Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session B54: Focus Session: Cooperative Phenomena: Phase Transitions and Magnetocalorics |
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Sponsoring Units: GMAG Room: Mile High Ballroom 1B |
Monday, March 3, 2014 11:15AM - 11:27AM |
B54.00001: Use of Analysis of Variance to Analyze Universal Scaling in Magnetocaloric Materials Dustin D. Belyea, Casey W. Miller Universal scaling analysis of magnetocaloric materials is a tool used to remove the temperature and field dependence in the measured M(H,T) space, and should in theory result in a single universal curve for all materials within a single universality class. This can be used to compare materials and predict behavior beyond measurable field ranges [1,2]. However, in many cases the scaled curves show wide variations in collapsing onto the universal curve, leading to a noisy collection of scaled curves. Hypotheses as to the origin of such scaling violations include minority phases, either magnetic or structural, or both [3]. In this work we show that it is possible to measure the degree to which a material follows universal scaling using tests for homoscedasticity, namely the Brown-Forsythe test. We use experimental magnetic phase transitions as well as some computational models and then apply this test to show general trends in the changes of magnetization with the inclusion of such magnetic and structural minority phases. [1] V Franco et al, J. Phys.: Condens. Matter 20 285207 (2008) [2] V. Franco et al, J. Appl. Phys. 106, 103911 (2009) [3] V. Franco et al, J. Magn. Magn. Mater., 321(9):1115-1120 (2009) [Preview Abstract] |
Monday, March 3, 2014 11:27AM - 11:39AM |
B54.00002: Magnetocaloric and Exchange Bias effects in Ni$_{50}$Mn$_{35.8}$Sn$_{14.2}$ alloy Vijaysankar Kalappattil, A. Biswas, T.L. Phan, S.C. Yu, N.H. Dan, M.H. Phan, H. Srikanth Heusler alloys have been a subject of intense research because of their intriguing physical properties. We have studied magnetic and magnetocaloric properties of such an alloy with composition Ni$_{50}$Mn$_{35.8}$Sn$_{14.2}$. The system undergoes two transitions upon cooling: (i) a ferromagnetic transition in austenite phase at T$_{\mathrm{C}}^{\mathrm{A}} \sim $310 K and (ii) the martensitic transition at T$_{\mathrm{MA}} \sim $ 165 K. Using Maxwell's equation, the magnetic entropy change ($\Delta $S$_{\mathrm{M}})$ is calculated from the is magnetization vs. field curves following heating, cooling, and loop protocols. A large inverse magnetocaloric effect (IMCE) is observed at T$_{\mathrm{MA}}$ in addition to the conventional magnetocaloric effect (CMCE) at T$_{\mathrm{C}}^{\mathrm{A}}$. While $\Delta $S$_{\mathrm{M}}$ is protocol dependent, we have demonstrated that a universal curve can be constructed to describe temperature (T) dependence of $\Delta $S$_{\mathrm{M}}$ corresponding to IMCE without rescaling of the T-axis and irrespective of measurement protocol. Such a universal behavior of IMCE is different from that of CMCE. The system also shows an exchange bias effect below T$_{\mathrm{MA}}$ due to phase coexistence. [Preview Abstract] |
Monday, March 3, 2014 11:39AM - 11:51AM |
B54.00003: Towards enhanced magnetocaloric effects: a combined experimental / computational methodology Karl Sandeman A simple thermodynamic analysis shows that there is scope to double the adiabatic temperature change of d-metal magnetocaloric materials in a fixed change of magnetic field [1]. If found in a a real material, such an improvement in the MCE will have two important effects: to increase the final efficiency of the cooling device and to reduce the use of rare earth materials in the magnet that is used to drive the MCE. I will give examples of a methodology for the search for new magnetocaloric materials, combining neutron scattering studies with materials modelling. We have used this approach to understand magneto-elastic coupling at a fundamental level in both Mn-based and Fe-based tricritical metamagnets [2,3]. \\[4pt] [1] K.G. Sandeman, Scr. Mater. 67 566 (2012).\\[0pt] [2] Z. Gercsi, K. Hono and K.G. Sandeman, Phys. Rev. B 83 174403 (2011) and references therein. \\[0pt] [3] Z. Gercsi et al., Phys. Rev. B 88 024417 (2013). [Preview Abstract] |
Monday, March 3, 2014 11:51AM - 12:03PM |
B54.00004: Vector boson excitations near deconfined quantum critical points Yejin Huh, Philipp Strack, Subir Sachdev We show that the N\'eel states of two-dimensional antiferromagnets have low energy vector boson excitations in the vicinity of deconfined quantum critical points. We compute the universal damping of these excitations arising from spin-wave emission. Detection of such a vector boson will demonstrate the existence of emergent topological gauge excitations in a quantum spin system. [Preview Abstract] |
Monday, March 3, 2014 12:03PM - 12:15PM |
B54.00005: First-principles study of the magnetic and electronic properties of NiAs-type FeSe Weidong Luo, Xiaole Zhang FeSe exhibits the NiAs-type hexagonal structure under external pressure or under certain growth conditions at ambient pressure. The superconducting transition temperature of alpha-FeSe initially increases and then decreases when external pressure is applied, in which the NiAs-type FeSe is considered as the competing phase to the superconducting alpha-FeSe. We study the magnetic and electronic properties of the NiAs-type FeSe using first-principles calculations. The ferromagnetic (FM) and several antiferromagnetic (AFM) configurations of NiAs-type FeSe have been studied, and we observe dependence of the electronic structure on its magnetic configuration of NiAs-type FeSe. The results of theoretical calculations are compared to experimental observations. [Preview Abstract] |
Monday, March 3, 2014 12:15PM - 12:27PM |
B54.00006: Phase diagram of an ideal three-dimensional antiferromagnet with random magnetic anisotropy Pavel Borisov, Felio A. Perez, Trent A. Johnson, Tudor D. Stanescu, David Lederman, Michael R. Fitzsimmons, Adam A. Aczel, Tao Hong The magnetic phase diagram of three-dimensional (3D) antiferromagnets with random magnetic anisotropy (RMA) is not well understood because systems studied experimentally to date have complicated magnetic structures with competing two-dimensional and three-dimensional exchange interactions. The properties of epitaxial thin films of the 3D RMA antiferromagnet Fe$_{x}$Ni$_{1-x}$F$_{2}$ thin films grown on (110) MgF$_2$ substrates were investigated by magnetometry and neutron scattering. Fe$_{x}$Ni$_{1-x}$F$_{2}$ is an ideal 3D antiferromagnet with which to study this problem due to the single-ion anisotropy energies of the transition metal site which tend to order Ni$^{2+}$ and Fe$^{2+}$ spins perpendicular to each other. The magnetic phase diagram determined from these measurements was analyzed using mean field theory. Regions with uniaxial, oblique and easy plane anisotropy were identified. An anisotropy glass region was discovered where a Griffiths-like breakdown of long-range spin order occurs. [Preview Abstract] |
Monday, March 3, 2014 12:27PM - 12:39PM |
B54.00007: Spin Jams, frustration, and exotic entropy scaling Israel Klich, Seung-Hun Lee, Kazuki Iida When spins are regularly arranged in a triangular fashion, the spins may not satisfy simultaneously their antiferromagnetic interactions with their neighbors. This phenomenon, called frustration, usually leads to a large set of ground states and to exotic states such as spin ice and spin liquid. Here we report a novel situation: a system governed by simple Heisenberg interactions that freezes into a glass like~state induced by quantum fluctuations, in contrast to the usual mechanisms for classical spin-glass which rely on the presence of disorder. At the heart of the effect is an unusual scaling of the number of local minima, with a scaling extensive in the boundary length rather than the volume, which we describe in terms of a new tiling problem. [Preview Abstract] |
Monday, March 3, 2014 12:39PM - 12:51PM |
B54.00008: Antiferromagnetism and a Dimensional Crossover in the Model Quantum Magnet SrCu$_{2}$(BO$_{3}$)$_{2}$ Sara Haravifard, Arnab Banerjee, Jasper van Wezel, Daniel Silevitch, Antonio Moreira Dos Santos, Jonathan Lang, Edwin Kermarrec, George Srajer, Bruce Gaulin, Hanna Dabkowska, Thomas Rosenbaum SrCu$_{2}$(BO$_{3}$)$_{2}$ has corner-sharing Cu$^{2+}$ S=1/2 dimers lying on a square lattice, corresponding to the two-dimensional Shastry-Sutherland model. We conduct high resolution x-ray and neutron diffraction measurements as a function of pressure, tuning the ground state from a singlet ground state to long-range order. We report a change in crystal symmetry as a function of temperature for pressures above 4 GPa, linked to antiferromagnetism and the tilting of the dimers out of the plane. The inclusion of Dzyaloshinskii-Moriya interactions in the Shastry-Sutherland Hamiltonian helps explain the experimental results. [Preview Abstract] |
Monday, March 3, 2014 12:51PM - 1:03PM |
B54.00009: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 1:03PM - 1:15PM |
B54.00010: Violating of the Essam-Fisher and Rushbrooke formulas: quantum phase transitions and 1D Ising model Vladimir Udodov The classical Essam-Fisher and Rushbrooke relationships (1963) that connect the equilibrium critical exponents of susceptibility, specific heat and order parameter are shown to be valid only if the critical temperature $T_{C}$ \textgreater\ 0 and $T \to T_{C}$. For quantum phase transitions (PT's) and 1D Ising model with $T_{C} = $ 0$K$, these relations are proved to be of different form. This fact has been actually observed experimentally, but the reasons were not quite clear. A general formula containing the classical results as a special case is proposed. This formula is applicable to all equilibrium PT's of any space dimension. The predictions of the theory are consistent with the available experimental data and do not cast any doubts upon the scaling hypothesis. [Preview Abstract] |
Monday, March 3, 2014 1:15PM - 1:27PM |
B54.00011: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 1:27PM - 1:39PM |
B54.00012: Phase Diagram for Magnon Condensate in YIG Fuxiang Li, Wayne Saslow, Valery Pokrovsky Recently, magnons, which are quasiparticles describing the collective motion of spins, were found to undergo Bose-Einstein condensation (BEC) at room temperature in films of Yttrium Iron Garnet (YIG). Unlike other quasiparticle BEC systems, this system has a spectrum with two degenerate minima, which makes it possible for the system to have two condensates in momentum space. Recent Brillouin Light scattering studies for a microwave-pumped YIG film of thickness $d=5$ $\mu$m and field $H=1$ kOe find a low-contrast interference pattern at the characteristic wavevector $Q$ of the magnon energy minimum. In this report, we show that this modulation pattern can be quantitatively explained as due to unequal but coherent Bose-Einstein condensation of magnons into the two energy minima. Our theory predicts a transition from a high-contrast symmetric phase to a low-contrast non-symmetric phase on varying the $d$ and $H$, and a new type of collective oscillations. [Preview Abstract] |
Monday, March 3, 2014 1:39PM - 1:51PM |
B54.00013: Magnetic-field-induced quantum criticality in a spin-1 planar ferromagnet with single-ion anisotropy Maria Teresa Mercaldo, Ileana Rabuffo, Luigi DeCesare, Alvaro CaramicoD'Auria The effects of single-ion anisotropy on field-induced quantum criticality in spin-1 planar ferromagnet is explored by means of the two-time Green's function method. We work at the Tyablikov decoupling level for exchange interactions and the Anderson-Callen decoupling level for single-ion anisotropy. In our analysis a longitudinal external magnetic field is used as the non-thermal control parameter and the phase diagram and the quantum critical properties are established for suitable values of the single-ion anisotropy parameter. We find that the single-ion anisotropy has sensible effects on the structure of the phase diagram close to the quantum critical point. Indeed, for values of the uniaxial crystal-field parameter above a positive threshold a re-entrant behavior appears for the critical line, while above this value the conventional magnetic-field-induced quantum critical scenario remains unchanged. \\ M.~T. Mercaldo, I. Rabuffo, L. De Cesare, A. Caramico~D'Auria, Eur. Phys. J. B {\bf 86}, 340 (2013) [Preview Abstract] |
Monday, March 3, 2014 1:51PM - 2:03PM |
B54.00014: Complex Magnetic behavior in CeGe$_{1.76}$ Single Crystals observed by Neutron Diffraction at Low Temperatures Wageesha Jayasekara, Wei Tian, Andreas Kreyssig, Halyna Hodovanets, Sergey L. Bud'ko, Paul C. Canfield, Robert J. McQueeney, Alan I. Goldman Magnetization measurements on CeGe$_{1.76}$ single crystals indicate two apparent magnetic transitions around 4 K and 7 K. A neutron diffraction experiment was performed at the instrument HB-1A, to study the magnetic order in detail and results will be discussed in this report. Surprisingly, several transitions have been observed: Below 7 K, incommensurate antiferromagnetic order appears with a propagation vector close to (0~0~0.25), that becomes commensurate below 5.2 K demonstrating a lock-in transition. Moreover, squaring-up behavior was also observed by the occurrence of higher harmonic Bragg peaks. An additional antiferromagnetic order was seen below 4.0 K by half-integer and integer indexed Bragg peaks. Further experiments are needed to verify the directions of the ordered magnetic moments. [Preview Abstract] |
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