic aggregate Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Spheres Morphology Table 1 Survey of nanoparticle materials produced by various chemical synthesis techniques (continued) TEM, XRD TEM, XRD, TGA, DTA, FTIR, UV-NIR SEM, XRD, TGA, VSM SEM, XRD, EDS. FMR, ACPerm TEM, XRD HRTEM, XRD, VSM SEM, XRD, FTIR, TPD TEM, XRD, EELS, ACPerm SEM, XRD, TGA SEM, XRD, VSM, Permeability SEM, XRD, TGA TEM, SEM, XRD, EDS, Microwave permeability TEM, XRD SEM, XRD, WAXS, VSM SEM, XRD, WAXS, VSM XRD, TGA, FTIR, VSM TEM, XRD TEM, SEM, ACPerm TEM, SEM, SQUID, ACPerm TEM, FMR, ACPerm SEM, XRD, XRF, TGA, GPC, TPD/MS, SQUID, ACPerm TEM, XRD, XPS, TGA TEM, XRD, NMR, EXAFS, VSM TEM, XRD, XANES, MS, DCSus SEM, XRD, FTIR TEM, XRD, FTIR SEM, XRD, DRS SEM, XRD TEM, HRTEM, VSM SEM, XRD, CV, VSM TEM, FMR, SQUID HRTEM, TEM, XRD, VSM TEM TEM TEM, SQUID SEM TEM, SEM, SQUID, VSM MFM, SQUID TEM, Conductivity SEM, TEM, CV XRD, cc angluar spectroscopy TEM, SEM, XRD, VSM TEM, STEM, XRD, EELS, EDAX TEM, XRD, VSM SEM, XRD, VSM Characterisation{ Published by Maney Publishing (c) IOM Communications Ltd 160 165 152 143 169 161 151 150 156 139, 157 144 148 167 478 479 142 145 146 147 164 480 137, 159 481 173 153, 154 177 178 201 184 186 202 203 196 179, 182 180 194 190 176 200 183 482 Ref. 185 155 138 140 Willard et al. Chemically prepared magnetic nanoparticles ED ED ED EC EC EC EC MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP MSP Fe, Fe2O3, Fe3O4 Fe2O3, NiO, Co3O4, CoFe2O4 SrFewOz Fe, Ni, (Co,Ni) a-Fe, c-Fe2O3, Fe3O4 MnxZnyFezOw c-Fe2O3 a-Fe, Co, Ni, FePt e-Co, hcp Co, mt-fcc Co (Fe,Pt) FePt FePt/Fe3O4 FePt (L10) FePt (L10) FePt (L10) FePt (L10) FePt (L10) FePt (L10) FePt (L10) FePt/Pt–Fe2O3 Pt–Fe2O3 CoPt3/ACA FePd, CoPt, (Fe,Co)Pt (L10) (Fe49Pt51)88Ag12 (L10) Mn52.5Pt47.5 (Sm,Co), (Nd,Fe) Fe3O4 4–12 2–30 2–50 10–10000 9–55 20–2000 1–25 6–10 2–10 2–5 2–50 2–10 3–6 3–10 2–8 2–6 1–4 2–5 3–5 8–12 y10 1–50 2–11 2–5 y3 y9 3–20 Size, nm Wires … Agglomerates Agglomerates … … Agglomerates Spheres, SA Spheres, SA Spheres, SA Spheres, triangular platelets Spheres, SA Spheres, SA Spheres, SA Spheres, SA Spheres Spheres, SA Spheres, SA Spheres Spheres Spheres Spheres, Wires Spheres, SA Spheres Spheres Clusters Spheres, SA Morphology TEM, VSM, SQUID TEM, XRD, DLS, BET SEM, XRD, ICP, SQUID TEM, SEM, XRD, XRF, MS, SQUID XRD, MS SEM, XRD, ICP–MS, SQUID TEM, XRD, FTIR, BET, Raman, MS, SQUID HRTEM, WAXS, VSM, SQUID HREM, WAXS, SAXS, XRD, SQUID TEM, PEELS, ICP, RBS, SQUID TEM, SEM, XRD HRTEM, XRD, VSM, SQUID HRTEM, XRD, SQUID HRTEM, EXAFS, SQUID TEM, SQUID TEM, XRD, RBS, XPS, MS TEM, VSM, SQUID TEM TEM, XPS, MS, VSM TEM, XPS TEM, XRD, XPS HRTEM, TEM, SEM, XRD, ICP, AES TEM, SEM, XRD, AGM, VSM TEM, XRD, VSM TEM, XRD, XRF, SQUID TEM, SQUID TEM, XRD Characterisation{ 483 189 192 195 249 193 191 211 206 3, 207 218 214 208, 213, 344 215 216 484 209 210 212 223 222 221 217, 220 485 219 204 486 Ref. *ACA: 1-adamentanecarboxylic acid; DSA: dodecylsulphonic acid; HPS: hyper-cross-linked polystyrene; LDH: layered double hydride; mt-fcc: multi-twinned face centred cubic; OPA: octylphosphonic acid; OTS: octadecyltrichlorosilane; PPO: poly(dimethylphenylene oxide); PS: polystyrene; PVA: polyvinyl alcohol; PVP: polyvinylpyrrolidone; RE: rare earth; SDS: sodium dodecylsulphate; TOPO: trioctylphospheneoxide; UDA: 10-undecanoic acid. {EC: electrochemical; ED: electrodeposition; H: hydrothermal; HR: hydride reduction; PPC: precipitation; M: micellar or microemulsion; MSP: multisynthesis processing; P: polyol; S: sonolysis; SG: sol–gel; T(CO): thermolosis – carbonyl decomposition; T(OM): thermolosis – organometallic; UV: photolysis. {AA: atomic absorption spectroscopy; ACPerm: alternating current permeametry; ACSus: alternating current susceptometry; AES: Auger electron spectroscopy; AFM: atomic force microscopy; AGM: alternating gradient magnetometry; BET: Brunauer–Emmett–Teller (a method of measuring surface area); CHN: carbon–hydrogen–nitrogen analysis; CV: cyclic voltammery; DLS: dynamic light scattering; DRS: diffuse reflectance spectroscopy; DSC: differential scanning calorimetry; DCSus: direct current susceptometry; DTA: differential thermal analysis; EDAX: energy dispersive analysis of X-rays; EDX/ EDS: energy dispersive X-ray spectroscopy; EELS: electron energy loss spectroscopy; EPR: electron paramagnetic resonance; ESR: electron spin resonance; EXAFS: X-ray absorption fine structure (spectroscopy); FB: Faraday balance; FMR: ferromagnetic resonance; FTIR: Fourier transform infrared spectroscopy; GPC: gas phase chromatography; HRTEM: high resolution transmission electron microscopy; ICP: inductively coupled plasma; IR: infrared spectroscopy; MFM: magnetic force microscopy; MS: Mössbauer effect spectroscopy; ND: neutron diffraction; NMR: nuclear magnetic resonance; PCS: photon correlation spectroscopy; PEELS: parallel electron energy loss spectroscopy; RBS: Rutherford back-scattering spectroscopy; SAD: selected area electron diffraction; SANS: small angle neutron spectroscopy; SAXS: small angle X-ray spectroscopy; SEM: scanning electron microscopy; SQUID: superconducting quantum interference device magnetometry; STM: scanning tunneling microscopy; TMA: thermal mass analysis; TEM: transmission electron microscopy; TGA: thermal gravimetric analysis; TPD: temperature programmed desorption; TPR: temperature programmed reduction; UVVis: ultraviolet-visible spectroscopy; UV-NIR: ultraviolet-near infrared spectroscopy; VSM: vibrating sample magnetometry; WAXS: wide angle X-ray spectroscopy; XANES: X-ray absorption near edge spectroscopy; XRF: X-ray fluorescence; XPS: X-ray photoelectron spectroscopy; XRD: X-ray diffraction. Method{ Compound* Published by Maney Publishing (c) IOM Communications Ltd Willard et al. Chemically prepared magnetic nanoparticles International Materials Reviews 2004 VOL 49 NO 3–4 135 Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd defined crystallinity, but lack controlled morphologies. This method is used in the precipitation of many ferrites such as barium and strontium hexaferrites. The size and morphology of precipitated nanoparticles can be tailored with limited success through the tight control of synthesis parameters such as pH, metal cation concentration, and the type of precipitating agent. These parameters allow the formation of nanoparticles that have a large size distribution (greater than 30% of the mean particle size)17 and are roughly spherical. The concentration of the metal species present in the initial reaction mixture has the largest effect on the overall nanoparticle size.18 Unfortunately, low concentrations result in limited particle growth, although the resultant particles are generally more uniform in size. As the metal concentration is increased, there is increased particle growth with a subsequent loss of size uniformity. A precipitating agent has a pronounced effect not only on the overall particle size, but also on the phase purity of the particle formed. This is especially true in the case of mixed metal spinels. Numerous precipitating agents have been employed, including citric acid19–21 and oxalic acid.22 In each case, there is a chelating effect that helps to facilitate complete precipitation. The precipitating agent is then evaporated through subsequent heating cycles. Subjecting the metal solutions to alkaline media facilitates the precipitation of metal hydroxides that result in a product with less agglomeration. The first controlled synthesis of magnetic nanoparticles utilising this alkaline precipitation technique was performed by Massart.17 Using this synthesis, Fe3O4 nanoparticles were precipitated from FeCl3 and FeCl2 at a slightly basic pH of 8.2. These particles were roughly spherical, 10 nm in diameter with greater than 50% size distribution. Through size selection titration the size distribution can be reduced to less than 5%.23 Size selection titration is a technique that disperses nanoparticles in a solvent to form a stable colloidal suspension. The colloidal solution is then systematically disrupted through titration with a non-solvent or electrolyte solution or temperature control, causing the larger nanoparticles to precipitate. The precipitate can be collected using centrifugation or filtration. This can reduce the size distribution to less than 5%, generating a monodispersed colloidal solution.24 The synthesis by alkaline precipitation was later expanded to include ferrites (MFe2O4, where M5Co,25–27 Mn,25,26 (Mn,Zn),28 and (Ni,Zn)29). The production of mixed ferrites presents additional difficulties due to the varying solubilities of the metal hydroxides. In the case of (Mn,Zn)Fe2O4, Fe(OH)3 starts to precipitate early at pH 2.6, while Mn(OH)2 precipitates at a much higher pH of 9.4. The Zn2z cations are amphoteric and precipitate as Zn(OH)2 at pH 7.6, but begin to redissolve at pH 9 forming Zn(OH)422 anions.30 For these reasons, to create uniform metal precipitates, the pH must be carefully controlled at 8.6. The synthesis is further complicated by the propensity of iron to oxidise, forming the c-Fe2O3 phase. The Fe3z cations oxidise at pH.9 producing this common impurity phase with corresponding poor magnetic properties. Titration of the metal by the base increases pH to the target value more slowly than does a titration of the base by the metal. A slow titration tends to cause additional problems due to inhomogeneity. As the pH is slowly 136 International Materials Reviews 2004 VOL 49 NO 3–4 2 TEM image of e-Co as deposited superlattice prepared by injection of lithium triethylborohydride into hot organic solution containing cobalt chloride and organic stabilisers TOP and oleic acid (reprinted with permission from Ref. 40) increased, the Fe3z approach complete precipitation before the Zn2z and finally the Mn3z begin to precipitate. This results in a central core which oxidises to form c-Fe2O3 coated by a shell of iron-deficient ferrite. Hydride reduction The reduction of metal salts using sodium borohydride can form very uniform spherical nanoparticles. The chemistry of the reduction can be very complex, however the reduction of iron,31 cobalt,32,33 and nickel34 has been explained in detail by Klabunde and co-workers. Although metal boride formation is a stable reaction, it can be eliminated with careful control of atmosphere and water content in the reaction. For this reason, the reactions are typically carried out in non-aqueous media (dimethylglyoxime, tetrahydrofuran, etc.) to obtain a pure phase metal. Mixing metal salts to form one reaction before reduction can form alloys,35–37 for example, (Fe,Ni,Cu) alloys were processed this way by Stolk and Manthiram.38 In this case, the particles reduced by borohydride are spherical, 10–20 nm in diameter, and of predominantly Invar phase with some impurities of Cu2O and Fe(BO2)2 present. This material can be heated where it undergoes a conversion to pure phase Cu–Invar with the particle size increasing to 200 nm. Cobalt nanocrystals can also be formed when lithium triethylborohydride is injected in a hot organic solution containing trioctylphosphene (TOP) and oleic acid.39–42 The e-Co crystal size can be controlled by the amount of metal precursor, the reducing agent concentration, and the amount of TOP and oleic acid used. The nanocrystals that are produced will self-assemble into superlattices, as can be seen in Fig. 2. This figure shows an example of the as deposited superlattice with the interparticle spacing controlled by organic capping groups. Hydrothermal Hydrothermal reactions are aqueous reactions carried out using autoclaves or high pressure reactors where the pressure can be over 2000 psi at temperatures above Published by Maney Publishing (c) IOM Communications Ltd Willard et al. 200uC. Water acts as a reactant at these supercritical conditions, accelerating the kinetics of the hydrolysis reactions. At increased temperatures, the solubility of most ionic species increases and, with the lower viscosity of water, exhibits greater mobility. The increased mobility allows Ostwald ripening to continue at a faster rate increasing the uniformity of the precipitates. Size and morphological control in hydrothermal reactions is achieved by controlling time and temperature. The reaction conditions of precursor material and pH have an impact on phase purity of the nanoparticles. There are two main routes for the formation of ferrites via hydrothermal conditions: hydrolysis and oxidation, and neutralisation of mixed metal hydroxides. Some lesser routes involve the hydrothermal treatment of mixed metal oxides,43,44 or the use of other solvents such as ethylene glycol at supercritical conditions.45,46 Hydrolysis and oxidation reactions are very similar to neutralisation reactions where the former uses ferrous salts as opposed to ferric salts in the latter.47 Rozman and Drofenik48 gave a detailed account of the synthesis of (Mn,Zn)Fe2O4 ferrite nanoparticles by the neutralisation of the mixed metal hydroxides. The hydrothermal parameters of temperature and time were varied resulting in 11 nm nanoparticles at both 95uC for 50 h and 140uC for 0.5 h. The shorter reaction time resulted in a nanoparticle surface with a greater amount of hydroxide groups, while reduced temperatures allowed additional water to be incorporated in the crystal structure that caused increased lattice distortions. Differential thermal analysis showed that the water is actually incorporated into the lattice with final dehydration at temperatures in excess of 700uC. This distortion and poor surface quality deteriorates magnetic properties. Rozman and Drofenik also noted that the spinel phase starts to crystallise at 90uC and continues through to 200uC. At temperatures greater than 200uC, the spinel phase can recrystallise as c-Fe2O3. The nanoparticle grains grow via Ostwald ripening. Strontium49 and barium50 hexaferrites can be more difficult to form due to the limited solubility of the strontium and barium precursors. Initial reactants have a greater impact on the final size, morphology, and phase of the hexaferrites than in other mixed metal ferrites. As with other ferrite preparations, temperature and reaction time influence size, as does the ratio of Fe/Ba. The choice of precipitating reagent has a pronounced effect on nanoparticle crystallinity. The larger the size of the spectator ion of the precipitating agent, the poorer the crystallinity. This phenomenon is presumably due to a change in the electrostatic potential of the metal hydroxides. The lower the electrostatic potential of the metal solution, the greater is the likelihood that the metal sols coagulate. This coagulation tends to facilitate the formation of c-Fe2O3.51 Since coagulation favours side reactions, stirring is often used to break up coagulating sols. Stirring the hydrothermal solution has resulted in improved nanoparticle uniformity and phase purity. Nanoparticles of barium hexaferrite have been synthesised hydrothermally under supercritical conditions using a rapid heat flow system.52 This allows the formation of barium hexaferrite in brief times (as short as 30 min) and in a non-stoichiometric, continuous flow synthesis. An example of the product from this synthesis is shown in Fig. 3. Chemically prepared magnetic nanoparticles 3 TEM micrograph of BaFe12O19 prepared by hydrothermal synthesis in supercritical water: thin hexagonal platelets are the typical morphology for M-type hexaferrites due to easier growth perpendicular to crystallographic c axis52 Micelle routes Surfactant molecules when in solution spontaneously form spherical aggregates called micelles or microemulsions. The difference between microemulsions and micelles is more than simple semantics, although recently these terms have been used interchangeably. The micelle aggregates have sizes of 1–10 nm in diameter, while microemulsions contain aggregates that are 10–100 nm in diameter.53 Direct micelles have the hydrophilic portion of the surfactant on the outside of the aggregate exposed to polar solvent, while reverse or inverse micelles have the hydrophobic portion on the outside exposed to a non-polar solvent. Micelles can form in the presence or absence of water. In the case of reverse micelles formed in hydrocarbon, water can be readily solubilised forming a ‘water pool’ where size is characterised by a water/surfactant ratio. In this fashion, the water pools within micelles impose kinetic and thermodynamic constraints on particle formation resulting in restricted ‘nano-reactors’. Aerosol OT or AOT (sodium dioctylsulphosuccinate) was the first and most characterised surfactant system used in the synthesis of magnetic nanoparticles.54–56 Other systems, such as cetyltrimethylammonium bromide (CTAB),57 sodium dodecylsulphate (SDS),58 and polyethoxylates (Igepal, Brij, Tween, C12E5)59 have been used, and more are being developed to optimise morphology and chemical parameters. Many of the reactions carried out in micelles are very similar to those performed in bulk aqueous reactions, but with the added morphological controls afforded by the surfactant system. The first magnetic nanoparticles formed in micelles were from the oxidation of Fe2z salts to form Fe3O4 and c-Fe2O3.60 This reaction was carried out in an AOT/ isooctane system and formed spherical nanoparticles with surprisingly tight size distributions of less than 10%. Later, other reactions using hydrogen peroxide were used to form MnFe2O4. The initial reaction conditions not only controlled the particle size, but also the cation occupancy.61 An interesting phenomenon seen with iron reductions is the influence of the surfactant on the iron crystal International Materials Reviews 2004 VOL 49 NO 3–4 137 Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd 5 TEM image of cobalt hexacyanoferrate synthesised using reverse micelles69 a polyhedral 13 nm average particle size synthesised by quenching reaction after short reaction time; b spherical 39 nm particles formed after reacting for 2 h 4 Antiferromagnetic KMnF3 nanoparticles synthesised by reverse micelle reactions (TEM)67,68 structure. If anionic surfactants (such as AOT) are used, a-Fe is formed with the body centred cubic (bcc) crystal structure expected from thermodynamic equilibrium of the bulk metal at room temperature.62 Conversely, if a non-ionic surfactant is used (for instance, nonyl phenol polyethoxylate), a face centred cubic (fcc) crystal structure forms.63 The process for the formation of metals can be expanded to form metal alloys. Instead of using a single metal salt, mixed metal salts are used and reduced simultaneously.64 It is essential that the reduction is carried out simultaneously or mixed phase products will be formed. The precipitation of precursors that are subsequently fired to produce an oxide end product is an important synthetic process. This has been used in the synthesis of many different ferrite materials: (Mn,Zn)Fe2O4, (Ni,Zn)Fe2O4, ZnFe2O4,65 and BaFe12O19.66 In these cases, the particles were formed with sizes between 5 and 50 nm. The particles had spheroidal morphology with typical size variations of 10%, although higher conversion temperatures generally widen the size distribution to 10–20%. Transmission electron microscopy images of antiferromagnetic KMnF3 nanoparticles synthesised using reverse micelles are shown in Fig. 4. The reaction time of the metathesis reaction controlled the morphology and size of the particles for this reaction. Particles synthesised by quenching the reaction after short reaction times developed polyhedral morphology and 13 nm average particle size (Fig. 4a). Similar reactions 138 International Materials Reviews 2004 VOL 49 NO 3–4 nanocubes carried out for 2 h resulted in particles with spherical morphology and 39 nm average grain diameters (Fig. 4b).67,68 Cobalt hexacyanoferrate nanocubes have been prepared using reverse micelles and AOT.69 Figure 5 shows the cubic morphology of the resulting cobalt hexacyanoferrate. Although there are many examples of inverse micelles used as microreactors, there are only a few examples of the use of direct micelles. Sodium dodecylsulphate is the principle surfactant used in these reactions due to the morphology of the aggregate. The most striking example of direct micelle synthesis was presented by Liu and co-workers with the synthesis of CoFe2O4.70,71 In this example, a chemometric model was created for predicting the size of a ferrite from the synthesis conditions of surfactant concentration, metal concentration, base concentration, and temperature. The synthesis was later expanded to include the mixed ferrites of MnFe2O4 and MgFe2O4.71,72 In order to control side reactions and precipitation common to aqueous systems, micelles are formed using alcohol as the polar phase. Fang et al.73 employed ethanol-based micelles to form SrFe12O19. In this case, Sr(OH)2 has such a high solubility in water that inhomogeneous and strontium deficient precipitates are formed, thus the change to ethanol was necessary. The particles display a large size distribution and a platelike morphology with an average particle size of 100 nm. Thermolysis, photolysis, and sonolysis methods One of the simplest methods to prepare nanoparticles is the decomposition of organometallic precursors. This decomposition may be driven by heat (thermolysis), light (photolysis), or sound (sonolysis). The relatively low decomposition temperature of organometallic compounds is a distinct advantage over other processing techniques. The decomposition temperature controls the nanoparticle growth. Since size and morphology have an effect on the properties of the nanoparticles, control of these properties is a primary goal. In many cases, polymers, organic capping agents or structural hosts are used to limit the size of the nanoparticle.74–85 The Willard et al. Published by Maney Publishing (c) IOM Communications Ltd 6 TEM micrograph of hcp cobalt rods, prepared by rapid thermal decomposition of Co(CO)8 in hot organic solution containing TOPO, oleic acid and o-dichlorobenzene76 polymers and capping agents are generally used to stearically protect the particles so that they do not coalesce. None the less, the particles synthesised are often agglomerated and have a large size distribution. Even if they are stabilised with surfactants or other organics to prevent agglomeration, they can interact with the surface, changing the magnetic properties. As an example, it has been shown that the magnetic properties of nickel are affected by the presence of surface moieties.85–87 However, there has been a great deal of research on preparation of monodispersed, unagglomerated nanoparticles as outlined in the following subsections. As stated above, polymers can be used to limit particle growth. One polymer, polyvinylpyrrolidone (PVP), has been shown to interact minimally with the surface of cobalt.74,75 It is suggested that the polymer matrix determines the cluster size and efficiently protects the particles. Cobalt nanoclusters were prepared by reduction of organometallic compounds of Co(g3-C8H13)(g4C8H12) in hydrogen and in the presence of PVP.74 Dried colloids were made up of non-interacting superparamagnetic particles. By varying the decomposition temperature, the size of particles was controlled. Organometallic precursors were also used to prepare other colloidal metals with a variation of size and structure. An example of this approach is the CoxPt12x alloy, prepared from an organometallic precursor with PVP.88 In most cases, when carbonyls are decomposed in the presence of stabilising polymers, spherical nanoparticles are formed. However, Alivisatos et al., have shown that cobalt nanorods could be prepared by modifying the thermal decomposition of cobalt carbonyl in the presence of oleic acid and trioctylphospheneoxide (TOPO).76–78 They rapidly injected the organometallic precursor into a hot surfactant mixture. This rapid injection of the precursor allowed for a ‘size distribution focusing’ by separating nucleation from growth.77,78 The decomposition of the precursor and nucleation occurs immediately upon injection, and metal nuclei are coated with the coordinating ligands. When a surfactant mixture is used (such as oleic acid and TOPO) and quenched shortly after injection, nanoparticles (4625 nm) of hexagonal close packed (hcp) cobalt are formed, as can be seen in Fig. 6. This morphology indicates that the surfactants link to different crystal faces with different strengths.77,78 If the reaction is allowed to proceed, the Chemically prepared magnetic nanoparticles 7 c-Fe2O3 nanoparticles formed when organometallic precursor CupFe (iron cupferron complex) was injected into solution of hot surfactants (TEM)91 rods transform to monodispersed spherical e-Co nanoparticles. If the coordinating ligand is a linear amine, both spherical e-Co and hcp cobalt discs are simultaneously formed.89 Increasing the amount of surfactant or adding the amine with the carbonyl leads to more disc formation. This e-Co crystal structure has been reported for cobalt nanoparticles prepared by a variety of solution techniques.76–79 TOPO along with other organic capping and stabilising agents has also been used to synthesise nanorods of iron.90 Spheres of nanosized c-Fe2O3 are easily formed when the iron cupferron complex is rapidly injected into trioctylamine at 300uC.91 The results of this injection are shown in Fig. 7. When the organometallic precursor, nickel bis-cycloocta-1,5-diene (Ni(COD)2) undergoes thermolysis in the presence of hexadecylamine (HAD) or TOPO, nanorods, spheres or tearshaped nanorods (Fig. 8) are formed, the shape being related to the concentration of the organic species.92 When [Co(g3-C8H13)(g4-C8H12)] is decomposed in anisole in the presence of an amine under 3 bar of H2, the resulting nanoparticles formed have greatly varying morphologies. For instance, they can be anywhere from 3 nm and relatively monodisperse to agglomerates with 8 TEM image of nickel nanorods prepared by decomposition of Ni(COD)2 in presence of 10 equivalent of hexadecylamine (HAD)92 International Materials Reviews 2004 VOL 49 NO 3–4 139 Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd a large size distribution to the formation of wires, having a 4 nm diameter and 1–3 mm in length.93 The type of amine and its concentration with respect to oleic acid are determining factors as regards wire formation. Using cobalt carbonyl under similar conditions results in 10 nm nanoparticles, while varying the organic stabilising agents used, results in other morphologies. Bracelet arrays of 27 nm cobalt particles are formed in the presence of resorcinol tetraphosphonate94 and tetrahedral structures95 are found when cobalt nanoclusters are reacted with NaAOT/toluene at 130uC. CoPt alloys and cobalt with a platinum shell have been prepared using transmetallation and CO decomposition.96 Alloys are prepared by the addition of Co2(CO)8 to a hot solution of an organic solvent containing a platinum organometallic compound and oleic acid. As the carbonyl decomposes to form cobalt nuclei, it also undergoes a transmetallation reaction with the platinum organometallic compound. To prepare a core/shell material, cobalt colloids are first formed by the decomposition of the carbonyl and then they are refluxed with the platinum organometallic compound with dodecane isocyanide as a stabiliser. The decomposition process may also be applied to form magnetic oxides.97 This can be done by either oxidising the metal particles or through an oxidative decomposition. In the first case, an iron oleate precursor is formed from the decomposition of iron carbonyl in the presence of octyl ether and oleic acid at 100uC. This solution is cooled to room temperature and the mild oxidising agent, (CH3)3NO, is added and the solution refluxed. This results in 11 nm Fe2O3 particles that will also form a two-dimensional hcp lattice. When the solution is refluxed, the iron oleate complex breaks down and iron particles are formed and then oxidised by (CH3)3NO. Variations in particle growth may occur by changing the amount of reactants or adding already prepared 11 nm nanoparticles into a fresh solution and reheating. Oxidative decomposition occurs when an oxidising agent and surfactant are added to the original solution. If a bimetallic organometallic precursor is used, such as (g5-C5H5)CoFe2(CO)9, CoFe2O4 nanoparticles with sizes of 4–9 nm are formed.98 Sonolysis or sonochemistry uses ultrasound or acoustic waves to decompose the organometallic precursors. The formation, growth, and collapse of bubbles in a liquid, drive high energy sonochemical reactions, without any molecular coupling of the ultrasound with the chemical species. This acoustic cavitation provides a localised hot spot with temperatures of about 5000 K, and pressures of y1800 atm. Subsequent cooling rates of about 109 K s21 are produced by the implosive collapse of bubbles in the liquid.99–109 Generally, volatile precursors in low vapour pressure solvents are used to optimise the particle yield. Acoustic irradiation is carried out with an ultrasound probe, such as a titanium horn operating at 20 kHz. Nanostructured particles are easily produced by sonochemically treating volatile organometallic precursors.99–117 The powders formed are usually amorphous, agglomerated, and porous. To obtain the crystalline phases, these powders may be annealed at relatively low temperatures. Highly porous amorphous powders are formed when organic solvents with high boiling points are used for 140 International Materials Reviews 2004 VOL 49 NO 3–4 sonolysis. For example, an amorphous iron powder was produced by the sonocation of iron carbonyl in decalin,99–101 yielding powder with a surface area of 120 m2 g21. Annealing this powder at 350uC under nitrogen resulted in a2Fe that was 50 nm in diameter. (Fe,Co),102,103 (Fe,Ni),104 and (Co,Ni)110 alloys have been prepared by this method. The as synthesised powders are amorphous. Annealing these powders in argon at 450uC for 5 h provides crystallisation. The (Fe,Co) is comprised of bcc iron and a mixture of fcc and hcp cobalt, while the (Fe,Ni) and (Co, Ni) alloys are fcc. Starting with the organometallic precursor, Fe(tBu)2(THF)2, Biddlecombe et al. were able to use ultrasound to prepare nanocrystalline Fe3O4 or Fe2O3, depending on the post-preparative conditions.118 If the as prepared powders are dried in vacuo, approximately 19 nm Fe3O4 particles are formed. On the other hand, if the as prepared powder is dried in air at 200uC, then 9 nm c-Fe2O3 is formed. The dried Fe3O4 nanoparticles form micrometre sized plate-like structures, whereas the dried c-Fe2O3 nanoparticles form needlelike structures. When stabilisers or polymers are added during or after sonication, then metal colloids result.101,102,105,111 These stabilisers could be alkyl thiols,103,106,111 PVP,101 oleic acid,101 octadecyltrichlorosilane (OTS),111 and sodium dodecylsulphate (SDS).84 Gedanken and coworkers104,105,112 have also used sonochemical routes to prepare nanoparticles of Fe, Fe3O4, Fe2O3, CoFe2O4, and CuFe2O4. If the sonication is carried out in the presence of oxygen, then oxides are formed.112,113 In the case of Fe2O3, when the coating solution was present during sonolysis, amorphous powder resulted, that was then converted to c-Fe2O3 upon annealing at 300uC for 3 h under an argon atmosphere. If the powder is first converted to c-Fe2O3, the OTS does not coat the powder. In general, nanoparticles with dimensions less than 10 nm will self-assemble to the thermodynamically stable form, usually an hcp assembly of particles. By pressing the nanoparticle solution on a substrate or interface, careful removal of the solvent will result in self-assembled monolayers (SAM). The size of the SAM coated nanoparticles is determined by the surfactant concentration in the coating solution. Oxide formation (Fe3O4, Co3O4) was also seen if the sonication was carried out in aqueous conditions and with noncarbonyl precursors, however the resulting powders were crystalline.113–116 When cyclodextrin is added to the aqueous solution, Fe3O4 nanorods are formed.119 An example of the nanorods formed by this process is shown in Fig. 9. If the sonolysis is carried out in the presence of a support or porous host, then colloidal metal particles are formed. These powders have a surface area and catalytic activity greater than those of commercially available or conventionally prepared materials. For example, 3– 8 nm amorphous iron particles on silica support were synthesised at 20uC from iron pentacarbonyl (Fe(CO)5), decane and silica gel.102 The sonochemically prepared Fe/SiO2 has an order of magnitude more active surface than the conventionally prepared material for the Fischer–Tropsch reaction. However, these materials are generally considered for catalytic reactions and not for magnetic applications. Willard et al. 9 Fe3O4 nanorods prepared sonchemically in aqueous solution containing iron (II) acetate and b-cyclodextrin (TEM)119 Published by Maney Publishing (c) IOM Communications Ltd Sol–gel methods Sol–gel processing can be used to prepare a variety of materials, including glasses, powders, films, fibres, and monoliths. Traditionally, the sol–gel process involves hydrolysis and condensation of metal alkoxides. Metal alkoxides are good precursors because they readily undergo hydrolysis, i.e. the hydrolysis step replaces an alkoxide with a hydroxide group from water and a free alcohol is formed. Once hydrolysis has occurred the sol can react further and condensation reaction (polymerisation in some cases) occurs. Factors that need to be considered in a sol–gel process are solvent type, temperature, precursors, catalysts, pH, additives and mechanical agitation. These factors can influence the kinetics, growth, and hydrolysis and condensation reactions.120 The solvent influences the kinetics and conformation of the precursors, and the pH affects the hydrolysis and condensation reactions. The pH also affects the isoelectric point and the stability of the sol. These in turn affect the aggregation and particle size. By varying the factors that influence the reaction rates of hydrolysis and condensation, the structure and properties of the gel can be tailored. Because these reactions are carried out at room temperature, further heat treatments need to be carried out to achieve the final crystalline state. Due to the metastability of the as synthesised particles, annealing and sintering can be performed at low temperatures. Sol–gel routes have been used to prepare pure, stoichiometric, dense, equiaxed and monodispersed particles of TiO2 and SiO2,120 but this control has not been extended to the ferrites. Generally, the particles produced are agglomerated. Ultrafine powders of CoFe2O4 and NiFe2O4 have been synthesised after being calcined at 450 and 400uC, respectively.121,122 Most of the ferrite sol–gel synthesis focus has been on cobalt ferrite compositional substitution studies with Mn,123 Cr,124 Bi,125 Y, La,126 Gd, Nd,127 and Zn.128 Sol–gel routes have also been attractive for the preparation of hexagonal ferrites. For example, the M-type hexagonal ferrite, Ba12xSrxFe12O19, formed 80– 85 nm hexagonal platelets after a 950uC calcination for 6 h.129 And nanospheres of the W-type ferrite, BaZn22xCoxFe16O27, resulted after calcination in air for 4 h at 650uC.130 The particle size ranged from 10 to 500 nm corresponding to annealing temperatures of 650 Chemically prepared magnetic nanoparticles and 1250uC, respectively, and increased with increasing calcination temperatures. U-type hexagonal ferrites were also prepared, with 10–25 nm spherical particles formed at 750uC.131 The grain size could be changed by increasing the calcination temperature. These calcined powders had an amorphous layer coating. Sol–gel routes have also been used to prepare precursors of BaFe12O19, which upon calcining at 850uC, yields the hexagonal ferrite. Yttrium iron garnets with particle sizes from 45 to 450 nm have also been prepared.132 Mathur and Shen133 have prepared the manganite perovskite La0.67Ca0.33MnO3 by dissolving the metal precursors in an acidic ethanolic solution. Drying the solution at 120uC and calcining at 300–400uC leads to preceramic foam which forms nanocrystalline La0.67Ca0.33MnO3 (40 nm) after a heat treatment at 650uC. It should be noted that the sol–gel process is particularly attractive for the synthesis of multicomponent particles with binary or ternary compositions using double alkoxides (two metals in one molecule) or mixed alkoxides (with mixed metaloxane bonds between two metals). Atomic homogeneity is not easily achieved by coprecipitating colloidal hydroxides from a mixture of salt solutions, since it is difficult to construct double metaloxane bonds from metal salt.120 Hybrid materials such as metal oxide–organic nanocomposites can be prepared using the sol–gel approach. For example, controlled nanoheterogeneity can be achieved in metal/ceramic nanocomposites.120 Reduction of metal oxide particles in hydrogen provided the metal–ceramic nanocomposite powders such as iron in silica,134 Fe2O3,135 and NiFe2O4.136 The metal particles, a few nanometres in size with a very narrow size distribution even for high metal loading, were statistically distributed in the oxide matrix without any agglomeration, as a result of anchoring the metal complexes to the oxide matrix. The narrow particle size distribution could not be achieved if the sol–gel processing was performed without complexation of metal ions. Polyol The polyol method, in which the polyol acts as solvent, reducing agent, and surfactant, is a suitable method for preparing nanophase and micrometre size particles with well defined shapes and controlled particle sizes.137–170 By this method, precursor compounds such as oxides, nitrates, and acetates are either dissolved or suspended in a diol, such as ethylene glycol or diethylene glycol. The reaction mixture is then heated to reflux between 180 and 199uC. During the reaction, the metal precursors become solubilised in the diol, form an intermediate, and then are reduced to form metal nuclei, which form metal particles. Submicrometre size particles can be synthesised by increasing the reaction temperature or inducing heterogeneous nucleation via adding foreign nuclei or forming foreign nuclei in situ. Nanocrystalline powders such as Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Sn, Re, W, Pt, Au, (Fe,Cu), (Co,Cu), (Co,Ni), and (Ni,Cu) were also synthesised using different salt precursors by this method.137–165 In many cases, use of nucleating agents to assist formation of nanoparticles was not required. For example, nanostructured powders of CoxCu1002x (4(x(49 at.-%)137,138 were synthesised by reacting cobalt acetate tetrahydrate and copper International Materials Reviews 2004 VOL 49 NO 3–4 141 Willard et al. Chemically prepared magnetic nanoparticles 10 Fe48Co52 nanoparticles prepared by a polyol method (SEM)149 Published by Maney Publishing (c) IOM Communications Ltd acetate hydrate in various proportions in ethylene glycol. The mixtures were refluxed at 180–190uC for 2 h, the powders precipitated out of solution, and were subsequently collected and dried. Since copper was more reducible than cobalt, nucleation of copper occurred first, and cobalt subsequently nucleated on copper crystallites. X-ray diffraction showed evidence for formation of a metastable alloy. Diffraction peaks due to fcc copper were detected in all samples, however diffraction peaks due to cobalt were not detected until x519. To confirm the structure of powders, studies of local atomic environment were performed using extended X-ray absorption fine structure (EXAFS) spectroscopy and solid state nuclear magnetic resonance (NMR). The results from these investigations and vibrating sample magnetometry (VSM) ruled out the formation of metastable alloys, but confirmed the synthesis of nanocomposites of (Co,Cu). The polyol method has also been a useful preparative technique for the synthesis of nanocrystalline alloys and bimetallic clusters. Fiévet and co-workers have extensively studied the ferromagnetic system of (Co,Ni), (Fe,Co), (Fe,Co,Ni).139–148 The (Co,Ni) alloy particles had densities and saturation magnetisation close to the bulk values, and showed a shift to higher FMR resonance frequencies as the Co/Ni increased. This was also observed in the (Fe,Co,Ni) particles that were 50–150 nm in size. An example of the morphology of alloy nanoparticles formed by the polyol technique is shown in Fig. 10. The particles depicted by the SEM image consist of Fe48Co52 and possess a mean particle size of 30 nm.149 Nanocrystalline Fe10Co90 powders with grain size of 20 nm were prepared by reducing iron chloride and cobalt hydroxide in ethylene glycol without nucleating agents.150 Nickel clusters were prepared using platinum or palladium as nucleation agents.151 The nucleating agent was added 10 min after the nickel hydroxide–PVP– ethylene glycol solution began refluxing. The nickel particle size was reduced from about 140 to 30 nm when a nucleating agent was used. Reduction of particle size was also obtained by decreasing the nickel hydroxide concentration and by the use of PVP. Nickel prepared without nucleating agents oxidised at a temperature of 370uC. Smaller nickel particles synthesised with nucleating aids oxidised at a lower temperature of 260uC, as expected from the higher surface area of finer particles. Desorption studies showed the adsorbed surface species 142 International Materials Reviews 2004 VOL 49 NO 3–4 11 TEM image of Co nanoparticles prepared by polyol method from acetate medium171 were CO moieties and H2O, and nitrogen containing species were not observed. This indicated that ethylene glycol, not the polymer, was adsorbed on the surface of particles. The ethylene glycol had only half monolayer coverage. When this protective glycol was completely removed from the surface, oxidation occurred. It was suggested that the Ni–Pd and Ni–Pt particles had a 7–9 nm palladium and a 6–8 nm platinum nucleus, respectively. Oxidation studies showed that some alloying of nickel with platinum occurred. Cobalt nickel alloys of 210–260 nm particle sizes were also prepared using either silver or iron as nucleating agents.152 Recently, anisotropic particles of cobalt and (Co,Ni) have been prepared.171 By adding sodium acetate or sodium hydroxide to the polyol solution, or ruthenium as a nucleating agent, non-spherical particles resulted. If sodium acetate is added to the solution, agglomerates of rods (25 nm dia.) formed. Addition of the hydroxide to the polyol solution leads to 25–40 nm hexagonal platelets. An example of the hexagonal structure can be seen in Fig. 11. Polymer protected bimetallic clusters were also formed using a modified polyol process.152 The modification included addition of other solvents and sodium hydroxide. In the synthesis of (Co,Ni) with average diameters between 150 and 500 nm, PVP and ethylene glycol were mixed with cobalt and nickel acetate with PVP. The glycol and organic solvents were removed from solution by rinsing in acetone or filtration. The PVP covered particles were stable in air for months. Compared to aqueous methods, the polyol approach resulted in the synthesis of metallic nanoparticles protected by surface adsorbed glycol, thus minimising the oxidation. The use of non-aqueous solvent such as polyol also further reduced the problem of hydrolysis of fine metal particles as often occurred in the aqueous case. By modifying the polyol method with the addition of water to act more like a sol–gel reaction (forced hydrolysis), oxides can be prepared.153–155,172 For example, 6 nm CoFe2O4 was prepared by the reaction of ferric chloride and cobalt acetate in 1,2-propanediol with the addition of water and sodium acetate. In this case, spherical particles formed, however the reaction did not use the ruthenium nucleating agent. Soluble c-Fe2O3 nanoparticles can be prepared similarly to the method of the CoFe2O4, however an amine capping agent (n-octylamine) must be added to the solution.173 Willard et al. Other heating sources for the polyol method have been reported besides the conventional heating mantle. Alternative sources include a 2.45 GHz microwave gyrotron169 and millimetre wave beam sources.170 One technique, the laser–liquid method, uses a laser to heat the solution.174,175 This has been used to prepare submicrometre nickel particles. Published by Maney Publishing (c) IOM Communications Ltd Electrochemical/electrodeposition Electrochemical and electrodeposition routes are generally used to prepare nanocrystalline coatings, and are not the focus of this paper. However, electrodeposition has been used to prepare wires of magnetic materials176–186 and recently it has been used to prepare particles.187–195 Grain growth is favoured at low overpotentials and high surface diffusion rates, and nuclei formation is favoured at high overpotentials and low diffusion rates. These conditions are experimentally achieved when using pulsed modes. Under pulsed conditions, the peak current density can be considerably higher than the limiting current density attained for the same electrolyte during direct current plating. It has been shown that nanostructured materials will result when the deposition variables (i.e. bath composition, pH, temperature, current density, overpotential, and additives) are chosen so that nucleation is favoured with reduced grain growth. Recently, Penner187 has carried out a detailed study and modelling of the parameters that affect the nucleation and growth in electrodeposited nanoparticles and wires. He has followed the LaMer and Dinegar model11 to explain the formation of electrodeposited platinum nanoparticles on graphite electrodes. Penner and co-workers found that when instantaneous nucleation and diffusion controlled growth conditions were satisfied, the particles formed were 7 nm in diameter with a standard deviation of 3.2.196 From these results, two factors were considered for the broadening of the particle size distribution. In electrodeposition, the nuclei are distributed on the electrode surface in a ‘pseudorandom’ process. On the terraces, the nucleation process is random, yet, while on the step edges, the nuclei are aligned along the edge. The growth step is dependent on the number and proximity of the neighbouring nuclei. Penner points out that interparticle diffusion coupling (IDC) is the most important mechanism for broadening of the size distribution for randomly nucleated particles. This condition also exists in solutions, but instead of nucleating heterogeneously, the particles in solution collide, react, and move on. Experimentally, to overcome IDC, the slow growth method187,188,196,197 or H2 coevolution method was used. In the slow growth method, a high overpotential is applied to provide nuclei on the electrode. A second low overpotential pulse is then applied to promote the growth step. Nickel nanoparticles were synthesised using this slow growth method.189 In H2 coevolution, convective mixing equalised the growth rate of the nanoparticles. The formation of gas bubbles and their release and movement through the solution caused convective mixing. By varying the deposition time and potential, nickel nanoparticles of 2–250 nm were synthesised.187,188 Another electrochemical method that has been used to prepare nanoparticles involves the use of a sacrificial anode.189–191,198 Based on a method developed by Reetz Chemically prepared magnetic nanoparticles et al.,199 Pascal et al.191 have prepared 3–8 nm iron oxide nanoparticles from a sacrificial iron electrode in an aqueous solution of dimethylformamide (DMF) and cationic surfactants. Adjusting the current density controls the particle size. The as prepared nanoparticles showed a broad scattering peak by X-ray diffraction, consistent with an amorphous phase. Dierstein et al. have used a similar strategy to prepare other metal oxides.189 This technique is termed ‘electrochemical deposition under oxidising conditions’ (EDOC). Here the anode is oxidised to produce Mnz species in solution, which are then reduced by electrons generated by the cathode, in the presence of stabilisers. These stabilised metal species are then oxidised by oxygen that is bubbled into the system.189 The EDOC technique has been used to prepare nanoparticles of Fe2O3, Fe3O4, Co3O4, NiO, and CoFe2O4.176 The difference between the method used by Pascal et al. and EDOC, is that, in EDOC, oxygen is bubbled in, whereas, in the Pascal et al. synthesis, oxygen is produce from the electrochemical breakdown of water. Mixed oxides of strontium hexaferrites,192 and (Mn,Zn) spinel ferrites193 have also been prepared using sacrificial electrodes. Nanowires are easily formed in porous templates by electrodeposition. Unlike precipitation methods and chemical vapour deposition, where great care must be taken so as not to plug the pores during wire formation, templated electrodeposition provides material growth from the base, thus plugging is avoided. This technique has been used to prepare nanowires of cobalt (Refs. 177–181, 199–201), nickel (Refs. 178–182, 190, 194, 202, 203), FePt (Ref. 183), FeCo (Ref. 184), FeNi (Ref. 176), CoPt (Ref. 183), with the common template materials, polycarbonate,204 anodised alumina,177,182 and mica.176 Multisynthesis processing methods The IBM nanoparticle synthesis route3 is a combination and variation of the polyol method and the thermal decomposition routes used by Alivisatos et al.77–79 This multisynthesis process (MSP) involves a high temperature organic or solution phase synthesis followed by a size selective separation technique to obtain relatively monodispersed (,5%) nanoparticles. More specifically, rapid injection of the organometallic precursor into a hot solution containing the stabilising agents allows nucleation immediately upon injection. Because the capping agents and surfactants are present, the size and shape of the nanoparticles are controlled. Size and morphology can be controlled by adjusting the reaction conditions such as time, temperature, precursor concentration, surfactant type, and surfactant concentration. Sun et al.3 have prepared monodispersed FePt nanoparticles and self-assembled superlattices by the high temperature reduction of platinum acetylacetonate and the thermal decomposition of iron carbonyl in the presence of stabilising agents. In the case of FePt, both reactions were carried out together in the presence of oleic acid and oleyl amine. The composition was controlled by the ratio of the iron precursor to that of the platinum precursor. The nanoparticles were produced with sizes tunable in the range 3–10 nm. A colloidal solution of the nanoparticles was prepared by flocculating the particles and redispersing them in a nonpolar solvent. This solution was put on a substrate and International Materials Reviews 2004 VOL 49 NO 3–4 143 Published by Maney Publishing (c) IOM Communications Ltd Willard et al. Chemically prepared magnetic nanoparticles 12 a Superlattice of Fe50Pt50 prepared by multisynthesis process (MSP) and b resulting superstructure when oleic acid/oleyl amine is replaced with hexyl analogs (TEM)3 the solvent allowed to evaporate. This led to the selfassembly of the nanoparticles. An hcp three-dimensional superlattice was formed by the self-assembly of 6 nm Fe50Pt50 with oleic acid and oleyl amine stabilisers (Fig. 12). Changing the alkyl group on the stabilisers can change the interparticle distance and superlattice symmetry. This is evident in Figure 12b, in which the oleic acid/oleyl amine was replaced with hexanoic acid/ hexylamine. Changing the alkyl group from dodecyl to hexyl leads to a particle spacing of 1 nm, and a superlattice having a cubic close packed structure. This hcp to cubic transition was also seen with e-Co. The superlattices show no aggregation at temperatures up to 600uC. The as synthesised 4 nm Fe52Pt48 has a fcc structure and annealing at 560uC for 30 min leads to particles with an ordered L10 crystal structure. Particle superlattices of these particles exhibit a similar trend. The as synthesised assembly has a chemically disordered fcc structure where annealing results in the iron and platinum atoms rearranging into the chemically ordered L10 structure. (The term fct – face centered tetragonal – has been used to describe the ordered phase of FePt, CoPt, FePd, and CoPd intermetallics by the nanoparticle community. This is strictly incorrect, as the L10 phase is primitive tetragonal with ordered layers of atoms along the [001], e.g. Fe atoms on z50 plane and Pt atoms on z51/2 plane.205) The amount of ordering can be controlled by temperature and annealing time. Since the initial paper on FePt synthesis,3 there has been an explosion of research on FePt prepared by this method206–216 or variations on this method217,218 and other magnetic materials, i.e. Mn52.5Pt47.5 (Ref. 219), CoPt (Refs. 220, 221), FePd (Ref. 220), and SmCo (Ref. 204). Recently, core/shell particles of platinum on iron and platinum on 144 International Materials Reviews 2004 VOL 49 NO 3–4 Fe2O3 have been prepared using this method.222,223 In this case, the Pt(acac)2 solution was injected in the hot organic solution, then the temperature was lowered to 70uC and the iron carbonyl was added to the Pt–organic solution. After the iron carbonyl addition, the temperature was raised again. This route yielded 10 nm cores of platinum and an iron shell that was 2–3 nm thick. Even with the presence of oleic acid in the organic solution, the iron shell oxidised. Powder XRD could not distinguish between c-Fe2O3 and Fe3O4, so XPS was used to determine the nature of the oxide formed. It was found to be c-Fe2O3 which was also confirmed by HRTEM lattice imaging. One of the greatest advantages of chemical routes is that they are carried out in solution. This allows a great deal of versatility and compatibility. Since many of the chemical routes use similar solvent systems, they may be interchanged for one another or carried out concurrently. Therefore, combining one or more techniques is relatively simple. For instance, micellar techniques are often used to regulate the size of the nanoparticles in sol–gel synthesis. The versatility also comes from the fact that a broader spectrum of materials can be prepared by combining techniques. In addition, because this is a solution technique, additives, capping agents, and functionalities are easily added to the reaction. These additives can be added pre-synthesis, in situ, or post-synthesis. It is also possible to prepare nuclei or cores by one technique and inject them into the solution of a second, thereby either forming particles or core/shell structures. Structural characterisation The magnetic properties of nanoparticles are determined by their physical structure: the size and shape of the particles, their microstructure, the chemical phase or phases that are present, the defects, and differences between the crystal structure of nanoscale phases and their bulk counterparts. Although individual structural probes are sometimes capable of providing definitive information about some aspect of the particles, in many cases the results of a single probe are ambiguous, inconclusive, or difficult to interpret. This may be true even when the technique is reliable for bulk measurements. Consider, for example, an X-ray diffractogram of nanoparticles believed to consist of a single phase (see Fig. 13). Compared to a bulk standard, the peaks will be broadened. In the case of subnanometre particles, structural relaxation may change the lattice constant measurably. Surface effects, negligible in a bulk crystal, may play a significant role. These effects will result in differences between the nanoparticle diffractogram and the diffractogram of a standard bulk material, reducing the utility of diffraction as a ‘fingerprinting’ technique. Accordingly, a suite of complementary tools is generally used to determine the physical characteristics of nanoparticles. Self-consistence between these techniques increases the confidence in the accuracy of a given characteristic. In some cases, the use of parameters found by one technique can be used to refine parameters found by another technique, thereby providing a better picture of the particle assembly. This is especially true of diffraction techniques (XRD and TEM) aiding Mössbauer effect spectroscopy or synchrotron radiation probes. Willard et al. Chemically prepared magnetic nanoparticles complete descriptions of their use and applications, interested readers should consult the references cited. Conventional techniques Published by Maney Publishing (c) IOM Communications Ltd Mean particle and crystallite size 13 Diffraction patterns for sample of manganese zinc ferrite nanoparticles 12 nm in diameter (pattern a) and bulk manganese zinc ferrite sample (pattern b): although diffraction patterns look similar, EXAFS analysis of samples indicated that particles were multiphase; broad peaks of manganese zinc ferrite largely mask iron oxide impurity phase The advent of increasingly sophisticated synchrotron radiation probes has been important in this regard. Concurrent with the rapid increase in the intensity of ultraviolet and X-ray beamlines available to the scientific community has been the development of increasingly powerful experimental and analytical techniques for structural determinations. In the subsections below, the use of conventional tools for determining the structure of magnetic nanoparticles is discussed followed by the application of synchrotron radiation to the problem. Finally, a few examples of state of the art analyses are given. A subjective summary of the relative applicability of the various probes discussed here is given in Table 2. (A similar table of structural probes, but without the emphasis on magnetic nanoparticles, can be found in Ref. 224.) Note that while this review includes a discussion of several methods of structural characterisation, it is not detailed enough to teach the use of these techniques. For more The defining characteristic of nanoparticles is, of course, their size. In principle, most structural probes are sensitive in one way or another to the size of the particles. In practice, however, nanoparticle ‘size’ is an ambiguous concept. An assembly of nanoparticles generally consists of some distribution of sizes, approximating a log normal distribution. This distribution has a significant affect on magnetic properties; for example, the superparamagnetic blocking temperature is strongly dependent on the size and size distribution of the particles. This distribution is also likely to be skewed, often a log normal distribution approximates the data.225 A log normal distribution is one in which the number of particles is normally distributed as a function of the logarithm of some measurement of the particle size (typically the diameter for spherical particles). Two parameters are required to specify a log normal distribution. Often, the mean and standard deviation of the distribution as a function of the logarithm of the measured particle size are used; these are known as the geometric mean and geometric standard deviation, respectively. It should be noted that the geometric mean is equivalent to the median of the size distribution as a function of measured particle size. Using N for the number of particles with radii r¡Dr, r0 for the geometric mean, and s for the geometric standard deviation, the formula for a log normal distribution may be written " # 1 {(ln r=r0 )2 N(r)! exp Dr (1) r ln s 2(ln s)2 Please note that the literature does not always agree regarding the meaning of the symbol s for log normal distributions. In this paper, s is always the quantity used in equation (1). Figure 14 shows typical volume-weighted log normal distributions for small nanoparticles. Different techniques will thus yield different values for the same sample, depending on the weighting scheme implicit in the technique. For example, mean diameters calculated from TEM images generally weight each particle equally (‘number weighting’). This is in contrast to techniques which weight each atom equally (‘volume Table 2 Relative utility of various structural probes for characterising magnetic nanoparticles: one check indicates technique that provides uncertain or incomplete information for wide range of materials or is only applicable in especially favourable situations; three checks indicates technique that is widely (but not universally) applicable and often gives fairly reliable information; two checks, of course, provides intermediate level of information Size Morphology Phase identification Site occupancy Chemical composition Kinetics EM DLS XRD ND MS ICP XAS XPS EDXAS DAFS 333 333 33 … 333 … 33 … … … … 33 33 … 33 3 … 3 33 … 33 33 … 3 … … 3 33 … 3 … … … … 333 … 33 … 33 33 3 3 … … 3 3 3 33 3 … 33 33 3 333 3 … 33 333 3 3 EM: electron microscopy; DLS: dynamic light scattering; XRD: X-ray diffraction; ND: neutron diffraction; MS: Mössbauer effect spectroscopy; ICP: inductively coupled plasma; XAS: X-ray absorption specroscopy; XPS: X-ray photoelectron spectroscopy; EDXAS: energy dispersive X-ray absorption spectroscopy; DAFS: diffraction anomalous fine structure. International Materials Reviews 2004 VOL 49 NO 3–4 145 Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd 15 Nanoparticles comprising iron oxide shell on iron core (TEM): difference in contrast between oxide and metal allows shells to be clearly visible241 a r051.91 nm, s51.35; b r054.78 nm, s51.35; c r054.98 nm, s51.11 (in c, there is not room to label volume weighted mean size; it is identified by unlabelled line between Scherrer and EXAFS sizes) 14 Volume weighted log normal size distributions for nanoparticles: mean sizes as found using volume, Scherrer (XRD), EXAFS (extended X-ray absorption fine structure), and # (number) weightings are indicated (note scales on x axes differ) weighting’). For volume weighting, equation (1) must be multiplied by an additional factor of r3. If particles are not single crystals, either because of aggregation, multiple phases, or amorphous regions, it is also important to note whether a technique yields a crystallite size or the physical size of the particle. For determinations of crystallite size, Scherrer analysis of X-ray diffractograms (XRD) is commonly used.48,226,227 This technique relies on the broadening of diffraction peaks due to the limited number of diffracting planes. Because other factors, such as strain, can broaden XRD peaks, the claim is often made that Scherrer analysis provides a lower limit on mean crystallite size. It must be emphasised, however, that this is not the case for small nanoparticles in polydispersed samples. The X-ray diffractogram intensities for a given phase are proportional to the square of the volume of the particle, thus selecting a ‘mean’ weighted heavily 146 International Materials Reviews 2004 VOL 49 NO 3–4 toward the high end of the distribution. In fact, it is straightforward to demonstrate that for log normal distributions with s>exp(!6/6)<1.5, the Scherrer formula (properly corrected for strain and instrumental broadening) diverges, yielding an infinitely large mean diameter! Values of the geometric standard deviation of this size or greater are not unusual.225,228–230 Of course, although the log normal distribution given in equation (1) extends to arbitrarily large values of r, distributions of actual particles do not. Nevertheless, the divergence of Scherrer determined sizes for moderately broad ideal log normal distributions should be taken as an indication that Scherrer analysis is not reliable for moderately polydispersed samples, yielding more information about the size of the largest crystallites than about the mean of the distribution. Although more sophisticated techniques for extracting information regarding particle sizes from XRD have been known for decades and are capable of analysing polydispersed samples,231 the requisite data quality and complexity of the techniques have prevented them from being utilised in the area of magnetic nanoparticles. One of the most powerful tools for determining particle size and morphology is transmission electron microscopy (TEM). This technique will generally report the total particle size (as well as crystallite size) and has the virtue of providing details of the size distribution. In many cases, aggregates of smaller particles can be discerned. If the nanoparticles consist of more than one phase and the phases provide enough contrast, then the individual phases may also be visible (Fig. 15). For further discussion of the capabilities of electron microscopy for investigating the physical and magnetic structure of nanostructured materials, see the review article by Thomas and Hütten;232 imaging of small metallic nanoparticles is discussed by Ascencio et al.233 A few representative examples of the application of TEM imaging to magnetic nanoparticles are given in Refs. 191, 234–240. Dynamic light scattering242 (DLS), also known as photon correlation spectroscopy (PCS) or quasi-elastic Published by Maney Publishing (c) IOM Communications Ltd Willard et al. light scattering (QELS), is an important supplemental technique for determining the sizes of particles in solution, particularly when the size distribution is narrow and approximately log normal. Because of the ability to size particles in solution (including aerosols), DLS shows promise for in situ studies of nanoparticle synthesis.243 The DLS technique is sensitive to total particle size, and like Scherrer analysis, yields information which is weighted by the square of the particle volume, although results are generally ‘converted’ to volume or number weighting. It is important to note that these converted values assume a particular form for the distribution, and may differ substantially from the true volume or number weighted mean size. Also, like Scherrer analysis, DLS is not appropriate for samples with broad log normal distributions. Finally, extended X-ray absorption fine structure (EXAFS), see below, also contains information about particle size, particularly for small particles. As with XRD, sophisticated techniques exist for extracting size information from samples comprising well characterised particles, such as metal structures on supported catalysts,244,245 but these techniques are not generally applicable to moderately polydispersed samples of magnetic nanoparticles. In analogy with Scherrer analysis, a simple model based on the assumption of spherical particles has been used.241,246–248 It has been shown that this technique yields values smaller than the volume weighted mean diameter for the crystallites, and is thus a suitable complement to Scherrer analysis. The ‘EXAFS size’ is therefore a well defined value for all log normal distributions for which the volume weighted mean diameter is well defined, i.e. for all log normal distributions with s>exp(!3/3)<1.78. Figure 14 compares theoretically calculated mean sizes, weighted in the same way as the techniques described above, for typical log normal distributions of small nanoparticles. Figure 14a is for small (4.0 nm number weighted mean diameter) nanoparticles with a moderately broad size distribution. Figure 14b and c both exhibit number weighted mean diameters of 10.0 nm, but in the latter case the distribution is relatively narrow. In all cases, the weighted means follow the order: number weighted,EXAFS,volume weighted,Scherrer. This order will be followed for all distributions, even those which are not log normal, except that it is possible for distributions with large numbers of very small particles to yield an EXAFS value smaller than the number weighted value. As a practical matter, it should also be noted that in some cases it is possible for the TEM to ‘miss’ small crystallites, leading to a value somewhat greater than the true number weighted mean. As can be seen from Fig. 14a–c, the fractional difference between the mean diameter as determined by the various techniques is strongly dependent on the width of the distribution, but only weakly dependent on its number weighted mean. Indeed, the ratio of the Scherrer determined mean to the EXAFS determined mean is a useful measure of width for moderately broad distributions. Like the mean, the width of a distribution can be defined in a variety of ways. It may be taken as twice the standard deviation of the distribution, or as half of the range which encompasses 95% of the particles. These Chemically prepared magnetic nanoparticles definitions, although similar, are not identical if the distribution is not Gaussian. Similarly, the width is often specified as a fraction of ‘the’ mean size, which is of course somewhat dependent on the way in which the mean is defined and measured. These differences in definition introduce difficulties in comparing the width of size distributions presented in the literature. As a rule of thumb, however, these definitions rarely differ by more than a factor of two from one another. For example, a distribution with a reported width of 5% of the mean can be safely said to be narrower than one with a reported width of 15% of the mean, regardless of the definitions in use. If syntheses by different groups yield widths reported as 20 and 30% of the mean, however, the definitions used must be considered before concluding that the first distribution is actually narrower. Phase identification Although XRD is often considered the gold standard for determination of crystal structure, it does possess significant limitations. As discussed in the introduction to this section, differences between nanoparticle and bulk crystal structures in combination with peak broadening and multiple phases may make XRD results inconclusive. Sometimes, it yields almost no information: very small particles less than a nanometre across, core/shell structures with shells a few monolayers thick, and amorphous phases all are difficult to discern using conventional XRD. Certainly, XRD is still the method of choice for confirming the presence of a single expected phase with crystallite size greater than about 10 nm, but the nature of magnetic nanoparticles often yields samples that do not meet those criteria. The high intensities and small spot sizes of electron beams may make electron diffraction useful.236,240,249,250 Selected area electron diffraction (SAED or SAD) sometimes allows discrimination of phases as small as 1–2 nm in size, but at this scale double diffraction, calibration issues, and the close proximity of diffraction spots arising from different phases often prevents an unambiguous identification.232 Site occupancy Magnetic materials with the same nominal chemical composition, even with very high phase purity, often differ markedly from one another due to a different distribution of species or defects (i.e. atoms, ions, or vacancies) among the available crystallographic sites. This is especially true for ferrimagnetic materials containing multiple sublattices. Spinel ferrites, for example, have two sublattices with parallel coupling of the magnetic moments within each sublattice and antiparallel coupling between them. The result is that the magnetisation of the material arises from the difference in magnetisation of the sublattices. Often, different species reside on each sublattice. If vacancies are found preferentially on one sublattice, or if some of the atoms on one sublattice are switched with those on the other, the magnetisation may be altered substantially.251 It is thus very important to determine site occupancy in these materials. If the material possesses atomic ordering and the crystallites are large enough that the peaks are not prohibitively broad, then diffraction techniques are often applicable. Even if the occupancy is random or International Materials Reviews 2004 VOL 49 NO 3–4 147 Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd quasirandom in nature, diffraction can often be used to obtain the average structure. For example, XRD can reveal the percentage of sites in the octahedral sublattice of a spinel compound that are vacant.252 It cannot, however, conclusively reveal the local distortions caused by these vacancies, nor the tendency of vacancies to cluster or disperse. In the case of one atom substituting for another, however, XRD is only useful if the substituted atom differs substantially in its atomic scattering factor from the one being substituted. The atomic scattering factor is primarily a function of the number of electrons in the atom. Since in ferrimagnetic oxide materials the substitution is often one first-row transition metal for another (particularly manganese, iron, cobalt, nickel, and zinc), this prevents XRD from being used to find average site occupancy in many cases. The same restrictions apply to electron diffraction. Neutrons, on the other hand, are uncharged, and thus interact predominantly with the nucleus of atoms rather than the surrounding electrons. Since the coherent scattering cross-section of nuclei is not correlated with atomic number, elements with very similar atomic scattering factors for X-rays may have very different scattering cross-sections for neutrons. For example, iron (with its isotopes present in their natural abundance) has about 6 times the cross-section for coherent scattering of thermal neutrons as does manganese, and more than 14 times that of cobalt. The atomic scattering factors for X-rays for these three elements, in contrast, are within a few per cent of each other. In some cases, therefore, neutron diffraction (ND) can be useful for probing differences in site occupancy to which XRD is not sensitive.253,254 Since this is a diffraction technique, only the average structure can be obtained. In addition, neutrons possess a magnetic dipole moment, and thus are sensitive to magnetic ordering (for some examples of this use, see Refs. 255, 256). Unfortunately, due to the small number of neutrons available for experimentation, relatively large sample sizes are necessary for this technique. This is sometimes difficult for investigative studies in which nanoparticles synthesised by chemical means, however it is not prohibitive.70,72,103 Mössbauer spectroscopy depends on the ability of 57 Fe nuclei embedded in a crystal to emit ‘recoil-free’ gamma radiation (i.e. momentum is conserved by the recoil of a macroscopic number of atoms, rather than an individual nucleus). Because essentially no energy is lost to recoil, the energy distribution of the c-rays is extremely narrow, and is dependent on the chemical and magnetic environment of the 57Fe nuclei. To create a spectrum, the source of the photons is vibrated, imparting a time dependent Doppler shift to the photons. These photons can then undergo the inverse process in a sample, in which those of the appropriate energy are absorbed by 57Fe nuclei (‘resonant absorption’). Thus, information is conveyed regarding the local environment of 57Fe in the sample, generally including coordination number and oxidation state. Because Mössbauer spectroscopy also depends on the magnetic environment, it is an important tool for investigating iron containing nanoparticles.227,239,257–264 Remarkably, thermal analysis techniques such as thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) have been used to determine site occupancies. 148 International Materials Reviews 2004 VOL 49 NO 3–4 Each technique measures the value of a physical parameter as the temperature is changed in a controlled fashion. For TGA, the parameter is the mass of the sample (outgassing or oxidation accounts for the mass changes); for DSC the parameter is the power required to keep the sample at the same temperature as a reference material; for DTA the temperature of the sample relative to a reference material is measured. Some site occupancies in magnetic nanoparticles have been deduced from DSC and TGA,265 since the temperature at which ions oxidise (e.g. Fe2z going to Fe3z) is dependent on the local environment of the ion.266 Depending on the synthesis technique and particle size, however, this effect may be obscured by the effects of materials adsorbed on to the surface of the nanoparticles. Chemical composition In addition to size, phase, and site occupancy, the magnetic properties of nanoparticles depend on the chemical composition, i.e. the fraction of each element that is present. There are a number of techniques for determining chemical composition accurately. Inductively coupled plasma–atomic emission spectroscopy267 (ICP–AES) is widely used because of its sensitivity, wide applicability, and ease of use. In this technique, a plasma torch is used to atomise, ionise, and excite the atoms in a solutionised sample, which are then quantified by the intensity of their characteristic emissions. The sensitivity of the technique varies considerably between elements, but is generally accurate to better than 2% for the main constituents of magnetic nanoparticles. A notable exception is oxygen: environmental oxygen prohibits the direct measurement of the amount of oxygen in the sample. If all other elements can be accounted for, it is possible to estimate the amount of oxygen present by subtracting the mass of the other elements from the total mass of the sample. Because of the difficulties in measuring the mass of sample in solution, this method frequently yields only a rough estimate of oxygen content. Another family of techniques for determining chemical composition employs the high energy electron beam of an electron microscope to generate secondary electrons in the sample, causing the formation of holes and the subsequent emission of X-rays when the holes are filled. These X-rays are measured by energy dispersive spectroscopy (EDS or EDX, the X standing for X-ray analysis) or by wavelength dispersive spectroscopy (WDS or WDX). When an instrument is optimised for determining chemical composition rather than for imaging, this technique is sometimes called electron probe microchemical analysis (EPMA). These techniques are more sensitive to heavier elements than lighter, with the limit for quantitative analysis depending on the instrument, sample, and detection method (oxygen and carbon fractions are often achievable). Accuracy of better than 2% is readily obtainable. Recently, with a field emission gun electron source, EDX has been shown to have subnanometre spatial resolution.268 More generally, resolutions of under 10 nm are readily obtainable, allowing the chemical composition of individual nanoparticles to be determined. For further discussion of this technique as well as other methods for determining chemical composition using electron Willard et al. Chemically prepared magnetic nanoparticles microscopy (e.g. electron energy loss spectroscopy), see the review by Mackenzie.269 Published by Maney Publishing (c) IOM Communications Ltd Synchrotron based techniques The suite of techniques discussed above can provide considerable structural information about many types of magnetic nanoparticles. There are, however, significant gaps. Diffraction, for example, can provide only very limited information about amorphous phases. Likewise, the structure of nanoparticle materials consisting of multiple chemical phases may resist solution via the above techniques. Fortunately, techniques that utilise synchrotron radiation are well suited to both of these scenarios. About forty dedicated ‘light source’ synchrotrons on four (soon to be five) continents are currently available to the scientific community. Although each facility has different capabilities, they generally provide broad spectrum ultraviolet and X-ray radiation that is more than 6 orders of magnitude brighter than a conventional X-ray tube. These characteristics allow high resolution spectroscopy to be conducted in a reasonably short time frame, sometimes even allowing the kinetics of a synthesis to be observed in situ. Although there are a variety of modes in which this spectroscopy can be conducted, many of which will be discussed below, the essential feature of these spectra is that they provide information about electronic structure and/or local environment that is completely element specific. Because the information is inherently local, amorphous materials can be investigated; because the information is element specific, the structures of materials consisting of multiple phases, or of those exhibiting point defects, can often be clarified. Modes of data collection X-ray absorption spectroscopy (XAS) experiments filter the photons emerging from the synchrotron with a monochromator so that only a narrow band of energies impinge on the sample at any given time; this energy is then scanned through the absorption edge to produce the spectrum. The intensity of transmitted photons, yield of fluorescent photons, and/or total electron yield, may be measured independently. Total electron yield is surface sensitive, with most of the signal coming from the first few tens of nanometres of material (depending on the material and the energies involved). Fluorescence is less surface sensitive, typically probing to a depth on the order of ten micrometres, and is suitable for extremely dilute samples or those that are too thick for transmission. Transmission is sensitive to the entire thickness of the sample, but requires the sample to be on the order of several micrometres thick. In emission techniques, a monochromator is still used, but the energy is left fixed. Instead, the distribution of kinetic energies produced by the emitted electrons is recorded, providing information about the electronic structure of the material. This is known as X-ray or ultraviolet photoelectron spectroscopy (XPS, UPS), or sometimes as electron spectroscopy for chemical analysis (ESCA). Energy dispersive X-ray absorption spectroscopy270,271 (EDXAS or DXAS), utilises a curved polychromator to send photons through the sample with a range of energies corresponding to different diffracted angles, so 16 X-ray absorption spectroscopy of iron foil near K edge: dotted line divides XANES (X-ray absorption near edge structure) from EXAFS (extended X-ray absorption fine structure) region that an entire spectrum can be collected simultaneously with a linear or areal detector. The EDXAS technique holds promise for investigating the kinetics and mechanisms of reactions on the sub-second time scale.272–276 The high intensity and selectable energy of synchrotron radiation can also be used to advantage in XRD; this can be combined with XAS in a number of ways. In diffraction anomalous fine structure277,278 (DAFS) experiments, for example, the angle of diffraction is varied simultaneously with the incident photon energy in such a way that the same Bragg peak is being sampled at different energies. The result is an element specific spectrum that is sampling only atoms in sublattices that obey the Bragg condition for that peak. In this way, the local environment of an element in a particular phase, or, under favourable conditions, a particular type of lattice site,279–282 can be probed. A comparison of DAFS and XAS analyses of nanoparticulate metals is given by Bazin et al.283 In DAFS, many energies are sampled at one Bragg peak. In anomalous diffraction, entire diffractograms are sampled at a small number of energies. This technique has recently been used to determine the site occupancy not only by element, but by valence as well.284 Data interpretation and analysis The interpretation of synchrotron based X-ray absorption spectra is often not straightforward. For example, a portion of the X-ray absorption spectrum for iron is shown in Fig. 16. The binding energy of the K electrons in iron is 7112 eV; therefore, the absorption shows a large jump (the ‘edge’) as the energy of the incident photon exceeds that energy. This binding energy will increase by a few electronvolts if the iron is oxidised and the edge will shift accordingly; thus the position of the edge can be used to estimate the oxidation state. The detailed structure within about 30 eV of the edge, known as X-ray absorption near edge structure (XANES), is generally interpreted in terms of core electron transitions to available energy states (either above or below the Fermi level); XANES therefore yields information on bonding and the local symmetry International Materials Reviews 2004 VOL 49 NO 3–4 149 Willard et al. Chemically prepared magnetic nanoparticles of the atoms surrounding the absorber. The features revealed by extended X-ray absorption fine structure (EXAFS), i.e. the quasi-oscillatory features above 30 eV, are due to coherent scattering off of atoms in the local environment up to a distance of several angstroms from the absorbing atom. Depending on the sample, type and region of spectrum, and desired information, several techniques are used to analyse this information. (References immediately after the technique name are to applications of the technique to magnetic nanoparticles.) Published by Maney Publishing (c) IOM Communications Ltd Fingerprinting285,286 The simplest method of analysis is to compare the spectrum with a known reference material (the ‘empirical standard’). Of course, if the material is crystalline and sufficiently similar to the reference, XRD can perform this task without the need for a synchrotron. On the other hand, nanoparticle materials are often similar on a local scale to a bulk analogue, but lacking (or different) in long range order. If the XANES (or the XPS) of the material is similar to the reference, it suggests that the immediate environment of the atoms is similar; if the EXAFS is similar, then the local environment may be similar out to several angstroms. Comparison to theory264,287,288 This may represent either qualitative assignments of particular spectral features to particular causes (e.g. a particular electronic transition or backscattering from a particular coordination shell) or comparison to an ab initio calculation289,290 of a spectrum (a ‘theoretical standard’). Curve fitting to theoretical standard291,292 This technique is used most often to analyse EXAFS, including the oscillatory portion of DAFS.293 In this technique, a model of the material, with several parameters left free, is hypothesised. For example, a sample may be hypothesised to consist of nanoparticles composed of bcc iron mixed with Fe2O3. Free parameters might include the size of the particles and the fraction of each phase present. A theoretical standard is then calculated for reasonable values of the free parameters; theoretical expressions can be used to compute how the spectrum changes for different values of the parameters. Subsequently, the values of the free parameters are optimised by a least squares fit of the modified theoretical standard to the data. A poor fit indicates that the model is poor; a good fit indicates that the model may be good. If the model is good, then the values of the free parameters that produce the fit are expected to reflect the actual structure of the material. When used judiciously, this technique is among the most powerful and can be used profitably even when the material is made up of multiple phases. It is, however, prone to misinterpretation if used carelessly, partly because more than one model may correspond to the same spectrum or similar spectra (the model that does not correspond to the material is often termed a ‘false minimum’). For this reason, an EXAFS fit of a single coordination shell of a single sample under a single set of conditions is generally suspect. There are, however, 150 International Materials Reviews 2004 VOL 49 NO 3–4 several methods of reducing the likelihood of false minima. For example, the probability of a good fit being achieved with a poor model is considerably reduced if more than one absorption edge is refined simultaneously. Likewise, fitting more than one coordination shell simultaneously is advisable, at least for crystalline phases, because this will reduce the correlation between parameters (a single-shell fit, for example, cannot distinguish the coordination number from the EXAFS quantum efficiency S 2o ) and because it will act as a powerful constraint on valid models. Of course, it is helpful if the number of free parameters can be reduced by using the results of other probes. Chemical composition, for example, can be determined reliably by methods given in the subsection of that name, above. The chemical composition, in turn, can be used to constrain site occupancies of materials exhibiting substitutional disorder. It is also worth noting that analyses of EXAFS by the curve fitting method are almost always dominated by systematic error; this error may stem from limitations of the ab initio calculation, the choice of free parameters, the values chosen for fixed parameters, the method used to extract the EXAFS oscillations from the raw data, or the way in which the samples were prepared for measurement. Fortunately, these sources of systematic error do not vary much for a set of samples prepared and analysed in the same way. Thus, if a series of measurements and fits are performed on a sample while varying some extrinsic parameters (e.g. temperature, pressure) or measurements are performed on a series of samples differing in some way (e.g. nanoparticle size, composition variation), the differences between the values of the parameters as determined by the fits are much more reliable than the values themselves. This is a particularly valuable strategy if one of the samples has been well characterised by another method (i.e. it is an empirical standard), since the degree of systematic error can then be ascertained. Principal component analysis294 (PCA) This technique requires a family of samples presumed to have differing (but unknown) proportions of chemical phases present, some (or all) of which may be unknown. For example, a sequence of core/shell nanoparticles could be prepared in such a way that the ratio of shell volume to core volume varies. As another example, an aging study could be performed on metallic nanoparticles: the ratio of oxide to metal could be expected to increase over time. The PCA technique decomposes the spectra of such families into components: mathematically orthogonal functions from which all of the sample spectra can be constructed. The number of components required to reproduce all of the spectra to within reasonable experimental error is the number of distinct phases present in the family of samples. If all the phases are unknown, PCA will yield only the number of phases present. If the identity of each of the phases is known and empirical or theoretical standards are available, then PCA reveals the fraction of each phase that is present. Finally, if the structures of some phases are known and others unknown, PCA may be combined with curve fitting to a theoretical standard; this combination is sometimes called ‘residual phase analysis’. Willard et al. Chemically prepared magnetic nanoparticles The PCA technique has recently been used to analyse EXAFS,294–296 XANES,274,276,297,298 and XPS.299,300 Selected examples of synchrotron characterisation Published by Maney Publishing (c) IOM Communications Ltd Multi-edge modelling of site occupancy As described above, ferrimagnetic materials derive their magnetic properties from the interaction between magnetic cations located on two sublattices. In the case of manganese zinc ferrite nanoparticles, XRD is not well suited for determining on which sublattice each metal species resides, since the atomic scattering factors of manganese, zinc, and iron are very similar. Mössbauer spectroscopy can determine the site occupancy of the iron atoms, but not the other two species. In the past several years, EXAFS has been used to observe differences in site occupancies for all elements between samples (e.g. as a function of synthetic method,291 annealing,301 or milling302), but comprehensive quantitative occupancies for this three-cation mixed ferrite have been difficult to obtain. For materials containing multiple metals, the EXAFS of each edge has traditionally been analysed separately. Ideally, this approach provides site occupancies that are consistent with the stoichiometry of the sample. In practice, uncertainties and correlations present in the analysis may lead to ‘best-fit’ occupancies which are prima facie incorrect, requiring, for example, more than 100% of one kind of site to be occupied. To take full advantage of the redundant information present in multiple spectra, a simultaneous refinement of multiple spectra can be performed.302 Calvin et al.292 have applied this to manganese zinc ferrite nanoparticles by first using ICP–AES to determine the stoichiometry of the samples. Assuming sample composition and site occupancy did not vary significantly from particle to particle, this stoichiometry was then used to reduce the number of free parameters related to site occupancy to two: the fraction of manganese ions on tetrahedral sites and the fraction of zinc ions on tetrahedral sites. By reducing the number of free parameters, the precision of the results is improved: site occupancies for most samples were determined to within ten percentage points. DAFS and site specific valences Another approach to investigating ferrimagnetic materials is offered by DAFS. Because the symmetry of the sublattices in a ferrimagnetic material differ, each contributes to different (albeit overlapping) subsets of the diffractogram. For example, the octahedrally coordinated sublattice in magnetite (Fe3O4) contributes to the (222) and (444) reflections, while the tetrahedral site contributes to the (022), (224), and (444) reflections. Thus, XAS collected at the (222) reflection contains information about the octahedral sublattice only, while the (022) and (224) reflections contain information about just the tetrahedral sublattice. Frenkel et al. used this property to study the valence distribution in magnetite.280 Figure 17 shows their data in the XANES region of the spectra. The difference between the spectra is notable, and reflects the differences in symmetry of the local environment and the different valences of cations on the two sublattices. In 17 DAFS (diffraction anomalous fine structure) analysis of magnetite: trough near 7118 eV in (222) reflection corresponds to trough near 7130 eV in other reflections – large shift is indicative of lower average valence in octahedral sites (figure adapted from Ref. 280) particular, the higher average valence of the iron on the tetrahedral sublattice is clearly indicated by the large energy shift between the octahedral and tetrahedral spectra. The authors of the study then proceeded to use curve fitting of the (222) spectra to a theoretical standard to yield quantitative information on the local environment of the iron cations on the octahedral sublattice. Using PCA and empirical standards to analyse mixtures Although it is possible to analyse mixtures of a few components with methods such as fitting EXAFS to theoretical standards, it can be quite tedious if there are a large number of possible components to choose from. One solution is to use PCA to identify the component phases, and then proceed with more traditional fitting techniques to refine the parameters. Ressler et al. recently used this approach on mixtures of nanoparticulate manganese compounds generated by internal combustion engines.298 Although their interest was environmental, the challenges are not unlike those sometimes encountered in magnetic nanoparticle syntheses: according to the authors, ‘X-ray diffraction measurements yielded little or no information due to a combination of insufficient sample mass, amorphous nature of the particulate matter, and/or small particle size.’ Principal component analysis requires samples that differ in non-trivial ways; the study authors accomplished this by collecting 12 samples from engines operating under various conditions. The PCA indicated that the 12 XAS spectra could be constructed out of just three components, implying just three manganese containing compounds were present in the exhaust. The study authors then examined the XANES of each sample to establish the average valence of the samples; this, along with ESCA results, allowed them to narrow the likely compounds to 10. Of these 10, only Mn3O4, MnSO4, and Mn5(PO4)[PO3(OH)]2?4H2O could be constructed from the components present in the samples, suggesting that these were the three compounds present. International Materials Reviews 2004 VOL 49 NO 3–4 151 Willard et al. Chemically prepared magnetic nanoparticles Future trends in synchrotron characterisation of magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd As the synthesis of magnetic nanoparticles has become more sophisticated, moving from ball milling methods which generate a broad size distribution of disordered single phase particles to wet chemical techniques which show promise for producing nearly monodisperse particles with a high degree of structural order, so have the techniques used to characterise them. Increasingly, synchrotron radiation has played a role, with both experimental and analytical techniques rapidly advancing and proliferating. High intensity sources, for example, have stimulated the development of DAFS, enhancing the element specificity of XAS with the ability, in many cases, to probe individual phases. The EDXAS technique is now allowing the progress of synthesis reactions to be monitored at a scale of seconds or below; this technique has as yet been little used with magnetic nanoparticles, but that is likely to change as researchers broaden their focus from the end-product of a synthesis to its mechanism and kinetics. Curve fitting to theoretical standards is becoming increasingly sophisticated, with simultaneous multi-edge refinements292,303 and models that account for the size and shape of nanoparticles244–246 beginning to appear. Finally, PCA promises to yield significant structural information from materials that had previously resisted characterisation. Magnetic properties of nanoparticles This section of the review emphasises the magnetic characteristics important to nanoparticles. The following subsections provide a general overview of the relevant parameters, behaviour, and experimental tools used to examine magnetic properties. Additionally, selected examples from recent literature have been included as examples of the types of research being conducted and to emphasise the strengths of many characterisation techniques. Magnetism of fine particles In general, the magnetic behaviour of nanocrystals is dependent on the physical properties of individual particles and their environments. The most important physical characteristics include chemical composition, particle size, particle morphology, intrinsic materials parameters (e.g. magnetocrystalline anisotropy, saturation magnetisation, etc.), surfaces/interfaces, and particle size distribution. Most of these parameters are easily obtained (or are relatively unimportant) for bulk magnetic materials; however, as described in the previous section, this is not necessarily true for nanoparticles. The terms pertaining to the environment of the nanoparticles are generally controllable during experimentation (albeit occasionally with difficulty). These terms include temperature, applied magnetic fields, measurement time, and interparticle distance. In many experiments, even the sequences in which fields and temperatures are changed are quite important. All of these factors lead to potentially confusing results. Due to the large amount of existing data, comparisons of bulk magnetic material behaviour can be made to alleviate some of the confusion in nanoparticle analysis. Although, when the particles are isolated and sufficiently 152 International Materials Reviews 2004 VOL 49 NO 3–4 small, magnetic responses similar to bulk magnetic materials fail to describe their performance. When the critical length scales for physical properties are similar in size to the structural length scales of the material, interesting physical phenomena arise. In the case of exchange coupled magnetic materials, this length is 1–100 nm and called the ‘exchange correlation length’. For this reason, nanoparticulate materials have provided an interesting magnetic phenomenon that has been studied over the past half century. However, the technological importance of miniaturisation (especially in the burgeoning fields of nanoelectronics and bioelectronics) requires a better understanding and control of producing these materials with reliable properties. Size, shape, composition, and distributions of these qualities about their means will determine the success of these materials for application. In recent years, new processing techniques and novel use of proven characterisation techniques have advanced our knowledge of magnetic nanoparticles. However, the recent technological attention adds to over half a century of basic research in this field. A number of excellent reviews have been published over the years that encompass many of the aspects of the topics contained herein.304–306 The following subsections introduce definitions of magnetic quantities (see Appendix also) and forms of magnetic behaviour that are important to the further discussion of magnetic nanoparticles. Magnetic materials parameters Intrinsic and extrinsic materials properties are generally easy to distinguish from each other for bulk materials. Intrinsic characteristics are generally invariant with respect to microstructure of the material, depending mainly on the crystal structure and chemical composition (or stoichiometry) of the material. Examples of intrinsic magnetic properties include Curie temperature Tc, saturation magnetisation Ms, exchange stiffness A, and magnetocrystalline anisotropy K. Extrinsic parameters, on the other hand, depend upon the microstructure of the material and are naturally affected by the size, shape, and morphology of the particles. Extrinsic magnetic properties include coercivity Hc, remanent magnetisation Mr, and magnetic susceptibility x. However, this clear demarcation is not necessarily the case for nanoparticles, where the large amount of surface area may convert conventionally intrinsic properties into extrinsic properties due to surface relaxation and broken bonds at the particle surfaces. This has been demonstrated by Sun et al., where nickel nanowire arrays exhibit reduced Curie temperatures when the wire diameter is reduced below 100 nm.185 As might be expected, the deviations from the bulk intrinsic properties increase as the particle size is reduced. Paramagnetism All materials respond to applied magnetic fields. Those that possess strong interaction with magnetic fields are generally called magnetic, however many other types of phenomenon are more common than this class, more properly called ferromagnetic or ferrimagnetic materials. Paramagnets are one of the other classes of magnetic materials that are especially important due to the close relation of paramagnetism to ferromagnetism. Willard et al. Paramagnetic materials possess unfilled electronic shells giving them a permanent magnetic moment. This moment consists of contributions from the spin and orbital momentum of the electrons of the atom. Paramagnets differ from the more technologically useful ferromagnetic materials in the fact that their magnetic moments do not interact strongly with each other or with applied magnetic fields. The magnetic moments tend to align with an applied magnetic field in a stochastic manner, yielding a slightly positive magnetic susceptibility (y1026). The temperature dependence of the susceptibility for many paramagnetic materials follows the well known Curie law (x5C/T, where C is the Curie constant – a term dependent on the properties of the paramagnet). Chemically prepared magnetic nanoparticles 18 Non-interacting particles (compositionally invariant and single phase) as function of particle size, indicating magnetisation reversal mechanism regimes at isothermal temperature T0 Published by Maney Publishing (c) IOM Communications Ltd Ferromagnetism/ferrimagnetism More complex, and interesting, magnetic phenomena are found when the magnetic moments on adjacent atoms interact. Ferromagnetic materials have interactions causing the alignment of the magnetic moments. These materials have high susceptibility (up to 106) and net magnetic moments even in the absence of an applied field. Still more complex, yet equally interesting from a technological standpoint, are ferrimagnetic materials. These materials consist of antiparallel arrangements of the magnetic moments, yet the material maintains a net magnetisation. This is due to either the numbers of moments being different in each direction, or the size of the magnetic moments in alternating directions being different (or both). Ferrimagnets have high susceptibility (up to 106) and net magnetic moments even in the absence of an applied field, much the same as ferromagnets. At sufficiently high temperatures, the magnetic ordering of both ferromagnetic and ferrimagnetic materials is disturbed, causing a change to paramagnetic behaviour (at the Curie temperature, as shown in Fig. 19c). The most important technological bulk materials have ferrimagnetic or ferromagnetic behaviour with their high susceptibility and net magnetisation. Common examples of technologically important ferromagnetic materials include iron, cobalt, nickel, and their alloys and rare earth intermetallics (i.e. Nd2Fe14B and Sm2Co17). Prevalent ferrimagnetic materials include iron based oxide materials with the inverse spinel crystal structure (called ferrites), barium based or strontium based magnetoplumbites, and rare earth/iron based oxides (called garnets). As a result of their excellent bulk magnetic properties, these materials are synthesised as nanoparticles for magnetic applications. Ideal particle systems The magnetic behaviour of nanoparticles is diverse and complicated. The characteristics of particles and their environments will be discussed below in the simplest manner. This means a discussion of spherical, monodisperse particles without magnetic interactions between adjacent particles or agglomerations. Then, the ‘Mechanisms for complicating magnetic behaviour’, e.g. interparticle interactions, particle size distributions, will be discussed. Single domain particles Bulk ferromagnetic materials are generally polycrystalline with each grain consisting of thousands of magnetic domains separated by boundaries called domain walls. In the simplest case, the magnetisation in adjacent domains continuously changes direction by 180u over the width of the domain wall. The domain wall is a defect in the material that requires energy to form. As mentioned above, the multiple domains form in a grain to alleviate the magnetostatic field exiting the grain. Figure 18a shows a multidomain particle with 90u domain walls. It should be noted that the magnetic domain walls have significant width, generally in the tens to hundreds of nanometres. In many respects, the most technologically important characteristic of magnetic materials is the process of magnetisation reversal. Whether the material requires very little field to reverse the magnetisation (i.e. a soft magnet) or maintains a single magnetisation direction to very high magnetic fields (i.e. a hard magnet), the magnetisation reversal determines the performance of the material. This is true for bulk magnets and magnetic nanoparticles alike. In bulk magnetic materials, the nucleation and motion of domain walls through the material control the reversal. The ease with which the domain walls move through the material when a magnetic field is applied determines the application for which the material is best suited. Pinning of domain walls on grain boundaries, voids, or inclusions causes an increase in the field necessary for magnetisation reversal by hindering the motion of domain walls. The nucleation of magnetic domains arises to minimise the magnetostatic energy. As the size of a bulk ferromagnet is reduced, the nucleation of reverse domains becomes more difficult because of the large magnetostatic energy. A small ferromagnetic particle made up of a single spherical grain has a magnetostatic energy with the form (pm0M 2s r2w)/9 and the domain wall energy (2pr3c)/w, where m0 is the permeability of free space, r is the particle radius, c is the domain wall energy per unit area, and w is the domain wall width. Minimising the total energy with respect to the particle radius yields a relation, in terms of the materials parameters, for the critical radius under which a domain wall will not form309 rcrit ~(324AK)1=2 =(m0 Ms2 ) In a practical sense, particles below this limit consist of a single domain that thermodynamically cannot support the formation of a domain wall (as indicated by Fig. 18b). This requires magnetisation reversal by International Materials Reviews 2004 VOL 49 NO 3–4 153 Willard et al. Chemically prepared magnetic nanoparticles rotation of the magnetisation into the applied field direction. The preferred direction of the magnetisation of a ferromagnetic material for instance, in the absence of an applied field, is determined by magnetic anisotropies, which arise from many sources, including the shape of the magnet, its crystalline structure, chemical composition, the strain in the material and effects of surface chemistry. Their origins are quite different and affect the characteristics of magnets of all sizes, particles and bulk alike. Generally, many types of anisotropy are combined to fully describe nanoparticles.310 Shape anisotropy arises from the field formed by the magnetisation of the material (a consequence of the Maxwell equations).311,312 It is described analytically for ellipsoids of revolution, with a uniaxial form for prolate ellipsoids (i.e. acicular particles) 1 Ms2 (Nb {Na ) sin2 h E~ 2 Published by Maney Publishing (c) IOM Communications Ltd where h is the angle between the magnetisation and the applied field, Ms is the saturation magnetisation, and Nb and Na are the demagnetising factors for the long and short axes of the ellipsoid, respectively. Spherical particles have isotropic shape anisotropy, which is easy to visualise since all directions are equal. Slight aspheric distortions of near spherical particles yield inconsequential differences in the shape anisotropy until the length of the aspheric axis exceeds approximately 30% of the sphere radius. The magnetocrystalline anisotropy forms due to spinorbital coupling and, in general, possesses the symmetry of the crystal structure of the magnet. Uniaxial anisotropy for tetragonal, rhombohedral, and hexagonal crystal structured materials has the form E~K1 sin2 hzK2 sin4 h while cubic materials have energies described by E~K1 (a21 a22 za22 a23 za23 a21 )zK2 (a21 a22 a23 ) where ai are the direction cosines for the magnetisation with respect to the Cartesian directions of the cubic primary axes and h is the direction of the magnetisation with respect to the uniaxial direction. The angles used for magnetocrystalline anisotropy are in relation to the lattice vectors with energy minima corresponding to specific crystallographic directions. An example of the uniaxial energy density is shown in Fig. 19a and b. The lowest energy corresponds to the unique crystalline axis found in the tetragonal, rhombohedral, or hexagonal crystal systems. The strain derivative of the magnetocrystalline anisotropy provides additional anisotropic response through the magnetostriction coefficient ls. For a single axis of stress, this term is uniaxial, with the form 3 E~ l s sin2 h 2 s where s is the stress on the material. Although shape, magnetocrystalline, and magnetostriction anisotropies are the most common terms, additional forms of magnetic anisotropy have been examined, including one important to nanoparticles. Due to the large surface/volume ratio, the relaxation of atoms at the particle surfaces and the interaction of 154 International Materials Reviews 2004 VOL 49 NO 3–4 a uniaxial anisotropy energy density surface for blocked single domain particle (K is magnetic anisotropy, V is volume, and kT0 is Boltzmann energy term); b cross-section of energy density surface showing two energy wells at magnetic easy axes; c typical saturation magnetisation as function of temperature (H5constant); d hysteresis loops for ferromagnets below Curie temperature Tc 19 Schematic diagrams for single domain particles ligands with atoms on the surfaces of the particles can have a significant effect on the anisotropy of particles. A simple, elegant model of the hysteretic behaviour of single domain particles was developed in the late 1940s. This Stoner–Wohlfarth model313,314 considers the energy due to the magnetocrystalline anisotropy, shape anisotropy, and magnetostatic interactions of the particle as a function of a magnetic field applied at a fixed angle from the magnetic easy axis. As the applied field is increased, the magnetisation is coherently rotated into the field direction. Calculations based on the Stoner–Wohlfarth model are presented in Fig. 20. The ideal cases of isolated, monodisperse, non-interacting nanoparticles, both single domain and superparamagnetic are shown. For an assembly of non-interacting particles with aligned uniaxial anisotropy directions, a square hysteresis loop with high coercivity and remanent magnetisation is found (Fig. 20, curve a). An assembly of randomly oriented uniaxial anisotropy directions, on the other hand, exhibits lower remanence and coercivity (Fig. 20, Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd 20 Stoner–Wohlfarth model calculations of hysteresis loops for assembly of aligned single domain particles (curve a) and randomly oriented single domain particles (curve b): Langevin function calculation of assembly of superparamagnetic particles is also shown (curve c) curve b). The Stoner–Wohlfarth model works well for single domain particles; however, as the particle size is reduced further, the magnetostatic and anisotropic magnetic energies are no longer the only two of consequence – thermal energy plays an increasing role. As demonstrated in Fig. 20, curve c and discussed in the next subsection, this thermally activated magnetisation reversal, or superparamagnetic behaviour, exhibits no remanent magnetisation, no coercivity, and the necessity of very large fields for saturation of the magnetisation. Superparamagnetism Superparamagnetism is a magnetisation reversal mechanism for fine particles driven by thermal energy.315 At large particle sizes, the magnetisation is confined to specific directions determined by the shape, crystal structure, lattice strain, etc. As the particle size is reduced, stochastic thermal fluctuations exceed the energy barrier keeping the magnetisation in its energy well (Fig. 18d). Similar to a ferromagnetic material, the magnetic moments of adjacent atoms remain aligned and acting as one large magnetic moment for the whole particle (i.e. exchange coupled), however their direction is not fixed (as indicated by the isotropic energy density in Fig. 21a). An assembly of these particles with ‘super moments’ acts like a paramagnetic material, hence the term superparamagnetism. However, this should not be confused with the magnetic material having ‘supermagnetic’ properties. In the end, superparamagnetism is not terribly useful for technological applications. The thermal activation over the magnetic anisotropy energy barrier takes the form f ~f0 exp({Eb =kT) with Eb as the energy barrier (equal to KV in the absence of an applied field), f0 is an attempt frequency on the order of 109 Hz, and kT is the Boltzmann energy term. When the time for activation over the energy barrier is long compared to the experimental measurement time, the superparamagnetic particles are ‘blocked’. The blocking of superparamagnetic particles occurs below a temperature aptly named the blocking temperature TB. However, the blocking temperature is dependent on the time of measurement, applied magnetic field, and particle size distribution, making it an ill defined parameter. a nearly isotropic energy density surface; b typical magnetisation as function of temperature for small fixed fields, H5constant (ZFC is zero field cooled; FC is field cooled) – particles are considered blocked below TB; c ‘hysteresis’ loop for superparamagnets above blocking temperature TB 21 Schematic diagrams for superparamagnetic particles Below the blocking temperature, the magnetisation as a function of temperature and magnetic field are similar to the curves found in Fig. 19c and d, respectively. The particles possess coercivity, remanent magnetisation, and saturation magnetisation. However, in contrast to the single domain particles, the Curie temperature is rarely exceeded prior to the blocking temperature upon heating. Therefore, superparamagnetic behaviour takes over long before significant reduction of saturation magnetisation as the temperature is raised. Figures 21b and 21c show the magnetisation changes typical of superparamagnetic particles above the blocking temperature and the characteristic magnetisation versus magnetic field curve with neither remanence nor coercivity. The size scale between superparamagnetism and single domain particles is best described by its magnetisation reversal behaviour. The constant fluctuation of the superparamagnetic particles is not entirely present, however magnetic relaxation does occur. The coherent rotation mechanism of the single domain particles does not describe the particles, although they maintain hysteretic behaviour. This size range intermediate to superparamagnetic and single domain particles exhibits incoherent rotation of the magnetisation, one example International Materials Reviews 2004 VOL 49 NO 3–4 155 Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd 22 Diagram for blocking temperature as function of particle diameter for constant measurement time t and magnetic anisotropy K: if measurement time is decreased, then blocking temperature is increased for given grain size (indicated ‘a’) or if magnetic field H is raised then blocking temperature is lowered for given grain size (indicated ‘b’) of which is the magnetisation curling mechanism (see Fig. 18c).316 Magnetic curling has been examined by Ferré and co-workers on nickel and cobalt nanowires where the competing shape and magnetocrystalline anisotropies are at 90u angles to each other.182 Isolated particles The magnetisation of an assembly of superparamagnetic particles has the same dependence on applied magnetic field as a paramagnetic material, replacing the atomic magnetic moment of the paramagnet with the particle magnetic moment of the superparamagnet. The Langevin function describes this behaviour L½b~coth ½b{1=b where b5mH/kT (m5MsV). The magnitude of the magnetic moment m of an individual particle is a strong function of particle radius (V3r3). This is important when particle size distributions are considered. The Langevin function as applied to superparamagnetism reveals the chief characteristics of the superparamagnet response to a magnetic field, namely, no coercivity, no remanent magnetisation, and very high field saturation of the magnetisation. Since superparamagnetism is a stochastic phenomenon, the direction of particle magnetic moment fluctuates with time. Any specific particle will change its direction of magnetisation during the time of magnetisation measurement. For example, measurement by SQUID (superconducting quantum interference device) magnetometry has a measurement time of roughly 102 s, yielding a blocking temperature defined by 25~KV =kTB whereas Mössbauer spectroscopy has a measurement time of 1028 s, providing a blocking temperature of 2:7~KV =kTB In other words, when the measurement times are much greater than the relaxation time, thermal activation will provide a statistical average of the magnetisation (for temperatures above blocking temperature of a particle). 156 International Materials Reviews 2004 VOL 49 NO 3–4 As illustrated in the schematic Fig. 22, the perceived blocking temperature can be manipulated by changing the measurement time t or the applied field H for a given set of nanoparticles (possessing constant magnetic anisotropy K). Decreasing the measurement time increases the blocking temperature when the particle assembly and magnetic fields are kept constant (Fig. 22, indicated ‘a’). This is illustrated by the comparison in the previous paragraph between SQUID magnetometry and Mössbauer spectroscopy. When the magnetic field is increased (for a single measurement time and particle assembly) the blocking temperature is reduced (Fig. 22, indicated ‘b’). This is due to the stabilising effect of the magnetic field on the particle assembly. The general trend of larger blocking temperatures at large particle size has been demonstrated experimentally, e.g. MgFe2O4 and CoFe2O4 (Refs. 70, 71) and cobalt (Ref. 317). The reduction of TB when the magnetic field is increased has been shown, e.g. (Fe,Cu),317 Fe3O4, CoFe2O4, MnFe2O4 (Ref. 318), and MgFe2O4 (Ref. 319). Dilution of samples also has an effect on the blocking behaviour of nanoparticles. Separate studies on cobalt (Ref. 39) and Fe3O4 (Ref. 320) have shown reduction in the blocking temperature as the interparticle interactions are increased. This is supported by energy calculations for self-assembled monolayers containing particles with uniaxial anisotropy and magnetostatic interactions.321 The blocking temperature of the nanoparticle array decreased as the inverse cube of the interparticle spacing. The smallest nanoparticle materials, with sizes below 1–2 nm, behave in a manner that is difficult to relate to their bulk counterparts. Superparamagnetic materials have enough particle volume to remain bulk-like in the case of magnetocrystalline anisotropy and magnetisation. However, when the fraction of surface atoms becomes too great, these intrinsic material properties are highly influenced by surface relaxation effects, thereby modifying their values. This in turn makes comparisons to the models for superparamagnetism difficult. This has been indicated in Fig. 18e, with the title ‘sub-superparamagnetic’ particles for lack of a better term. Mechanisms for complicating magnetic behaviour Up to this point, the magnetic properties have been simplified to a large degree to avoid confusing issues. These ideal particle systems have avoided the effects of oxidation, organic ligands bound to the particle surface, particle shape, agglomeration, multiphase materials, etc. All of these have contributed to the difficulty of characterisation and analysis of the magnetic characteristics of nanoparticles. The magnetic behaviour of the nanoparticles is sensitive to the physical characteristics of individual particles (including morphology, intrinsic materials parameters, and size) and their environments (such as, interparticle interactions, material at the particle surfaces, temperature, and applied magnetic fields). Therefore, a complete characterisation of the particles and their surroundings should accompany any magnetic study. Failure to provide these prerequisites makes meaningful determination of the intrinsic magnetic properties of the nanoparticles difficult. Willard et al. 23 Schematic phase diagram for interparticle interactions as function of particle size Chemically prepared magnetic nanoparticles 1: shell exterior surface; 2: shell relaxation shell material; 4: shell interphase interface core interphase interface; 6: core material 24 Schematic diagram of metallic nanoparticle dation at surface, indicating various regions region; 3: region; 5: with oxistructural Published by Maney Publishing (c) IOM Communications Ltd Interparticle interactions Interparticle interactions cause a multitude of complications in assessing the magnetic properties of nanoparticle assemblies. Each particle generates a local magnetic field that can change the local energy minima for magnetisation of adjacent particles. This has been examined by Zeng et al., where an effective demagnetisation factor has been used to describe increased particle packing densities.177 The magnetostatic field generated by the particles can promote chaining of particles and agglomeration if the particles are free to move (as found by Burke et al.322 for example). As schematically shown in Fig. 23, interparticle interactions play an increasing role as the distance between particles decreases. Particles are considered to be isolated when they have a sufficiently small interaction, as the magnetic field generated by adjacent particles is generally the longest range interaction, this limit is determined by some small arbitrary field generated by the particles in the assembly. The particles shown in Fig. 18 are considered isolated; this is indicated in Fig. 23 by a dashed line. At large separation distances d12, the simple cases of superparamagnetic and single domain particles are found. The shaded region between the two indicates the region where incoherent reversal modes make significant contributions to the magnetisation reversal. When the particles touch each other, short-range exchange interactions are possible. Depending on the amount of interface, this might be considered a polycrystalline ferromagnetic material at this point (assuming a large fraction of particle surface is represented by interparticle interfaces). This is the desirable case for using nanoparticles as precursors for bulk compacted material, although there are great difficulties in achieving this goal. Intraparticle effects Other forms of anisotropy may form when copious surface area is available. Anisotropy due to surface termination of the particle, exchange coupling to antiferromagnetic oxides, or bonding to surfactants alter the magnetic behaviour and, in some cases, the anisotropy of the material directly. In many cases, a passivating coating has been applied to protect metallic nanoparticles from oxidation, as a means of making them compatible with specific environments (biological or chemical) or to avoid agglomerations (ferrofluids). The addition of shells to nanoparticles adds new dimensionality to the complexity of the magnetic response. In Fig. 24, the simplest form of a core/shell nanoparticle is represented schematically. Six regions are enumerated for their potential to affect the magnetic properties of the composite material. It is noteworthy that some of these regions may share a coherent interface making them difficult to distinguish from one another. The shell surface has been shown to modify the ‘magnetic volume’ of the particle, making it differ from the structural volume (as found by TEM for example). This might be due to the direct bonding of ligands to the surface, missing bonds at the particle surface, or development of a non-magnetic phase at the surface.323,324 Reduced magnetisation due to the expansive surface of nanoparticles has been explained by spin canting or pinning at the surface of the nanoparticles,325 and the introduction of a ‘dead’ layer at the surface (y5 Å thickness).326,327 Surfactants coating the surfaces of particles can have significant impact on the magnetic properties of the nanoparticles.27,322,328 The effect of various surfactants bound to the surfaces of Fe2O3 nanoparticles has been examined by Shafi et al.113 They found phosphonate coated nanoparticles had much reduced magnetisations compared to carboxylate and sulphonate coated particles. Modelling of the surface effects on the anisotropy of magnetic nanoparticles has been approached by adding radially symmetric anisotropy terms to compete with uniaxial anisotropy terms for the bulk of the particles.329 Depending upon the thickness of the shell, there may be a region with near bulk properties for the shell material. However, that region is sandwiched between a surface dominated region and an interphase interfacial region near the core material. The shell material within a few unit cells of the surface of the particle presumably has a relaxation of the bonds due to the missing bonds at the surface, which can significantly change the magnetisation, especially in magnetic oxides. The interdiffusion of material from the core into the shell material has significant feasibility, depending on how the particles were passivated. This region may also have significant strain produced during the formation of the shell and the interphase interface. Similarly, the diffusion of oxygen into the core material is possible, with the same International Materials Reviews 2004 VOL 49 NO 3–4 157 Willard et al. Chemically prepared magnetic nanoparticles consequences as for the shell material. Finally, presuming that the particles do not fully oxidise, a metallic core with intrinsic magnetic properties similar to the bulk resides at the centre. Intentional encapsulation of magnetic nanoparticles into a matrix can have profound effects on the magnetic behaviour of the array. A non-uniform change in the saturation magnetisation as a function of silica coating thickness was observed for iron nanoparticles by Atarashi et al., for example.330 Another complication to understanding the magnetic properties is found when the crystal structure accommodates a large range of compositions (such as metallic alloys or spinel ferrites). In these cases, the compositions of the particles may deviate significantly from their parent solutions.141,193 The assumption that the solution composition and the particle composition are identical should be avoided as this will make meaningful magnetic characterisation difficult to interpret. Published by Maney Publishing (c) IOM Communications Ltd Imbedded and consolidated particles Magnetic nanoparticles are designed for a variety of applications, as described at the start of this review. The applications mentioned there indicate the necessity for both single particle and consolidated particle systems. Single particle applications include ferrofluids and targeted drug delivery. Consolidated particle applications include inductor applications and, in some cases, magnetic recording media. Thus far, the present section has addressed single particle magnetic behaviour. In this subsection, consolidated nanostructured materials will be discussed. In many cases, these materials possess unique magnetic properties due to the structural and magnetic correlation lengths being similar in size. Individual nanoparticles brought into close proximity to each other experience magnetostatic interactions from their neighbours’ local fields (produced by the existence of the magnetisation). The magnetostatic energy is a major driving force for agglomeration. This can cause chaining of magnetic particles into long ‘pearl necklace’ like structures to minimise the magnetostatic fields for the whole system. This usually occurs for freely mobile particles that do not have a capping layer to limit the distance between particles, thereby reducing the energy well for minimisation of the magnetostatic energy. Consolidated nanoparticles differ from the freely moving nanoparticles in the respect that they are rigidly affixed to one another and they share substantial interfaces. Ideally, the nanoparticles could be compressed into completely dense bulk forms, however this has proven quite difficult in practice. Generally, consolidation to a fully dense form requires physical compaction followed by annealing. If the annealing step is too short or at too low a temperature, the compact maintains voids. When the annealing process is too long, significant grain growth occurs. In many cases both of these problems arise concurrently. There has been some success in consolidated nanoparticle materials. Inductor core materials have been consolidated using a microwave sintering process. Exchange spring nanocomposite materials have also been produced by using two types of nanoparticles.331 In both of these cases, the development of the nanocrystalline microstructure is advantageous for improved magnetic properties. 158 International Materials Reviews 2004 VOL 49 NO 3–4 a 0.024 powder/binding polymer ratio (Tmax540 K); b 1.2 powder/binding polymer ratio (Tmax565 K); c highly agglomerated powders of oxide (Tmax588 K) 25 Zero field cooled (ZFC)/field cooled (FC) results for 9 nm diameter c-Fe2O3 nanoparticles with increasing amounts of interparticle interactions:334 applied field for all three experiments was 50 Oe The substantial interfaces in the consolidated nanoparticle materials change the interparticle interactions from magnetostatic to exchange coupling. The exchange coupling is essential for the improved magnetic properties found in many nanostructured materials (i.e. exchange spring magnets332 and nanocrystalline soft magnets333). The nanocomposite materials possess hysteresis loops similar to those of single phase magnets when exchange coupled. However, if the phases are not fully exchange coupled for whatever reason, the hysteresis loop develops a shoulder in its second quadrant. This indicates the phases have different coercivities and are switching independently. Measurement techniques Due to the stochastic nature of many magnetic nanoparticle effects, the magnetic properties obtained by different measurement techniques are dependent on the time scales of measurement, the measurement temperature, and the applied magnetic field strength. Complications arise when the effects of surface atoms, particle size distributions, and interparticle interactions are considered. The wide variety of measurement techniques described in this section highlight some of the research using each technique. Table 1 gives a comprehensive listing of characterisation techniques used for examination of nanoparticles produced by chemical routes. Direct current susceptibility Direct current susceptibility measurement is a common method of determining the superparamagnetic blocking temperature. This is accomplished by examining a Published by Maney Publishing (c) IOM Communications Ltd Willard et al. a 11 nm diameter particles measured at 5 K (Hc52000 Oe); b 9 nm diameter particles measured at 5 K (Hc5897 Oe); c 6 nm diameter particles measured at 5 K (Hc5291 Oe); d 9 nm diameter particles measured at 290 K (Hc50 Oe) 26 Typical hysteresis loops from blocked and superparamagnetic hcp cobalt nanoparticles produced by multisynthesis process206 particle system assembly under zero field cooled (ZFC) and field cooled (FC) experimental conditions (see e.g. Fig. 25). The ZFC experiment requires cooling the sample to a low temperature with subsequent application of a small, applied magnetic field. The temperature is then raised at a constant rate and susceptibility data are collected. The susceptibility reaches a maximum at a temperature near the blocking temperature, with superparamagnetic decay of the susceptibility above the blocking temperature. The experiment proceeds with a second cooling of the sample, this time in an applied field. The FC experiment provides more certainty in the temperature at which superparamagnetic behaviour sets in. This experiment requires cooling the sample in a small magnetic field of the same strength as the ZFC experiment, again collecting susceptibility data as the temperature is increased. The measurement relaxation time for these experiments is near 10 s. Sun and Murray have shown that ZFC experiments on monodisperse self-assembled nanoparticles can be used for examination of interparticle interactions.39 The magnetostatic fields generated by the nearby nanoparticles in the array shift the peak in the ZFC curve to lower temperatures in addition to sharpening the peak. It should be noted that these experiments were supported by evidence from X-ray diffraction, transmission electron microscopy, and inductively coupled plasma experiments, all of which added to the analysis of the magnetic properties. The work of Puntes and Krishnan shows interparticle dipolar interactions through the use of ZFC/FC experimentation.335 The experiments on e-Co self-assembled nanoparticles showed a broad peak in the ZFC susceptibility characteristic of dipolar interactions. These authors also observed a strong increase in the FC susceptibility at low temperatures, presumably due to Brownian motion of the particles in the viscous oleic acid when the field was applied at high temperature. Chemically prepared magnetic nanoparticles 27 Magnetisation as function of magnetic field/absolute temperature for PVP coated iron nanoparticles (3– 8 nm in diameter): data were collected at 200, 250, and 290 K – universal curve formed when magnetic field is normalised by absolute temperature is characteristic of superparamagnets101 The importance of shape anisotropy is shown by Park et al. in their study of 2611 nm iron nanorods and 2 nm diameter spherical particles.82 The ZFC/FC experiments show an order of magnitude lower blocking temperature for the spherical nanoparticles, consistent with the calculated anisotropy difference between the sphere and rod morphologies. Direct current hysteresis Lack of coercivity and remanent magnetisation in particulate samples are hallmarks of superparamagnetism. The hysteretic behaviour of a magnetic material can be probed by cycling a large magnetic field between opposite directions relative to the sample. The theoretical hysteresis curves shown in Fig. 20 compare the ideal cases of isolated, monodisperse, non-interacting nanoparticles, either single domain or superparamagnetic. More generally, a particle size distribution where some of the particles are large enough to be blocked while others remain superparamagnetic is necessary to describe a real material. As first demonstrated by Bean in 1955,336 a weighted average of contributions from the superparamagnetic (Langevin function) and blocked particles (Stoner–Wohlfarth model) describes the hysteretic behaviour, barring interparticle interactions. Realistic models of this behaviour have been developed in recent years to describe the variety of measured loop shapes.337,338 Typical hysteresis loops for magnetic nanoparticles are shown in Fig. 26. These loops were produced using samples of hcp cobalt synthesised from the multisynthesis processing techniques pioneered by the authors, Murray et al.206 The results show increased coercivity at 5 K for larger particles and superparamagnetism when the 9 nm sample was heated above the blocking temperature (in this case 290 K). An illustration of an effect unique to superparamagnetic particles is shown in Fig. 27. When the magnetisation is plotted against magnetic field normalised by absolute temperature, the data fall onto a universal curve.101,336 If an applied field is large enough, the magnetisation of the sample will stop increasing with applied field (saturate). This value of magnetisation is an intrinsic property and is most closely linked to the composition of the material. For comparison between different International Materials Reviews 2004 VOL 49 NO 3–4 159 Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd materials, the measured magnetic moment for the hysteresis loop at high fields (generally above 1.5 T) is normalised by volume or mass. Error bars should accompany these values for magnetic nanoparticles due to the small sample sizes used in many cases. Large errors will accompany samples normalised by the mass of the magnetic transition metal used in the sample. A basic type of hysteresis loop study for nanoparticle materials entails the measurement of loops above and below the blocking temperature of the material. This is illustrated by the work of Chen and Zhang for MgFe2O4 spinel ferrites.339 They demonstrated the increased susceptibility for large sized superparamagnetic particles at high temperatures and the coercivity dependence on particle size for blocked particles. Sun et al. used a superconducting quantum interference device (SQUID) magnetometer to examine the phase transformations via hysteresis loops of monodisperse self-assembled FePt nanoparticles.207 Their hysteresis results identify annealing conditions and composition ranges that were optimised for large coercivity. SQUID magnetometry studies of the surface effects on the magnetisation of nanoparticles were performed by Toneguzzo and co-workers.146,164 These experiments showed a common effect in nanoparticle systems, namely, that the saturation magnetisation varies linearly with the inverse of the mean particle diameter (or proportional to the surface area divided by the volume). This has been explained by spin canting or pinning at the surface of the nanoparticles,325 the introduction of a ‘dead’ layer at the surface,326 and the formation of surface oxides during various studies. Toneguzzo et al. used a metallo-organic and oxide phase shell model to analyse their data.146 A particle core/shell model was used by Gangopadhyay et al. to describe this reduction in magnetisation with decreasing particle size.340 Their parameters included magnetisation and densities of the core and shell materials (score, sox, rcore, rox), thickness of the shell tshell, and mean radius of the particle rmean, leading to the following equation rcore tshell spart ~score {3 score {sox (2) rox rmean This model has been shown effective for metals, alloys, and oxides, e.g. (Mn,Zn)Fe2O4 (Ref. 48), CoFe2O4 (Ref. 70), Y3Fe5O12 (Ref. 132), Fe,341 and (Ni, Co, Fe)146 (see Fig. 28). Additional complexity has been observed by Verelst et al. for Co/CoO nanoparticles, where the blocked particles exhibit exchange bias behaviour due to the antiferromagnetic coupling of the CoO with the metallic Co core.75 A similar study of yttrium–iron garnet (Y3Fe5O12) by Sánchez et al. showed the saturation magnetisation is lower than the bulk value by a term inversely proportional to the particle diameter.132 In this case, the oxide particles have a decreased magnetisation presumably due to either a non-magnetic surface layer or noncollinear spin arrangement. Shafi et al. have examined the surface effects of various surfactants on the surfaces of amorphous Fe2O3 nanoparticles.113 They attribute low magnetisation found in hysteresis loops for particles coated with octyl phosphonic acid as an indication of antiferromagnetic coupling to the nanoparticles. Sun et al. have studied the coercivity as a function of temperature for FePt nanoparticles with different 160 International Materials Reviews 2004 VOL 49 NO 3–4 28 Saturation magnetisation of (Co,Ni) and (Fe,Co,Ni) nanoparticles plotted against reverse mean particle radius: fits use equation (2), with extrapolation indicating near bulk values for saturation magnetisation146 isothermal annealing temperatures.207 They found higher coercivity at higher annealing temperatures, due in part to atomic ordering of the FePt into the L10 crystal structure with concomitant high magnetocrystalline anisotropy. Although the analysis was successful for this study due to careful experimentation and choice of materials, in general, the comparison of coercivity between samples is complicated by the changes in morphology producing changes in reversal mechanism. This problem was avoided by Sun et al. due to the charring of the surfactant coating during annealing, which prevented large changes in microstructure. In general, this type of analysis can be difficult, especially if agglomeration or oxidation of the samples occurs. Temporal remanent magnetisation The switching and relaxation behaviour of nanoparticles can be examined by a number of different temporal remanent magnetisation techniques. Thermoremanent magnetisation (TMR) studies provide relaxation information about particles that have been field cooled below the blocking temperature with ensuing removal of the magnetic field at the measurement temperature. Isothermal remanent magnetisation (IMR) experiments require zero field cooling to the measurement temperature with subsequent application and removal of a magnetic field. Finally, similar to the IMR curves, dc demagnetisation curves involve zero field cooling of the sample, however a large magnetic field is applied to the sample and then a field is applied in the opposite direction from which it is finally removed. The combination of dc demagnetisation and IMR experiments can provide interparticle interaction information through the calculation of the dM parameter.342,343 Since superparamagnetic particles do not have remanent magnetisation, the magnetic remanence will come from the blocked magnetic particles at a given temperature. The experiments of Zeng et al. have shown the interparticle interactions of FePt nanoparticles by use of dM curves.344 The particles in this case are annealed and thereby aggregate showing a trend of negative dM values indicating dipolar interactions for low annealing temperatures and large positive dM indicating exchange coupling at high annealing temperatures. Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd 29 Thermal remanent magnetisation (TRM) and zero field cooled (ZFC) data for MnFe2O4 sample with 8 nm dia. grains: TRM curve was produced by cooling in 100 Oe field and same field was used for ZFC curve – full relaxation of TMR curve coincides with blocking temperature72 Thermal remanent magnetisation was used to examine MnFe2O4 nanoparticles with a mean particle size of 8 nm (see Fig. 29).72 These experiments by Lui et al. showed a reduction of Mr to zero near the blocking temperature of the nanoparticle assembly. Thermal remanent magnetisation experiments have also been used by Held et al. to investigate the interparticle interactions in diluted samples of 7 nm diameter cobalt particles.345 An accompanying analysis provides context for a simple model of the TMR curves as a function of particle spacing. Alternating current susceptibility Alternating current susceptibility measurements are a valuable source of magnetic characterisation with measurement frequencies from 10 Hz to 10 kHz. This technique gives information about the dynamic behaviour of the nanoparticles with the capability for initial susceptibility measurements at very small fields. Alternating current susceptibility measurements are carried out as a function of temperature with fixed, small field amplitude. Multiple experiments at different frequencies indicate the changes in blocking temperature. Experiments show the real part of the susceptibility x9 is frequency independent at high temperatures due to thermal equilibrium of the superparamagnetic particles. When some of the superparamagnetic particles are blocked as the temperature is reduced, increased frequencies shift the x9 peaks to higher temperatures with a decreased magnitude. These attributes are clearly shown by the work of Dormann et al. on c-Fe2O3 nanoparticles (Fig. 30).346 Nanoparticles in varying sizes and degrees of interaction were examined by ac susceptibility. Particle samples (7 nm dia.) were examined as chains of particles and as entangled chain agglomerates. The particles with a chain-like environment possessed larger susceptibility than the agglomerated sample for all measurement frequencies. Additionally, the chained particle sample exhibited lower peak temperatures for x9, indicating lower stability of the particles to thermal fluctuations than the agglomerated sample that possesses more near neighbours (thus higher magnetostatic interparticle interactions). The effect of dilution on relaxation times of amorphous Fe0.78C0.22 particles as a function of temperature a chains of particles with 7 nm particle diameter; b highly agglomerated particles with 7 nm particle diameter: note higher blocking temperatures for higher frequencies and greater degree of agglomeration 30 Alternating current susceptibility data for samples of c-Fe2O3 at measurement frequencies of 10, 95, 1000, and 10 000 Hz and field amplitude of 1 Oe346 was examined by Djurberg et al.347 The more dilute sample showed much smaller relaxation times for a given temperature. This reflects the significance of interparticle interactions, where higher degrees of magnetostatic interaction provide stability to the system of particles. Ferromagnetic resonance Ferromagnetic resonance (FMR) experiments provide information about the anisotropy, the distribution of particle easy axis orientations, and surface effects of magnetic nanoparticles. The experiment uses a strong magnetic field to align the magnetic moments of the particles and a transverse high frequency field to produce a precession of the magnetic moment about the strong applied field. These experiments are generally accomplished by fixing the high frequency field component and measuring the magnetic susceptibility as a function of the direction and magnitude of the large applied field. In general, the resonance frequency is lowered for smaller magnetocrystalline anisotropy (at a constant external field). This has been shown by Fannin et al. for (Mn,Zn)Fe2O4 and (Ni,Zn)Fe2O4 nanoparticles with 9 nm mean diameters.348 The many GHz frequency range of the high frequency field International Materials Reviews 2004 VOL 49 NO 3–4 161 Willard et al. Chemically prepared magnetic nanoparticles Published by Maney Publishing (c) IOM Communications Ltd 31 Ferromagnetic resonance data for Co80Ni20 nanoparticles and microparticles – complex permeability m i0 of particles was measured in 0.1–18 GHz frequency range in absence of external magnetic field: 220 nm sample exhibited spin-wave resonance as indicated by numerous resonance peaks145 component gives a measurement time of greater than 1029 s. Viau and co-workers have examined (Ni,Co) and (Ni,Fe,Co) by FMR over a large particle size range (see e.g. Fig. 31).139,143–145,147 For micrometre sized, multidomain particles, they found a single broad resonance peak, while for particle below 50 nm, a single sharp resonance was observed. The large sized particles have resonance consistent with a curling process. The sharp peak for the sub-50 nm particles was consistent with non-uniform exchange resonance modes. In between these two extremes, multipeak resonance was observed, where the peaks have been correlated with spin-wave resonance modes for the particles (similar to those found in thin films). Spinel ferrites show similar behaviour. NiFe2O4 nanoparticles, 10–25 nm in diameter, show a single sharp resonance peak which broadens at reduced measurement temperatures.112 Studies carried out by Diehl et al. show the general trends of line sharpening for smaller particle size and higher measurement temperatures for e-Co and twinned fcc cobalt samples.349 These effects are due to the thermal relaxation of the nanoparticles, yielding a single sharp resonance peak. The peak shape changes were accompanied by shifts in the resonance lines due to changes in the presence or magnitude of the particle anisotropy. The effects of surface anisotropy were examined by Gazeau et al. for c-Fe2O3 nanoparticles below 10 nm diameter.350 Mössbauer spectroscopy Mössbauer spectroscopic measurements use the absorption and emission of c-rays to examine the local hyperfine fields at atomic nuclei. Generally, c-rays with frequencies in the range 1018–1019 Hz are used for these experiments to excite the 57Fe isotope within the sample. This technique can be used to reveal information about the crystal structure (refer to the subsection ‘Site occupancy’ above) and magnetic characteristics of the nanoparticles with roughly a 1028 s measurement time. Modelled Mössbauer spectra can be superimposed to incorporate many crystalline and amorphous phases, 162 International Materials Reviews 2004 VOL 49 NO 3–4 32 Mössbauer spectra from 57Fe in CoFe2O4 nanoparticles with 3 nm diameter at various measurement temperatures in zero field: solid lines are fits to data, including sextet terms from tetrahedral and octahedral spinel ferrite sublattices and superparamagnetic doublets353 distinguishing different oxidation states of iron and different site symmetries, including separate contributions from surface atoms when relaxation effects are not significant. Due to thermal activation, superparamagnetic particles exhibit fluctuations in the hyperfine field parameters as a function of time. Barium hexaferrites (BaFe12O19) have five incommensurate iron sublattices which produce complex Mössbauer spectra with a sextet for each sublattice and an additional doublet for any superparamagnetic contributions. These materials (as well as other complex oxides) have been probed for samples with different particle sizes and annealing temperatures.20,129,351 The simpler crystal structure of spinel ferrites reduces the number of incommensurate iron sublattices to two. For this reason, many Mössbauer effect spectroscopy studies have been conducted on these materials. Prasad and Gajbhiye examined NiFe2O4 nanoparticles by room temperature Mössbauer spectroscopy for samples isothermally annealed at temperatures ranging from 473 to 1173 K.352 The resulting data showed a transition in behaviour from superparamagnetism for samples annealed at 473 K to ferrimagnetism for samples annealed at 973 K due to grain growth. Intermediate to these results were samples annealed at 673 K, showing both a pair of sextets from the octahedral and tetrahedral sublattices of the ferrimagnetic phase and a doublet from the superparamagnetic phase. Another study by Chen et al. shows the temperature dependence (55–300 K) of the Mössbauer spectra for 6 and 12 nm diameter MgFe2O4 nanoparticles.319 These samples showed low temperature sextets with the addition of a doublet when the measurement temperature reached the Published by Maney Publishing (c) IOM Communications Ltd Willard et al. blocking temperature of some of the particles. At 300 K, only the superparamagnetic behaviour was observed. Samples of (Co,Zn)Fe2O4 nanoparticles of varying composition and size have been examined by Lee et al.128 Room temperature Mössbauer spectroscopy was carried out for Co0.9Zn0.1Fe2O4 particles annealed at temperatures between 473 and 1073 K. Results similar to those of Prasad and Gajbhiye were found. Figure 32 shows Mössbauer absorption spectra from 3 nm CoFe2O4 particles measured at temperatures from 4.2 to 180 K.353 The 4.2 K spectrum shows ferrimagnetic behaviour, while the 180 K spectrum shows superparamagnetic behaviour. The intermediate temperatures provide examples of the progression through the blocking of progressively smaller particles as the temperature is lowered. The effects of large surface area were examined by Bødker et al. by Mössbauer experiments on metallic iron nanoparticles.354 The anisotropy of the particles was determined in situ as a function of grain size from the observed hyperfine field. The results indicted an increase in the anisotropy as the particle size decreased which was mainly attributed to magnetocrystalline anisotropy of the particles. Closing remarks The goals of this review are to describe the chemical synthesis options available for the processing of magnetic nanoparticles, provide a review of useful and novel characterisation methodologies to better understand the structure and chemistry of these particles, and to broadly review the magnetic properties of these particles in terms of their intrinsic magnetism as well as cooperative effects. The review has been limited to the time period beginning in 1990 and extending through 2003. Where appropriate, the reader is directed to other reviews and papers that provide the needed background to understand the outstanding issues addressing magnetic nanoparticles. The scope has been focused on the chemical methodologies that have been used to synthesise magnetic nanoparticles. Since these techniques use similar solvent systems, they offer the widest versatility and flexibility in Chemically prepared magnetic nanoparticles processing. Combining one or more of these techniques is relatively simple and allows the synthesis of many types of oxides and alloys. Of particular interest is the use of surfactant-mediated synthesis, including micellar, sol–gel, and polyol techniques. These techniques offer the ability to control particle size to less than 10% and allow a wide range of materials and morphologies. Notwithstanding these powerful processing methods, gaining insight into the composition, phase, and structure of the particles remains elusive. To address this challenge, the use of synchrotron radiation techniques is proposed, to supplement the more routine laboratory characterisation tools. The X-ray absorption fine structure (EXAFS) technique has recently been used to analyse magnetic nanoparticles to determine element specific phase purity, cation disorder, and particle size. This powerful tool can address many of the difficulties that exist in determining the nature of nanoparticles. Finally, the intrinsic and extrinsic magnetic properties of nanoparticles are reviewed. Since the magnetism of these materials depends not only on particle chemistry and phase, but also on the particle size and environment, the roles of interparticle interaction and surface functionalisation are explored in determining the magnetic properties of the system. Appendix Units of measurement for magnetic applications suffer from unusual definition conventions, making it difficult to convert easily between CGS and SI systems. This appendix provides definitions and unit conversions for some of the salient terms necessary for understanding the magnetics of nanoparticles (see Table 3). More in depth descriptions of these terms and their uses can be found elsewhere.307,308 Curie temperature Tc is the phase transformation temperature from ferromagnetic (or ferrimagnetic) behaviour to paramagnetic behaviour. Saturation magnetisation Ms is the magnetic moment per unit volume of the material when a sufficiently large magnetic field is applied to remove all domain walls and align the magnetisation of the sample with the field. Table 3 Magnetic units table for CGS to SI conversion Quantity Symbol CGS unit SI unit Magnetic induction B gauss (G) Magnetic flux W maxwell (Mx) Magnetic field Magnetisation Magnetic polarisation H M 4p M J oersted (Oe) emu cm23 gauss (G) gauss (G) Specific magnetisation Magnetic moment Molar susceptibility Mass susceptibility Volume susceptibility Permeability Relative permeability Magnetic anisotropy Magnetostriction coefficient Exchange stiffness s m xm x k m mr K l A emu g21 emu emu Oe21 g21 mol21 emu Oe21 g21 Dimensionless G Oe21 Not defined erg cm23 ppm erg cm21 1024 tesla (T) (T5kg A21 s22) 1028 weber (Wb) (Wb5kg m2 A21 s22) 103/4p A m21 103 A m21 103/4p A m21 1024/4p tesla (T5kg A21 s22) 1 A m2 kg21 1023 A m2 4p 1026 m3 mol21 4p 1023 m3 kg21 4p dimensionless 4p 1027 kg m A21 s21 Dimensionless 1021 J m23 ppm J m21 International Materials Reviews 2004 VOL 49 NO 3–4 163 Willard et al. Chemically prepared magnetic nanoparticles Exchange stiffness A is the strength with which adjacent magnetic moments remain aligned due to exchange interactions. Magnetocrystalline anisotropy K is the internal energy density dependence on the direction of the magnetic moment with respect to the crystalline lattice. In this review, magnetic anisotropy is discussed in detail in the subsection ‘Single domain particles’, due to its strong influence on the magnetic properties of nanoparticles. Coercivity or coercive field Hc is the applied magnetic field required for reduction of a saturated magnetic material to zero magnetisation. Remanent magnetisation Mr is the magnetisation that remains after an applied field has been removed. Magnetic susceptibility x is the magnetisation normalised by the applied magnetic field. The magnetic susceptibility acts as a technological figure of merit for magnetic nanoparticles, since it links the particles (through their magnetisation) with a readily applied field. Published by Maney Publishing (c) IOM Communications Ltd Acknowledgements The authors gratefully acknowledge support for this work from the Office of Naval Research, Defense Advanced Research Projects Agency, and the National Research Council. The authors would like to thank Dr Bruce Ravel, Dr William O’Grady, Dr David Pena, and Dr Marc Raphael for many fruitful conversations. References 1. R. H. Kodama, A. E. Berkowitz, E. J. Mcniff and S. Foner: Phys. Rev. Lett., 1996, 77, (2), 394. 2. M. M. Miller, G. A. Prinz, S. F. Cheng and S. Bounnak: Appl. Phys. Lett., 2002, 81, (2), 2211. 3. S. Sun, C. B. Murray, D. Weller, L. Folks and A. 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