Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Multiferroics wikipedia , lookup
Strangeness production wikipedia , lookup
Degenerate matter wikipedia , lookup
Fluorescence correlation spectroscopy wikipedia , lookup
Superconductivity wikipedia , lookup
Bose–Einstein condensate wikipedia , lookup
State of matter wikipedia , lookup
Van der Waals equation wikipedia , lookup
Gibbs paradox wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Electron scattering wikipedia , lookup
APPLIED PHYSICS LETTERS 90, 062508 共2007兲 Lowering of the L10 ordering temperature of FePt nanoparticles by He+ ion irradiation U. Wiedwald,a兲 A. Klimmer, B. Kern, L. Han, H.-G. Boyen, and P. Ziemann Institut für Festkörperphysik, Universität Ulm, Albert-Einstein-Strasse 11, 89069 Ulm, Germany K. Fauth Max-Planck-Institut für Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart, Germany 共Received 22 December 2006; accepted 9 January 2007; published online 7 February 2007兲 Arrays of FePt particles 共diameter 7 nm兲 with mean interparticle distances of 60 nm are prepared by a micellar technique on Si substrates. The phase transition of these magnetic particles towards the chemically ordered L10 phase is tracked for 350 kV He+ ion irradiated samples and compared to a nonirradiated reference. Due to the large separation of the magnetically decoupled particles the array can be safely annealed without any agglomeration as usually observed for more densely packed colloidal FePt nanoparticles. The He+ ion exposure yields a significant reduction of the ordering temperature by more than 100 K. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2472177兴 Nanoparticles exhibiting a large magnetocrystalline anisotropy energy 共MAE兲 density have attracted enormous attention over the last years due to their potential use in magnetic data storage.1,2 The equiatomic alloy system FePt with a MAE of up to 6 ⫻ 106 J / m3 in its chemically ordered facecentered-tetragonal L10 phase is appealing for this purpose since high MAE values can be established over a relatively wide composition range.3 Moreover, self-assembled FePt nanoparticles can be prepared by chemical methods in large amounts at low cost.2,4 It turns out, however, that the assynthesized FePt particles exhibit the chemically disordered fcc phase with related low MAE values rather than the L10 phase which is kinetically blocked. Consequently, annealing at temperatures above 600 ° C is necessary to establish the technologically attractive, magnetically hard L10 phase.2,5 FePt nanoparticles prepared by means of colloidal chemistry are typically only 2 – 4 nm apart after their self-assembled deposition on a support. The organic ligands cannot withstand high temperature annealing, which therefore gives rise to particle agglomeration.6,7 Removal of the ligand shell by O and/or H plasma exposure results in an improved anchoring of the particles to the substrate.5,8 Thus, the formation of larger particles can be remarkably reduced, but not totally avoided. It is therefore desirable to reduce the annealing temperature at which the structural phase transition to the L10 phase occurs. Mainly two routes have been proposed: 共i兲 the incorporation of a third element by a few at. %,9 which has the undesired side effect that for pseudobinary alloys the MAE is significantly decreased;10 共ii兲 the enhancement of the number of vacancies within the particles aiming at lowering the activation energy for diffusion. Previous ion irradiation experiments performed on FePt films11,12 and agglomerated particles13 suggest the application of He+ bombardments to create defects while sputtering of atoms is almost completely suppressed. Thus, ion irradiation might also be a suitable method to reduce the transition temperature for well separated, individual nanoparticles which is in the focus of the current study. The FePt nanoparticles are prepared by a micellar technique.14 Based on the self-organization of the diblock a兲 Electronic mail: [email protected] copolymer polystyrene-block-poly-2-vinylpyridine, micelles loaded with Fe and Pt metal-salts are deposited on substrates by dip coating. Exposure to an oxygen plasma nucleates the precursor to particles and removes the organics. By subsequent hydrogen plasma treatment, the particles can be reduced to their pure metallic state. Details on the preparation have been published elsewhere.4 For the micellar particles the interparticle spacing can be controlled over a much larger range 共20– 100 nm兲 than for FePt colloids opening the possibility to anneal the resulting array without forming agglomerates. Thus, the effect of He+ ion irradiation on the ordering temperature of FePt nanoparticles can be determined on well separated, magnetically noninteracting particles for the first time. FePt alloy particles with an average diameter of 7 nm are prepared on Si共001兲 substrates covered by native oxide. Irradiations were performed at a pressure of 1 ⫻ 10−7 hPa and T = 300 K with 350 keV He+ ions at a fluence of 1016 He+ / cm2. Simulation by SRIM15 delivers an average number of displacements per target atom of 0.08 together with a projected range of 1.5 m for the present ions, i.e., practically all projectiles penetrate through the nanoparticles, produce defects there, and get stopped in the substrate. X-ray photoelectron spectroscopy 共XPS兲 is used to track the particle cleanliness throughout the preparation. The composition of the studied particles is Fe51Pt49 as determined from XPS Fe-2p3/2,1/2 and Pt-4f 7/2,5/2 line intensities with an estimated error of 3% assuming a homogeneous alloy. After irradiation, the samples are transported to beamline PM3 at BessyII synchrotron facility 共Berlin, Germany兲 in ambient conditions and, consequently, the particles are partially oxidized. Thus, a hydrogen plasma treatment 共50 W rf power, 20 min, 300 ° C兲 was applied to completely reduce the metal oxides prior to annealing.4 This process is performed in a custombuilt chamber attached to the beamline endstation and characterized by means of x-ray absorption near-edge spectroscopy 共XANES兲 and x-ray magnetic circular dichroism 共XMCD兲 in the total electron yield mode. All measurements are performed at normal incidence of x rays collinear to the external magnetic field of ±3 T with a beam of 93% circular polarization. Magnetic hysteresis loops are taken at the maximum dichroic signal of the Fe-L3 edge. The samples are 0003-6951/2007/90共6兲/062508/3/$23.00 90, 062508-1 © 2007 American Institute of Physics Downloaded 12 Mar 2007 to 131.152.32.83. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 062508-2 Wiedwald et al. Appl. Phys. Lett. 90, 062508 共2007兲 FIG. 1. 共Color online兲 SEM micrographs of 7 nm FePt nanoparticles prior 共a兲 and after He+ ion irradiation and annealing up to TA = 770 ° C 共b兲. The mean separation of the particles is 60 nm. The size histogram of the particles is shown in the inset. successively annealed from 300 up to 770 ° C in steps of about 100 K with a holding time of 30 min at each step. Before and after the complete annealing procedure, the particles are characterized by scanning electron microscopy 共SEM兲 and atomic force microscopy 共AFM兲 to determine particle diameters and to check for possible particle agglomerations. Figure 1 shows SEM images before the sample is exposed to 1016 He+ / cm2 共a兲 and after annealing and magnetic characterization 共b兲. The mean particle height is 7 nm with a full width at half maximum of 3 nm of a lognormal size distribution 共inset of Fig. 1兲 as determined by AFM. High-resolution SEM and transmission electron microscopy 共not shown兲 prove that the alloy particles have comparable lateral extensions, i.e., an approximately spherical shape. It is important to note that within the experimental error the size histograms are not changed, neither by annealing, in line with our previous observations,4 nor by the He+ bombardment. Next, the magnetic properties of FePt particles will be discussed. For this purpose, in panels 共a兲 and 共b兲 of Fig. 2, XANES and XMCD spectra are presented, which were taken at T = 11 K on He+ bombarded FePt particles 共a兲 and on a nonirradiated, but otherwise identically prepared sample 共b兲. In both cases, the particles were treated in a hydrogen plasma at T = 300 ° C prior to spectroscopy. The absorption spectra reveal the pure metallic character of the FePt particles and XMCD shows the typical metallic Fe-L3,2 signature with total Fe magnetic moments of 2.6B – 2.8B similar to recent results on corresponding colloidal nanoparticles but smaller than bulk samples.5 Panels 共c兲 and 共d兲 of Fig. 2 show the low temperature field dependence of the dichroic signal as related to 共a兲 and 共b兲. In this way, information on the hysteretic behavior of the particles is obtained and coercive fields of 600 and 1300 Oe are found prior to annealing for the irradiated particles and the reference, respectively. From simple Stoner-Wohlfarth model calculations we estimate an anisotropy energy density K of 共1 ± 0.1兲 ⫻ 105 J / m3 关共2.3± 0.3兲 ⫻ 105 J / m3兴 for the bombarded 共reference兲 sample, which compares reasonably well to the fcc FePt bulk value of 1 ⫻ 105 J / m3. The effect of annealing on the hysteretic behavior of the FePt particles is demonstrated in panels 共e兲 and 共f兲, where the field dependence of the dichroic signal taken at 300 K is presented after annealing at TA = 700 ° C for 30 min. As opposed to the nonannealed state, both samples exhibit now an open hysteresis loop at T = 300 K, indicating the presence of L10 particles. Most important, the He+ bombarded sample shows a room temperature coercive field HC of 800 Oe as compared to 200 Oe of the nonbombarded reference. The remanent magnetization, however, is found significantly lower than 0.5M S, which indicates that only a minority of particles has been fully transformed after this annealing step. For a more quantitative analysis of the bombardment effect on the magnetic behavior of the particles after annealing, in Fig. 3 the evolution of coercive fields as a function of annealing temperature is presented for both low 共11 K兲 and ambient temperatures 共300 K兲. Starting from the small fcc phase values, increasing annealing temperatures result in a clear enhancement of HC共11 K兲 in the case of the bombarded samples as opposed to the nonbombarded ones which show such a significant enhancement only after annealing at FIG. 2. Fe-L3,2 XANES and XMCD spectra for external fields of ±3 T for He+ ion irradiated 共a兲 and reference sample 共b兲 and element-specific hysteresis loops 关共c兲 and 共d兲兴 after H plasma reduction of oxides at T = 11 K. In 共e兲 and 共f兲 the room temperature hysteresis loops after annealing at TA = 700 ° C are plotted. Downloaded 12 Mar 2007 to 131.152.32.83. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp 062508-3 Appl. Phys. Lett. 90, 062508 共2007兲 Wiedwald et al. FIG. 3. 共Color online兲 Experimentally determined coercive fields as a function of 30 min annealing time at temperature TA for He+ ion irradiated and reference sample at T = 11 K and T = 300 K. Additionally HC is also given after 270 min annealing at TA = 775 ° C of the nonirradiated sample. 600 ° C. Room temperature ferromagnetism arises at TA close to 600 ° C for the irradiated sample while annealing at 700 ° C is necessary for the nonirradiated particles. In summary, the complete set of measurements of the ion irradiated sample is shifted by more than 100 K to lower annealing temperatures TA as compared to the reference. For comparison, the coercive field of the reference sample after 270 min annealing at TA = 775 ° C is added to Fig. 3. This long time annealing has a similar effect on the particle ordering as short time annealing of the bombarded sample. One should note that the maximum available external field of ±3 T is not sufficient to saturate the particle ensemble once a huge MAE 共HC ⬎ 5 kOe兲 is established. This leads to an underestimate of the corresponding HC values above TA = 700 ° C. The lower annealing temperatures necessary to form L10 ordered particles can be discussed in terms of a reduced activation energy for diffusion ED after He+ irradiation. The characteristic diffusion length depends on the diffusion coefficient D and the annealing time tA via = 冑DtA with D = D0 exp共−ED / 共kBTA兲兲. Here, D0 is the pre-exponential factor, and kB is Boltzmann’s constant.16 For alloys with a large uniaxial MAE, K, it has been experimentally demonstrated that K has a nearly linear dependence on degree of chemical order S.17,18 Thus, for Stoner-Wohlfarth particles with HC ⬀ K / M 共M standing for their magnetization兲, the coercive field is directly proportional to the degree of chemical order S of a particle. As a result, it becomes possible to estimate the activation energy for diffusion ED from hysteresis loops of noninteracting FePt particles assuming 共HC − HC0兲 / HC0 ⬀ S ⬀ , with HC0 denoting the initial coercive field. One should note that this assumption only holds well below full chemical order S Ⰶ 1. From Arrhenius plots of the quantity 共共HC − HC0兲 / HC0兲2 / tA 共not shown兲, activation energies of 0.7 eV for the He+ irradiated particles and 1.6 eV for the reference are derived. These energies are significantly smaller than the volume activation energy of 3.0 eV/atom reported for Pt in FePt.19 This may be related to the much stronger influence of surface diffusion in nanoparticles. Attributing the observed irradiation effect to defect enhanced diffusion rises the question as to why the bombarded particles exhibit a “defect memory” even though they are at least partly oxidized after the bombardment and subsequently reduced again prior to the magnetic measurements. The related defect behavior has still to be studied in more detail. In summary, we demonstrate that irradiation of an array of 7 nm magnetically decoupled FePt nanoparticles with 350 kV He+ ions at a fluence of 1016 ions/ cm2 results in a significant reduction of more than 100 K of the annealing temperature TA necessary for their transformation into the L10 phase. To arrive at this conclusion, XMCD hysteresis loops are determined as a function of annealing temperature and compared to a nonirradiated reference. Assuming a linear proportionality between the magnetic anisotropy and the degree of order, the TA reduction after irradiation can be mapped onto a related decrease of the activation energy for diffusion ED by more than 50%. The determination of activation energies by subsequent annealing will allow the direct comparison of hard magnetic nanoparticles prepared by different techniques in future studies. The authors thank P. Walther for access to the SEM and the BessyII staff for their support during beamtime. This work was supported by the DFG 共SFB 569, Zi317/21-1, GRK 328兲 and the Ulmer Universitätsgesellschaft. S. Sun, Adv. Mater. 共Weinheim, Ger.兲 18, 393 共2006兲, and references therein. 2 S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287, 1989 共2000兲. 3 B. Stahl, J. Ellrich, R. Theissmann, M. Ghafari, S. Bhattacharya, H. Hahn, N. S. Gajbhiye, D. Kramer, R. N. Viswanath, J. Weissmüller, and H. Gleiter, Phys. Rev. B 67, 014422 共2003兲. 4 A. Ethirajan, U. Wiedwald, H.-G. Boyen, B. Kern, L. Han, A. Klimmer, F. Weigl, G. Kästle, P. Ziemann, K. Fauth, C. Jun, R. J. Behm, A. Romaniuk, P. Oelhafen, P. Walther, J. Biskupek, and U. Kaiser, Adv. Mater. 共Weinheim, Ger.兲 19, 406 共2007兲. 5 C. Antoniak, J. Lindner, M. Spasova, D. Sudfeld, M. Acet, M. Farle, K. Fauth, U. Wiedwald, H.-G. Boyen, P. Ziemann, F. Wilhelm, A. Rogalev, and S. Sun, Phys. Rev. Lett. 97, 117201 共2006兲. 6 Z. R. Dai, S. Sun, and Z. L. Wang, Nano Lett. 1, 443 共2001兲. 7 Z. Jia, S. Kang, S. Shi, D. E. Nikles, and J. W. Harrell, J. Appl. Phys. 97, 10J310 共2005兲. 8 U. Wiedwald, K. Fauth, M. Hessler, H.-G. Boyen, F. Weigl, M. Hilgendorff, M. Giersig, G. Schütz, P. Ziemann, and M. Farle, ChemPhysChem 6, 2522 共2005兲. 9 S. Kang, J. W. Harrell, and D. E. Nikles, Nano Lett. 2, 1033 共2002兲. 10 G. Meyer and J.-U. Thiele, Phys. Rev. B 73, 214438 共2006兲, and references therein. 11 D. Ravelsona, C. Chappert, V. Mathet, and H. Bernas, Appl. Phys. Lett. 76, 236 共2000兲. 12 C.-H. Yang, C.-H. Lai, and C. C. Chiang, IEEE Trans. Magn. 40, 2519 共2004兲. 13 N. V. Seetala, J. W. Harrell, J. Lawson, D. E. Nikles, J. R. Williams, and T. Isaacs-Smith, Nucl. Instrum. Methods Phys. Res. B 241, 583 共2005兲. 14 G. Kästle, H.-G. Boyen, F. Weigl, G. Lengl, T. Herzog, P. Ziemann, S. Riethmüller, O. Mayer, C. Hartmann, J. P. Spatz, M. Möller, M. Ozawa, F. Banhart, M. G. Garnier, and P. Oelhafen, Adv. Funct. Mater. 13, 853 共2003兲. 15 J. F. Ziegler and J. P. Biersak, available online: http://www.srim.org. 16 B. Rellinghaus, S. Stappert, M. Acet, and E. F. Wassermann, J. Magn. Magn. Mater. 266, 142 共2003兲. 17 H. Kanazawa, G. Lauhoff, and T. Suzuki, J. Appl. Phys. 87, 6143 共2000兲. 18 S. Okamoto, N. Kikuchi, O. Kitakami, T. Miyazaki, Y. Shimada, and K. Fukamichi, Phys. Rev. B 66, 024413 共2002兲. 19 B. Rellinghaus, E. Mohn, L. Schultz, T. Gemming, M. Acet, A. Kowalik, and B. F. Kock, IEEE Trans. Magn. 42, 3048 共2006兲. 1 Downloaded 12 Mar 2007 to 131.152.32.83. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp