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Summer School of Advanced Functional Materials 2011
International Centre for Materials Physics, Chinese Academy of Sciences
ABSTRACTS
Multiferroic Vortices and Graph Theory
S-W. Cheong
Rutgers Center for Emergent Materials
Rutgers University, Piscataway, NJ 08854
[email protected]
The fascinating concept of topological defects permeates ubiquitously our
understanding of the early-stage universe, hurricanes, quantum matters such as superfluids and
superconductors, and also technological materials such as liquid crystals and magnets.
Large-scale spatial configurations of these topological defects have been investigated only in a
limited degree. Exceptions include the cases of supercurrent vortices or liquid crystals, but they
tend to exhibit either trivial or rather-irregular configurations.
Hexagonal REMnO3 (RE= rare earths) with RE=Ho-Lu, Y, and Sc, is an improper
ferroelectric where the size mismatch between RE and Mn induces a trimerization-type
structural phase transition, and this structural transition leads to three structural domains, each
of which can support two directions of ferroelectric polarization. We reported that domains in
h-REMnO3 meet in cloverleaf arrangements that cycle through all six domain configurations,
Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the
cycle of domain configurations is reversed. Vortices and antivortices are topological defects:
even in a strong electric field they won’t annihilate.
Recently we have found intriguing, but seemingly irregular configurations of a zoo of
topological vortices and antivortices in h-REMnO3. These configurations can be neatly
analyzed in terms of graph theory and this graph theoretical analysis reflects the nature of
self-organized criticality in complexity phenomena as well as the condensation and eventual
annihilation processes of topological vortex-antivortex pairs.
Ferroelectric Tunnel Junctions:
Controlling Electron and Spin Transport by Ferroelectric Polarization
Evgeny Tsymbal
Department of Physics and Astronomy, University of Nebraska-Lincoln
Tunnel junctions are useful electronic devices in which current-carrying electrons can
quantum-mechanically be transmitted between two metal electrodes across a very thin
insulating barrier layer. A particular example is a magnetic tunnel junction, where electrical
resistance depends on magnetization orientation of the two ferromagnetic electrodes – the
phenomenon known as tunneling magnetoresistance (TMR). So far, however, almost all the
existing tunnel junctions were based on non-polar dielectrics. An exciting possibility to extend
the functionality of tunnel junctions is to use a ferroelectric insulator as a barrier to create a
ferroelectric tunnel junction (FTJ). [1] The key property of FTJ is tunneling electroresistance
(TER) that is the change in electrical resistance of a FTJ with reversal of ferroelectric
polarization. Functional properties of a FTJ can be further extended by ferromagnetic
electrodes to create a multiferroic tunnel junction (MFTJ). In such a MFTJ the transport spin
polarization and TMR are affected by ferroelectric polarization of the barrier. [1] Thus, MFTJs
represent four-state resistance devices that can be controlled both by electric and magnetic
fields. This talk will address the physics of FTJs and MFTJs based on recent modeling and
experiments.
[1] E. Y. Tsymbal and H. Kohlstedt, Science 313, 181 (2006).
Emergent Phenomena in Spatially Confined Manganites
Jian Shen
Department of Physics, Fudan University, Shanghai 200433
Colossal electroresistance, colossal magnetoresistance, high Tc superconductivity and
the metal-insulator transition are some of the fascinating emergent behaviors found in complex
materials; however there is as yet no known model that is capable of fully explaining any one
of these behaviors let alone a unifying understanding capable of explaining the effects of
complexity on emergent behavior as a whole. One common trait that many of these complex
materials share is electronic phase separation. For this reason, a fuller understanding of
electronic phase separation should have far reaching implications across a wide range of
materials.
We will discuss recent work on a novel spatial confinement technique that has led to
some fascinating new discoveries on the role of electronic phase separation (EPS) in
manganites. In transport measurements on unconfined systems where device size is larger
than the inherent electronic phase domains, current bypasses regions of high resistance in favor
of regions with lower resistance, because the probing electrons will follow the path of least
resistance. By confining complex materials exhibiting EPS to length scales smaller than the
electronic phase domains that reside within them, it is possible to simultaneously probe
multiple resistive regions. This method allows for a much more complete view of the phases
residing in a material and gives vital information on phase formation, movement and
fluctuation. Since these phase separated regions also posses varied properties, this technique
promises to lead to unexpected functionalities for future device applications.
Reduced dimensionality studies on complex oxides sit at the intersection of
fundamental science and technological application. With the wide range of useful
materials—ferromagnets, antiferromagnets, high TC superconductors, multiferroics—present in
the complex oxides class, the burgeoning field of oxide electronics presents itself as the future
of device design. Not only will spatial confinement studies on correlated systems give us new
insights into fundamental physics, these studies are a vital step in the creation and
implementation of practical oxide electronic devices. Further, the formation and coexistence
of multiple electronic phases in a single system have been suggested as purely emergent
phenomena4; establishing a common language capable of discussing complexity in these
material systems could find overlap with other fields, such as Biology or Economics, where
complex systems are the norm.
Neutron scattering study on crystalline and magnetic structure and excitations of the
Fe-based superconductors
Wei Bao
Department of Physics, Renmin University of China, Beijing 100872
We will present our neutron scattering investigation on the recently discovered
Fe-based high transition temperature superconductor materials. Our neutron diffraction results
of magnetic order and lattice structure of the 1111 [1], 122 [2] and 11 [3] families of the
Fe-based superconductor materials uncover a close relation between lattice distortion and
magnetic interaction. The magnetostructural transitions resemble those previously discovered
in either vanandiumsesquioxide [4] or in manganites [5], which are naturally explained by an
orbital ordering transition. The prevailing theoretic explanation of the antiferromagnetic
transition in these parent materials, the spin-density-wave mechanism, however, is not
sufficient to explain our experimental results.
The antiferromagnetic phase can coexist with the superconducting phase in the 122
system [6], while a phase diagram more resembling that of cuprate superconductors exists for
the 11 system [7]. The symmetry of superconducting order parameter has strong signature in
magnetic excitation spectrum, and we observed the telltale spin resonance mode of the s+/symmetry and the accompanying spin gap in the superconducting state of the 11
superconductor [8]. The normal state was shown to exhibit single-lobed incommensurate
excitation continuum of a typical itinerant antiferromagnet, in contrast to spin-wave cone of a
localized antiferromagnet [8,9], supporting a Fermi liquid description of the normal state.
More recently a new family of alkali metal intercalated iron selenide superconductors
of Tc above 30 K is discovered. We will present the determination of the sample composition,
crystal structure and magnetic order using neutron diffraction technique. Contrary to previous
belief, the materials are mostly charge balanced, instead of heavily electron doped, with
appropriate chemical formula as AxFe2-x/2Se2 and Fe at valance +2 [10]. In superconducting
samples the Fe vacancies order into an almost perfect pattern in a five times larger unit cell
[11,12]. For all the superconductors, A=K, Rb, Cs, (Tl,K), and (Tl,Rb) and x~0.8, a large
moment block checkerboard antiferromagnetic order is found to coexist with superconductivity
[12,13]. These results demonstrate a very different kind of superconductors from all previous
iron based high Tc superconductors, and a new different mechanism is expected.
[1] Y. Qiu, W. Bao, Q. Huang et al., Phys. Rev. Lett. 101, 257002 (2008).
[2] Q. Huang, Y. Qiu, W. Bao et al., Phys. Rev. Lett. 101, 257003 (2008); M. Kofu, Y. Qiu, W.
Bao et al., New J. Phys. 11, 055001 (2009).
[3] W. Bao, Y. Qiu, Q. Huang et al., Phys. Rev. Lett. 102, 247001 (2009).
[4] W. Bao et al., Phys. Rev. Lett. 78, 507 (1997).
[5] W. Bao et al., Phys. Rev. Lett. 78, 543 (1997).
[6] H. Chen et al., EPL 85, 17006 (2009).
[7] T.J. Liu et al., Nature Materials 9, 718 (2010).
[8] Y. Qiu, W. Bao, Y. Zhao et al., Phys. Rev. Lett. 103, 067008 (2009).
[9] D.N. Argyriou et al., Phys. Rev. B 81, 220503 (R) (2010).
[10] W. Bao et al., arXiv:1102.3674 (2011)
[11] P. Zavalij et al., arXiv:1101.4882 (2011)
[12] W. Bao et al., arXiv:1102.0830 (2011)
[13] F. Ye et al., arXiv:1102.2882 (2011)
The effects of spin-orbit coupling in low-dimensional magnetism and spintronics
Ruqian Wu
Department of Physics and Astronomy, University of California, Irvine, CA 92697
Spin-orbit coupling is essential for various physical phenomena, from magnetic
anisotropy, magneto-optical effect, to quantum spin Hall effect. To appropriately integrate
spin-orbit coupling in modern density functional studies allow predicting and explaining new
properties and systems. I will discuss results of our recent studies for magnetic anisotropy,
Kondo effect and quantum spin Hall effect in magnetic thin films and other nanostructures.
Spin Dynamics in Rare Earth Titanates R2Ti2O7 (R = Dy/Tb, Nd) studied by ac
susceptibility
Hao Zeng
Department of Physics, University at Buffalo, the State University of New York
We present a serial of our recent studies on rare earth titanates R2Ti2O7 (R=Y, lanthanide),
with particular emphasis on the spin relaxation process with ac susceptibility as the main probe.
We studied the ac hybrid spin system DyxTb2-xTi2O7. The two compounds at the composition
boundaries are spin ice Dy2Ti2O7 and spin liquid Tb2Ti2O7, which are the two representative
geometrically frustrated pyrochlores. In addition to the known Dy3+ single-ion peak at Ts (Ts
peak), we identified a new freezing-like peak associated with Tb3+ spins (T* peak). We propose
that T* peaks correspond to the low-lying crystal field levels of Tb3+ spins, and the crystal field
scheme evolves as the composition x changes, which explains the phase diagram of T*(x). The
finding of the T* peak in DyxTb2-xTi2O7 demonstrates the rich dynamical magnetic behavior of
such systems, and provides experimental foundations for future explorations of hybrid rare
earth spin systems. It is known that the structure of R2Ti2O7 transforms into monoclinic as the
ionic radius of R increases. We have investigated Nd2Ti2O7 in this category. We showed
anisotropic paramagnetism arising mainly from the crystal electric field, and separated the
exchange contribution θex and the crystal field contribution θCF to the total Weiss temperature
θW. The θex orders of magnitude higher than the magnetic ordering temperature Torder suggests
strong spin frustration in the system, despite the absence of a pyrochlore structure. More
importantly, we found in Nd2Ti2O7 a novel field-induced slow spin relaxation in the
paramagnetic state. We propose that the field-induced slow spin relaxation is associated with
the cooperative behavior of the correlated regions formed by partially polarized spins through
spin correlations. We believe that the cooperative behavior can exist not only in geometrically
frustrated systems, but also in many more paramagnets with strong spin correlations, while
their observable microscopic effects are closely associated with the degree of frustration in the
system.
Shape-Controlled Metal Nanoparticles: Synthesis, Characterization and Applications
Jian-Guo Zheng
The Laboratory for Electron and X-ray instrumentation (LEXI)
California Institute for Telecommunications and Information Technology (Calit2)
University of California, Irvine, CA 92697-2800
It is well-known that when material size is reduced to nanometer scale, the physical and
chemical properties of nanoscale materials, such as luminescence, conductivity, and catalytic
activity, will be significantly different from their bulk counterpart, which may be attributed to
quantum confinement effect and large ratio of surface area to volume. Besides size and
composition, shape is also an important factor to control the properties of nanoparticles.
Colloid chemists have gained excellent control over particle size for several spherical metal
and semiconductor compositions. These colloidal spherical nanoparticles may be used as
probes for biological diagnostic applications, LED materials, lasers, and Raman
spectroscopy–enhancing materials. However, it was a challenge to synthetically control particle
shape. Some significant progresses have been made in synthesizing large quantities of
anisotropic nanoparticles - non-spherical structures with shape-dependent properties- in high
yield in last decade. This talk is going to give a brief review about shape-controlled metal
nanoparticles, including their synthetic strategies, characterization techniques and applications.
Recent progresses on the rare earth magnets made by rapid solidification process (RSP)
W. C. Chang
Physics Department, Chung Cheng University, Chia-Yi, Taiwan
E-mail: [email protected].
Since last two decades, rare earth permanent magnets (REPM) have been widely used in
office automation, computers and mobile communications, EV, HEV, wine turbines, etc. The main
processes for making REPM include sintering and epoxy bonding. The former is capable of
making anisotropic high performance magnets, while the latter is suitable for making thin wall
and tiny magnets, even though the magnetic property is usually isotropic and inferior to the
magnets made by the former. Nevertheless, the process is simple, easy and cheap. The original
materials for bonded magnets are normally produced by rapid solidification process (RSP),
especially for single phase nanocrystalline NdFeB ribbons. However, this process has also been
applied for making nanocomposite RFeB and some other permanent magnets. In this presentation,
I will introduce the recent progresses in our lab, on the permanent magnets made by RSP, the
materials include nanocomposites RFeB ribbons, nanocrystalline bulk RFeB magnets, Sm(Co,
M)7 (M=refractory elements) ribbons and Fe-B/FePt nanocomposites.
Probing Ferroelectric Functionality Cell-by-Cell by Aberration-Corrected STEM
S. J. Pennycook(1,2)*, H. J. Chang(1)**, D. N. Leonard(1), M. P. Oxley(1,2), J. He(2,1), S. T.
Pantelides(2,1) and A. Y. Borisevich(1)
(1) Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge,
TN 37831-6071, USA
(2) Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA
* E-mail: [email protected] ** now at Korea Institute of Science and Technology, 39-1
Hawolgok-dong, Seongbuk-gu, Seoul 136-791,Korea
The recent introduction of next-generation aberration correctors has propelled the
capabilities of the scanning transmission electron microscope (STEM) to a whole new level of
sensitivity. The resolution has increased to well below 1 Å, allowing atomic column positions
to be extracted with unprecedented precision, and making possible the mapping of ferroelectric
displacements on a unit cell by unit cell basis. Furthermore, the STEM allows multiple,
simultaneous images to be acquired, Z-contrast images, dominated by the heavy cation
columns can be obtained with either bright field images (sensitive to oxygen column positions)
or electron energy loss signals (allowing mapping of elemental composition, electronic
structure and dielectric properties. Correlating these signals at atomic resolution across
interfaces enables new insights into screening mechanisms and the role of lattice constraints,
such as suppression of octahedral rotations.
As a case study several examples will be shown of BiFeO3, mapping polarization
direct from a Z-contrast image [1], mapping octahedral rotations across interfaces with
electrodes, the observation of an induced dielectric anomaly, the influence of vacancy ordering
on polarization and the use of column shape analysis to locate and identify domain walls [2].
Experimental results will be complemented by model theoretical studies [3].
[1] A. Y. Borisevich et al., Phys. Rev. Lett. 105, 087204 (2010).
[2] A. Y. Borisevich et al., ACS Nano, 4, 6071 (2010).
[3] J. He at al., Phys. Rev. Lett., 105, 227203 (2010).
Solid Oxide Fuel Cells for Clean Energy Generation
Xingbo Liu
Mechanical & Aerospace Engineering Department, West Virginia University
Morgantown, WV 26506-6106
The first patent on fuel cells was filed in 1838, and they have received huge attentions
in recent years due to their many advantages over other energy generation devices such as
internal combustion engines and turbine systems. A fuel cell is an electrochemical cell that
converts chemical energy from a fuel into electric energy. Electricity is generated from the
reaction between a fuel supply and an oxidizing agent. It is made up of three segments which
are sandwiched together: the anode, the electrolyte, and the cathode. Two chemical reactions
occur at the interfaces of the three different segments. The net result of the two reactions is that
fuel is consumed, water or carbon dioxide is created, and an electric current is created, which
can be used to power electrical devices, normally referred to as the load. This talk will give an
overview of various types of fuel cells, with the focus on solid oxide fuel cell (SOFC). The
recent progress, as well as the challenges in both material and system levels in SOFC systems,
will be presented.
Irradiation Induced Structural and Magnetic Property Changes in Nanoparticle
Granular Films
You Qiang
Physics Department, University of Idaho, Moscow ID 83844
Future generation IV nuclear reactors in US and storage medium in irradiation
environment are expected to meet the standards of enhanced safety and economical
compatibility. This research contributes in part to meet this objective through the understanding
of irradiation induced structural and magnetic property changes in nanomaterials. Fe3O4 and
FeO+Fe3N granular films, created by the third generation nanocluster source were irradiated.
The pristine magnetite (Fe3O4) granular films with an average grain size of 3 nm are
superparamagnetic in nature. These films enriched ferromagnetic behavior under the irradiation
exposure of 5.5 MeV Si2+ ions to a fluence of 1016 ions/cm2 at room temperature. After
irradiation, the average grain size and magnetic domain size of the films showed dramatic
increase. On the contrary, the magnetic properties of FeO+Fe3N granular films were unaffected
even when irradiated with 2 MeV He+ ions to a fluence of 3 × 1015 ions/cm2 at room
temperature. Surprisingly, the saturation magnetization (30 emu/g), coercivity (87.9 Oe),
magnetic remanence (3.2 emu/g) and magnetic susceptibility measurements of these granular
films remained unaltered even after irradiation. This is a fascinating behavior because this
meets some of the requirements for advanced data storage applications in extreme environment.
Electron beam exposure on iron nanoparticles is well known to cause structural changes. The
mystery behind this behavior-particle size growth and oxide layer growth in core-shell iron
nanoparticles due to electron beam irradiation in TEM ultrahigh vacuum - was investigated in
this study.
Three-dimensional dislocation dynamics in epitaxial thin films
Lizhi Sun
Departments of Civil & Environmental Engineering and Chemical Engineering & Materials
Science, University of California, Irvine, CA 92697-2175; Email: [email protected]
Dislocation dynamics in three-dimensional setting has been the subject of extensive
and great interest since 1990 due to the significant roles it plays in plastic deformation, work
hardening and softening, fracture mechanics and electronic device fabrication. However, a
long-standing problem inherent to the classical dislocation dynamics is that simulation results
heavily depend on segment sizes, which substantially reduces the reliability of simulation
results. We present a new three-dimensional dislocation dynamics model together with its
physical background. This new model fully incorporates the interactions among differential
dislocation segments. The proposed model is applied to simulate the effect of dislocations on
the mechanical performance of epitaxial thin films. The interactions among the dislocation
loop, free surface and interface are rigorously computed by decomposing this complicated
problem into two relatively simple sub-problems. This model is allowed to determine the
critical thickness of thin films for a surface loop to nucleate and to simulate how a surface loop
evolves into two threading dislocations. Furthermore, the relationship between the film
thickness and yield strength is constructed and compared with the Hall-Petch relation.
Thermoelectric Nanocomposites and Microscale Modules
Jing-Feng Li
Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084
Thermoelectric materials are technically important for energy harvesting and
conversion technology, particularly for recovering an enormous amount of unused waste heat
produced by industrial processes and automotive exhaust. Good thermoelectric materials must
have high Seebeck coefficient, good electrical conductivity and low thermal conductivity.
However, it is very difficult to control the above three parameters independently, which often
counter each other. This talk will give two representative examples showing that thermoelectric
properties can be enhanced in nanocomposite materials. One example is AgPbmSbTem+2 system
bulk materials with intrinsically embedded nanoscale inclusions. Our work demonstrated that a
high ZT value up to 1.5 at 700 K can be achieved in this material fabricated by a facile process
combining mechanical alloying (MA) and spark plasma sintering (SPS) methods. Another
example is an extrinsic approach by incorporating nanoparticles into a thermoelectric
compound matrix to form composites, but the key point is how to effectively reduce thermal
conductivity to a greater degree than electrical conductivity for ZT enhancement. Our recent
studies revealed that nano-SiC dispersed Bi2Te3 polycrystalline materials show not only
enhanced thermoelectric performance but also better mechanical strength and fracture
toughness. By using such a kind of strengthened and fine-grained Bi2Te3-based materials
prepared by spark plasma sintering (SPS), miniaturized thermoelectric (TE) modules were
fabricated by combining mechanical cutting and photolithograph processes. A microscale
thermoelectric module with pillars that are as fine as 200 × 400 μm2 in cross-section with a
height up to 600 μm was fabricated, whose maximum open output voltage was about 20 mV
when heated under a100 W lamp.
Crystallization of ferroelectric lead zirconate titanate thin films under microwave
irradiation
Zhan Jie Wang
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences (CAS), 72 Wenhua Road, Shenyang 110016
E-mail: [email protected]
Lead zirconate titanate (Pb(ZrxTi1-x)O3: PZT) have excellent ferroelectric, pyroelectric
and piezoelectric properties. Techniques for the deposition of PZT thin films have been
developed rapidly in recent years because of the large number of potential applications of PZT
thin films in nonvolatile ferroelectric random-access memories (FeRAMs) and
micro-electromechanical systems (MEMS) such as micro-scanning mirror devices and atomic
force microscopy (AFM) cantilevers. It is imperative to decrease the thermal processing
temperature and time to prevent interdiffusion between the elements of the films and the
substrate and to prevent the evaporation of lead and lead oxide from the surface of the films. In
this study, PZT thin films were coated on Pt/Ti/SiO2/Si substrates by a sol-gel method and then
crystallized by multimode 28 GHz microwave irradiation or single-mode 2.45 GHz microwave
irradiation in the magnetic field. The crystalline phases and microstructures as well as the
electrical properties of the microwave-irradiated PZT films were investigated. Experimental
results indicated that microwave irradiation is effective for obtaining well-crystallized PZT thin
films with good electric properties at low temperatures or in a short time.
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