Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y22: Ferroelectric and Structural Phase Transitions |
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Sponsoring Units: DCMP Chair: Jason Lashley, Los Alamos National Laboratory Room: D163 |
Friday, March 25, 2011 8:00AM - 8:12AM |
Y22.00001: Magnetic and magnetoelectric excitations in multiferroic BiFeO$_{3}$ Diyar Talbayev, Stuart A. Trugman, Antoinette J. Taylor, Seongsu Lee, Sang-Wook Cheong Ferroelectric antiferromagnet BiFeO$_{3}$ combines ferroelectricity with an antiferromagnetic order at room temperature. A control of its magnetic state by voltage has been demostrated both in bulk and in thin film BiFeO$_{3}$. The distortion of the cubic perovskite lattice leads to two effects through the Dzyaloshinski-Moriya magnetic interaction: the ferroelectric distortion results in the observed incommensurate spiral spin structure, and the rotation of oxygen octahedra with alternating sense on neighboring Fe ions results in a local canting of spins. We present a terahertz spectroscopic study of magnetic excitations in BiFeO$_{3}$. We interpret the observed spectrum of long-wavelength magnetic resonance modes in terms of the normal modes of the material's spiral antiferromagnetic structure. We find that the modulated Dzyaloshinski-Moriya interaction and the local spin canting lead to a splitting of the out-of-plane resonance modes. We also assign one of the observed absorption lines to an electromagnon excitation that results from the magnetoelectric coupling between the ferroelectric polarization and the spiral magnetic structure of BiFeO$_{3}$. [Preview Abstract] |
Friday, March 25, 2011 8:12AM - 8:24AM |
Y22.00002: Size-dependent infrared phonon modes and ferroelectric phase transition in BiFeO$_3$ nanoparticles Peng Chen, Xiaoshan Xu, Christopher Koenigsmann, Alexander C. Santulli, Stanislaus S. Wong, Janice L. Musfeldt One emergent property of ferroelectric nanoparticles is the sized-induced structural distortion to a high-symmetry paraelectric phase at small particle sizes. Finite length scale effects can thus be advantageously employed to elucidate ferroelectric transition mechanisms. In this work, we combine infrared spectroscopy with group theory and lattice dynamics calculations to reveal the displacive nature of the ferroelectric transition in BiFeO$_3$, a room temperature multiferroic. Systematic intensity and frequency trends in selected vibrational modes show that the paraelectric phase is \emph{Pm}\=3\emph{m} and the lowest frequency A$_1$ feature is the soft mode that drives the first order transition. Finite length scale effects are also evident in the electronic structure with a red shifted band gap in nanoscale BiFeO$_3$ compared with that of the rhombohedral film, a result that can impact the development of ferroelectric photovoltaics and oxide- based electronics. Taken together, these findings demonstrate the foundational importance of size effects for enhancing the rich functionality and broad utility of transition metal oxides. [Preview Abstract] |
Friday, March 25, 2011 8:24AM - 8:36AM |
Y22.00003: Remarkably robust ferroelectric state in multiferroic Mn$_{1-x}$Zn$_x$WO$_4$ B. Lorenz, R.P. Chaudhury, Y.Q. Wang, Y.Y. Sun, C.W. Chu, F. Ye, H.A. Mook, J.A. Fernandez-Baca Zinc doping in Mn$_{1-x}$Zn$_x$WO$_4$ is equivalent to the removal of Mn spins and a dilution of the magnetic system. The multiferroic (ferroelectric) phase of MnWO$_4$ is stabilized through Zn substitution and the low-temperature commensurate phase (up-up-down-down phase) is completely suppressed at a Zn concentration of more than 5\%. The magnetic and ferroelectric phases as well as the multiferroic properties are studied through magnetic, heat capacity, polarization, and neutron scattering experiments. The multiferroic phase is remarkably stable and it still exists for Zn substitution levels up to and above 50\%. At low doping (2\%) the incommensurate helical and the commensurate low-T phases coexist. External magnetic fields do lift the phase degeneracy and stabilize either one of the two ground states, depending on the direction of the field. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y22.00004: Ferroelectricity in CaTiO$_{3}$ Single Crystal Surfaces and Thin Films and Probed by Nonlinear Optics and Raman Spectroscopy Eftihia Vlahos, Tom Lummen, Ryan Haislmaier, Sava Denev, Charles Brooks, Michael Biegalski, Darrell Schlom, Carl-Johan Eklund, Karin Rabe, Craig Fennie, Venkatraman Gopalan Bulk CaTiO$_{3}$ has a centrosymmetric point group and is \textit{not }polar or ferroelectric. However, we present surprising results that show highly regular polar domains in single crystals of CaTiO$_{3}$. Confocal Second Harmonic Generation (SHG) and Raman imaging studies were carried out on perovskite CaTiO$_{3}$ crystal surfaces. They reveal large, crystallographic polar domains at room temperature, with in-plane polarization components delineated by twin walls. SHG analysis indicates that the highest symmetry of the polar surface is $m $(space group P$c)$ with polarization in the $m$ plane. In addition, we present results of the polar domain structure imaged before and after the application of an external electric field. Finally, we present the SHG studies of CaTiO$_{3}$ thin films grown using reactive Molecular Beam Epitaxy (MBE); these films are predicted by theory to be ferroelectric and are shown experimentally, both with SHG and in-plane dielectric measurements, to be ferroelectric for temperatures less than $\sim $150 K with group symmetry \textit{mm}2. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y22.00005: Phase transitions in relaxors as seen by neutron scattering Severian Gvasaliya, Roger Cowley, Sergey Lushnikov, Bertrand Roessli, Gelu-Marius Rotaru Relaxors have a broad temperature and frequency-dependent peak in the dielectric permittivity that is not necessarly linked to a structural phase transition. A model relaxor is PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ (PMN) doped with PbTiO$_{3}$ (PT). We report neutron studies of the low-energy spectra of (1-x)PMN-xPT crystals. Apart from phonons which do not show a soft mode, there are two components of the diffuse scattering: one is quasi-elastic (QE) and the other static. The energy width of the QE scattering decreases as the peak of the susceptibility is approached. The static component behaves like an order parameter. In the crystals that become ferroelectric it is maximal at the ferroelectric phase transition, but in PMN it steadily increases on cooling. We discuss previously reported and new results in terms of a random-field model of the cubic crystal. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y22.00006: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y22.00007: Elastic collapse and avalanche criticality near a Mott transition J.L. Smith, D.J. Safarik, J.C. Lashley, E.K.H. Salje, C.P. Opeil, P.S. Riseborough We study some dynamic aspects of a Mott transition in a rare-earth alloy Ce$_{0.90}$Th$_{0.10}$ by resonant-ultrasound spectroscopy (RUS), electrical-transport, and thermal-expansion measurements. In the temperature range spanning the first-order transition, we observe a stiffening of the elastic response that is associated with a continuous front propagation ($e.g.$ solitons). A defining characteristic of a mixed phase regime, slow scanning rates (0.01 K/min) show these solitons to be superimposed with jerks and avalanches in all three data sets: RUS, resistivity, and thermal expansion data. Analysis of the avalanche data give power law distributions with critical exponents $P(E)=E^{n}$ for energy, in the case of thermal expansion data and length, in the case of electrical transport data. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y22.00008: Calculated temperature dependence of elastic constants and phonon dispersion of hcp and bcc beryllium Steven Hahn, Sergiu Arapan, Bruce Harmon, Olle Eriksson Conventional first principle methods for calculating lattice dynamics are unable to calculate high temperature thermophysical properties of materials containing modes that are entropically stabilized. In this presentation we use a relatively new approach called self-consistent \textit{ab initio} lattice dynamics (SCAILD) to study the hcp to bcc transition (1530 K) in beryllium. The SCAILD method goes beyond the harmonic approximation to include phonon-phonon interactions and produces a temperature-dependent phonon dispersion. In the high temperature bcc structure, phonon-phonon interactions dynamically stabilize the N-point phonon. Fits to the calculated phonon dispersion were used to determine the temperature dependence of the elastic constants in the hcp and bcc phases. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y22.00009: Influence of the Magnetic State on the Chemical Order-Disorder Transition Temperature in Fe-Ni Permalloy Marcus Ekholm, Helena Zapolsky, Andrei Ruban, Iryna Vernyhora, Denis Ledue, Igor Abrikosov In magnetic alloys, the effect of finite temperature magnetic excitations on phase stability below the Curie temperature is poorly investigated, although many systems undergo phase transitions in this temperature range. In this study [1], we consider random Ni-rich Fe-Ni alloys, which undergo chemical order-disorder transition approximately 100~K below their Curie temperature, to demonstrate from \textit{ab-initio} calculations that deviations of the global magnetic state from ideal ferromagnetic order due to temperature induced magnetization reduction have a crucial effect on the chemical transition temperature. We propose a scheme where the magnetic state is described by partially disordered local magnetic moments, which in combination with Heisenberg Monte-Carlo simulations of the magnetization allows us to reproduce the transition temperature in good agreement with experimental data. [1] Ekholm et al., Phys. Rev. Lett. 105:167208 (2010) [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y22.00010: Structural and magnetic properties of Ni$_2$MnGa from first-principles Vamshi Katukuri, Burak Himmetoglu, Matteo Cococcioni Ni$_2$MnGa is the prototype magnetic shape-memory alloy. In this work we use ab-initio calculations to characterize structural and magnetic transitions and to identify possible strategies to tune them towards the same critical point. To this aim both the austenite and the martensite phases of the Ni$_2$MnGa alloy are studied with particular attention to the electronic factors controlling their stability and the onset of the structural transition. Our results indicate that, in spite of its metallic character, electronic correlations play an important role in determining the behavior of this compound and, in particular, the entity (and sign) of the deformation accompanying the transition from the austenite phase to the martensite one. The vibrational properties of the austenite phase are also studied and structural instabilities (soft modes) are investigated as possible signatures of intermediate ``modulated'' structures. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y22.00011: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y22.00012: Origin of ``aging'' in shape-memory alloys Xiangdong Ding, Junkai Deng, Turab Lookman, Avadh Saxena, Xiaobing Ren For more than half a century it has been widely observed that a majority of shape-memory alloys exhibit a gradual change in physical properties with time in the martensitic phase, and this is referred to as ``aging.'' However, its microscopic mechanism has remained controversial due to lack of experiments that can probe atomic level processes. We clarify the atomic mechanism for how shape memory alloys ``age'' in time using a combination of molecular dynamics and Monte-Carlo simulations. Through analysis of the atomic configurations during aging, we find that the observed phenomenon is associated with a gradual change in the short range order of point defects so that the defect short range order tends to adopt the same ``symmetry'' as the crystal symmetry of the host martensite lattice. The results provide atomic-level evidence for the symmetry-conforming short-range order model, and may provide new insight into how to control aging to design aging-free shape memory alloys. Reference: 1). J. Deng, X. Ding, T. Lookman, et al, Physical Review B , \textbf{81}, 220101(R), 2010 2). J. Deng, X. Ding, T. Lookman, et al, Physical Review B, \textbf{82},184101, 2010 3). J. Deng, X. Ding, T. Lookman, et al, Applied Physics Letters, \textbf{97},171902, 2010 [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y22.00013: Microstructure from ferroelastic transitions using strain pseudospin clock models in two and three dimensions Turab Lookman, Romain Vasseur, Subodh Shenoy We show how microstructure can arise in first-order ferroelastic structural transitions, in two and three spatial dimensions, through a local mean-field approximation of their pseudospin Hamiltonians, that include anisotropic elastic interactions. Such transitions have symmetry-selected physical strains as their order parameters, with Landau free energies that have a single zero-strain ``austenite'' minimum at high temperatures, and spontaneous-strain ``martensite'' minima of structural variants at low temperatures. The total free energy also has gradient terms, and power-law anisotropic effective interactions, induced by ``no-dislocation'' St Venant compatibility constraints. In a reduced description, the strains at Landau minima induce temperature dependent, clocklike Hamiltonians, with strain- pseudospin vectors S pointing to discrete values including zero. We study elastic texturing in five such first-order structural transitions through a local mean-field approximation of their pseudospin Hamiltonians, that include the power-law interactions. The local mean-field solutions in 2D and 3D yield or oriented domain- wall patterns as from continuous-variable strain dynamics, showing the discrete- variable models capture the essential ferroelastic texturings. [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y22.00014: Symmetry breaking in amorphous solids undergoing martensitic phase transformation - a relation to Landau's theory Michael Fischlschweiger, Eduard Oberaigner Martensitic phase transformation can be classified as displacive solid -solid phase transformations, where the symmetry of the high temperature phase (austenite) breaks when phase transformation occurs. The martensitic phase (low temperature phase ) and its variants are products of symmetry breaking in solids. Based on a quasiparticle statistical mechanics approach the canonical free energy of a representative solid volume element consisting of several quasiparticles (representative mole number) can be derived. The symmetry breaking order parameter of the system is the total strain which is an ensemble mean value in the statistical mechanics concept. In the current theory the order parameter is a macroscopic strain in a sense that the representative volume element stands for the macroscopic level, whereas the lattice parameter changes are considered in the hamiltonian definition of each quasiparticle. Computational results of the developed theory correspond to experimentally observed phenomena in materials undergoing martensitic phase transformation. The present study is focusing the region nearby the phase transformation and shows how the developed theory for describing symmetry breaking and order parameter changes correspond to Landau's phenomenological theory of phase transitions. [Preview Abstract] |
Friday, March 25, 2011 10:48AM - 11:00AM |
Y22.00015: Probing driven first order structural transitions with resistivity noise U. Chandni, Arindam Ghosh We study the avalanche-mediated driven first order structural transition in nickel titanium shape memory alloys with time-dependent fluctuations in electrical resistivity. Higher order statistics of the fluctuations, or noise, has been used as a kinetic detector of the underlying two stage athermal phase transition. We have found that the non-gaussian component of the higher order statistics carries significant information about the transition parameters and is coupled to the microscopic origin of the phase transition. The results can be explained with a model based on three competing time scales dependent on avalanche relaxation, thermal fluctuations and drive rate. The transition temperature was found to decrease with increasing drive rate indicative of the increased possibility of the system being driven towards the athermal limit. Moreover, the magnitude of the non-gaussian component is found to have signatures of the extent of correlations in the system and hence a viable tool to detect any overlap of avalanches in space or time. The study establishes noise as a sensitive tool to probe the kinetics of driven structural transitions which can be exploited in a variety of other systems. References: U. Chandni et. al, Phys. Rev. Lett. 102, 025701 (2009) U. Chandni and A. Ghosh, Phys. Rev B. \textbf{81}, 134105 (2010) [Preview Abstract] |
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