Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V6: Bulk MultiferroicsFocus
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Sponsoring Units: GMAG DMP Chair: Srinivasa Singamaneni, North Carolina State University Room: 302 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V6.00001: Electric field control of magnetization dynamics in multiferroics Vetle Risinggard, Iryna Kulagina, Jacob Linder Multiferroics with a strong magnetoelectric coupling hold great promise in spintronics because they enable magnetic control of the electric polarization as well as electric control of the magnetization. We take an analytical approach, using the Landau-Lifshitz-Gilbert equation to describe the dynamic state of the magnetization. In particular, we show that in insulating multiferroics which exhibit the inhomogeneous magnetoelectric effect there exists an electrically controlled magnon-induced torque that acts even on a homogeneous magnetization. Unlike the magnon-induced torques that arise from Dzyaloshinskii-Moriya interactions or in the proximity of a topological insulator, the strength and direction of this torque is tunable by the externally applied electric field. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V6.00002: Understanding the spin-driven polarizations in Bi$M$O$_{\mathrm{3\thinspace }}(M=$3$d$ transition metals) multiferroics. Santosh KC, Jun Hee Lee, Valentino R. Cooper Bismuth ferrite (BiFeO$_{\mathrm{3}})$, a promising multiferroic, stabilizes in a perovskite type rhombohedral crystal structure (space group R3c) at room temperature. Recently, it has been reported that in its ground state it possess a huge spin-driven polarization [1]. To probe the underlying mechanism of this large spin-phonon response, we examine these couplings within other Bi based 3$d$ transition metal oxides Bi$M$O$_{\mathrm{3\thinspace }}(M=$ Ti, V, Cr, Mn, Fe, Co, Ni) using density functional theory. Our results demonstrate that this large spin-driven polarization is a consequence of symmetry breaking due to competition between ferroelectric distortions and anti-ferrodistortive octahedral rotations. Furthermore, we find a strong dependence of these enhanced spin-driven polarizations on the crystal structure; with the rhombohedral phase having the largest spin-induced atomic distortions along [111]. These results give us significant insights into the magneto-electric coupling in these materials which is essential to the magnetic and electric field control of electric polarization and magnetization in multiferroic based devices. [1] J. H. Lee, and R. S. Fishman, \underline {http://arxiv.org/abs/1504.07106} [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V6.00003: Spin Excitations and Phonon Anomaly in Quasi-1D Spiral Magneti CuBr$_2$ Yuan Li, Chong Wang, Daiwei Yu, Lichen Wang, Fa Wang, Kazuki Iida, Kazuya Kamazawa, Shuichi Wakimoto CuBr$_2$ can be considered as a model quasi-one-dimensional (quasi-1D) spin-1/2 magnet, in which the frustrating ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor exchange interactions give rise to a cycloidal magnetic order below $T_N = 73$ K. The removal of inversion symmetry by the magnetic order also makes the material a type-II multiferroic system with a remarkably simple crystal structure. Using time-of-flight inelastic neutron scattering spectroscopy, we have determined the spin-wave as well as phonon spectra throughout the entire Brillouin zone. The spin-wave spectrum exhibits pronounced anisotropy and magnon damping, consistent with the material’s quasi-1D nature and the non-colinear spin structure. The phonon spectrum exhibits dramatic discontinuities in the dispersion across the quasi-1D magnetic wave vector, indicative of strong magnetoelastic coupling and possibly of a spin-orbital texture that comes along with the spin correlations. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V6.00004: Magnetic and Magnetoelastic Excitations in the Multiferroic CuBr2 determined by Raman, Infrared and Neutron Spectroscopy Chong Wang, Daiwei Yu, Rongyan Chen, Xinyu Du, Lichen Wang, Xiaoqiang Liu, Kazuki Iida, Kazuya Kamazawa, Shuichi Wakimoto, Ji Feng, Nanlin Wang, Yuan Li Multiferroicity was recently discovered in anhydrous copper (II) bromide CuBr2 with a rather high transition temperature (TN $=$ 73.5 K). By the combination of the Raman, Infrared (IR) and inelastic neutron scattering (INS) experiments, evidences for strong magneto-elastic coupling and magneto-elastic excitations are found in CuBr2. In the Raman spectra, a range of broad peaks were observed with the indications of magnetic and phonon origin at the same time. The inelastic neutron scattering experiment reveals that those nontrivial broad peaks originate from the sites of the phonons at incommensurate Q vectors that correspond to the spiral magnetic order. These results strongly suggest the existence of hybrid excitations that involve both the spin and lattice degrees of freedom, and render CuBr2 a promising platform for studying dynamic magneto-elastic coupling. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V6.00005: Pressure effect on ferroelectric properties of multiferroics RMn$_{2}$O$_{5}$, (R = Gd, Tm) Narayan Poudel, Melissa Gooch, Bernd Lorenz, Ching-Wu Chu, Jaewook Kim, Sang-Wook Cheong The pressure effect on the ferroelectric properties of the multiferroics GdMn$_{2}$O$_5$ and TmMn$_{2}$O$_5$ is studied up to 18.2 kbar. Unlike in RMn$_{2}$O$_5$ (R= Tb, Ho, Y), no significant change in polarization is observed in TmMn$_{2}$O$_5$ up to 16.6 kbar . However, a new ferroelectric phase is observed in GdMn$_{2}$O$_5$ above a critical pressure, $P_c$ = 10 kbar at higher temperature. Our result indicates that pressure decouples the Gd moment from the Mn spin system and splits the ferroelectric phase. Thermal expansion data shows a large increase of the c axis at the ambient-pressure ferroelectric transition. The pressure-induced contraction of the $c$ lattice parameter is found to be the cause for splitting of ferroelectric phase by decoupling of two spin systems above $P_c$. The pressure-temperature phase diagram is derived based on dielectric and ferroelectric properties. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V6.00006: Doping-Tunable Ferrimagnetic Phase with Large Linear Magnetoelectric Effect in a Polar Magnet Fe$_{\mathrm{2}}$Mo$_{\mathrm{3}}$O$_{\mathrm{8}}$ Takashi Kurumaji, Shintaro Ishiwata, Yoshinori Tokura The magnetoelectric (ME) effect, i.e., cross control of magnetization (electric polarization) by an external electric (magnetic) field, may introduce a new design principle for novel spin devices. To enhance the ME signal, control of a phase competition has recently been revealed as a promising approach. Here, we report the successful chemical-doping control of the distinct ME phases in a polar magnet Fe$_{\mathrm{2}}$Mo$_{\mathrm{3}}$O$_{\mathrm{8}}$, in which an antiferromagnetic state is competing with a ferrimagnetic state. We demonstrate that Zn doping stabilizes the metamagnetic state to realize the spontaneous ferrimagnetic state and varies the ME coefficients from large negative to large positive values; for instance, the diagonal component of the ME coefficients under the magnetic field perpendicular to the polar axis varies from $-$142??ps/m to 107??ps/m by doping Zn from 12.5{\%} to 50{\%}. This remarkable doping control of the ME property originates from coexisting distinct ME mechanisms, which are selectively tunable by substituting one of the two distinct magnetic sites in the unit cell with nonmagnetic Zn. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V6.00007: \textbf{Field evolution of magnetism in multiferroic (ND}$_{\mathrm{\mathbf{4}}}$\textbf{)}$_{\mathrm{\mathbf{2}}}$\textbf{[FeCl}$_{\mathrm{\mathbf{5}}}$\textbf{(D}$_{\mathrm{\mathbf{2}}}$\textbf{O)] } Wei Tian, Huibo Cao, Jiaqiang Yan, Brian Sales, Jaime Fernandez-Baca (NH$_{\mathrm{4}})_{\mathrm{2}}$[FeCl$_{\mathrm{5}}$(H$_{\mathrm{2}}$O)] is a new organic multiferroic material that exhibits a very rich magnetic field versus temperature (\textit{B vs. T}) phase diagram. The material undergoes two successive magnetic transitions at 7.3K and 6.8K, with the onset of ferroelectricity at 6.8K at $B=$0T. Applying magnetic field with $B$// a-axis or $B$//c-axis induces transitions to different ferroelectric phases, and the electric polarization direction rotates from P//a-axis at $B=$0T to P//c-axis at $B=$5T. Here we report single crystal neutron diffraction results studied with $B$//a-axis that elucidate the field evolution of magnetism associated with different ferroelectric phases in (NH$_{\mathrm{4}})_{\mathrm{2}}$[FeCl$_{\mathrm{5}}$(H$_{\mathrm{2}}$O)]. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V6.00008: Mueller matrix ellipsometry studies of the optical phonons and crystal field excitations in multiferroic orthoferrites RFeO3 (R=Tb,Dy) V.A. Martinez, T.N. Stanislavchuk, A.A. Sirenko, A.P. Litvinchuk, Yazhong Wang, S.W. Cheong Optical properties of multiferroic orthoferrites RFeO3 (R=Tb,Dy) bulk crystals have been studied in the far-infrared range from 50 to 1000 cm-1 and temperatures from 7 K to 300 K. Mueller matrix and rotating analyzer ellipsometry measurements were carried out at the U4IR beamline of the National Synchrotron Light Source at Brookhaven National Lab. Optical phonon spectra and crystal field excitations were measured for all three orthorhombic axes of RFeO3. In the experimental temperature dependencies of the phonon frequencies we found non-Grüneisen behavior caused by the electron-phonon and spin-phonon interactions. We determined the symmetries and selection rules for the crystal field transitions in Tb3+ and Dy3+ ions. Magnetic field dependencies of the optical spectra allowed us to determine anisotropy of the crystal field g-factors for Tb3+ and Dy3+ ions. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V6.00009: Unusual ferroelectricity induced by the Jahn-Teller effect: A case study on lacunar spinel compounds Ke Xu, Hongjun Xiang The Jahn-Teller effect refers to the symmetry-lowering geometrical distortion in a crystal (or nonlinear molecule) due to the presence of a degenerate electronic state. Usually, the Jahn-Teller distortion is not polar. Recently, GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$ with a lacunar spinel structure was found to undergo a Jahn-Teller distortion from a cubic to ferroelectric rhombohedral structure at T$_{\mathrm{JT}} \quad =$ 38 K. Here, we carry out a general group theory analysis to show how and when the Jahn-Teller effect gives rise to ferroelectricity. On the basis of this theory, we ?nd that the ferroelectric Jahn-Teller distortion in GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$ is due to the noncentrosymmetric nature of the parent phase and a strong electron-phonon interaction related to two low-energy T$_{\mathrm{2}}$ phonon modes. Interestingly, GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$ is not only ferroelectric, but also ferromagnetic with a magnetic easy axis along the ferroelectric direction. This suggests that GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$ is a multiferroic material in which an external electric ?eld may control its magnetization direction. Our study not only explains the Jahn-Teller physics in GaV$_{\mathrm{4}}$S$_{\mathrm{8}}$, but also paves a way for searching and designing different ferroelectrics and multiferroics. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V6.00010: Interplay between magnetism and octahedra distortion in the hybrid improper multiferroic Ca$_{\mathrm{3}}$Mn$_{\mathrm{1.9}}$Ti$_{\mathrm{0.1}}$O$_{\mathrm{7}}$ Feng Ye, Jinchen Wang, Jaime Fernandez-Baca, Antonio dos Santos, Bin Gao, Sang-Wook Cheong A novel microscopic mechanism has been proposed to search for ferroelectric material for realistic application. The instability of the polar phonon mode is driven by the simultaneous condensation of two nonpolar lattice modes associated with oxygen octahedron rotation and tilt modes, and is responsible for the polar symmetry observed in the Ruddlesden-Popper compounds. We have used single crystal neutron diffraction to investigate the temperature and pressure dependence of these oxygen octahedron distortions in Ca$_{\mathrm{3}}$Mn$_{\mathrm{1.9}}$Ti$_{\mathrm{0.1}}$O$_{\mathrm{7}}$ which has a structural transition at 365 K and antiferromagnetic order at 120 K. We observed a strong interplay between magnetism and the local oxygen distortion near the magnetic transition. The control of the magnetism through octahedron rotation is also discussed. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V6.00011: Dynamics of the Ho$^{+3}$ magnetism in the multiferroic compound HoMnO$_3$ investigated via time domain terahertz spectroscopy N.P. Armitage, N.J. Laurita, Rongwei Hu, S-W Cheong The multiferroic insulator HoMnO$_3$ possesses a diverse array of magnetism due to both magnetically active Mn$^{+3}$ and Ho$^{+3}$ moments, the latter of which sit at two distinct sites within its non-inversion symmetric hexagonal crystal structure. While previous studies have focused on the ordering of the Mn$^{+3}$ moments, little is known about the magnetic structure below $5$K where it is believed that there is at least partial ordering of the Ho$^{+3}$ ions. In principle, magnetic exchange interactions exist between both distinct Ho$^{+3}$ and Mn$^{+3}$ ions, resulting in an complex phase diagram with as many as five distinct phases found below T $=5$K and H $=3$T. While previous infrared studies have focused on the Ho$^{+3}$ crystal field levels, the spin excitations in the low frequency end of the far infrared remain unknown. We report the finding of new infrared absorptions via time domain terahertz spectroscopy which we attribute to the Ho$^{+3}$ moments. The corresponding field dependence is studied. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V6.00012: Electric Field Effect on the Magnetic Order in Multiferroic LuMnO$_{3}$ Chunruo Duan, Junjie Yang, Leland Harriger, Despina Louca LuMnO$_{3}$ belongs to the family of hexagonal multiferroics in which ferroelectric and magnetic orders coexist and compete. The Mn$^{3+}$ ions reside on a triangular lattice that is geometrically frustrated but undergoes a Neel transition at T$_{N}$ \textasciitilde 90 K. Neutron experiments under electric field were carried out on a single crystal of LuMnO$_{3}$ at SPINS to investigate the coupling of the electric field to the magnetic order. The elastic and inelastic scattering around the commensurate (101) magnetic peak and the Mn trimerization induced (100) peak with and without electric field were investigated. When applying an E-field of 13.3 kV/cm along the (001) direction on an unpoled sample, an increase in (101) peak as well as a shift of the inelastic excitation near (100) to higher $\Delta $E have been observed. Once the sample is polarized, these effects exist without the field. On the other hand, an E-field along (110) direction shows almost no effect. The spin arrangement of the magnetic order is within the ab-plane, thus the Dzyaloshinskii-Moriya interaction explains why a polarization perpendicular to the magnetic moment gives a larger effect. The implication will be discussed. [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V6.00013: First-principles studies of magnetoelectric coupling in hexagonal LuFeO$_{3}$ under applied electric fields Yubo Zhang, Hongwei Wang, Pratikkumar Dhuvad, Xiaoshan Xu, Massimiliano Stengel, Xifan Wu The recently stabilized hexagonal LuFeO$_{3}$ thin-film provides an opportunity in realizing the magnetoelectric coupling in multiferroic materials, in which the weak ferromagnetism due to Dzyaloshinskii-Moriya interaction was found to be closely associated with the trimerization (K$_{3})$ mode. Here, we performed first-principles calculations in hexagonal LuFeO$_{3}$ and studied the variations of weak ferromagnetic moment under applied electric fields. It is found that the weak ferromagnetism is a property that can be directly tuned by the external electric fields. As an improper ferroelectric material, such a magnetoelectric coupling is realized by the strong interaction between the trimerization mode and ferroelectric mode. Under the electric field poling, ferroelectric mode will respond. A change in ferroelectric distortion will in turn affect the amplitude of trimerization mode, and therefore, the weak ferromagnetism. Interestingly, the magnetoelectric coupling in LuFeO$_{3}$ shows a strong nonlinear behavior originating again from the coupling between the trimerization and ferroelectric modes due to its improper nature. [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V6.00014: \textbf{Energetics of Intrinsic Defects in hexagonal LuFeO}$_{\mathrm{\mathbf{3}}}$\textbf{ \newline } Tula R. Paudel, Evgeny Y. Tsymbal The hexagonal Lutetium Ferrite ($h$-LuFeO$_{\mathrm{3}})$ is one of the few multiferroic materials where the spontaneous ferroelectric and magnetic ordering are simultaneously present at room temperature. Here, we investigate energetics of the intrinsic defects h-LuFeO$_{\mathrm{3}}$ using the first-principles supercell approach in the dilute limit. We find the possibility of intermixing, i.e., Lu replacing Fe when $h$-LuFeO$_{\mathrm{3}}$ is grown at the Lu rich conditions, and Fe replacing Lu when this compound is grown at the Fe rich conditions. In addition, our calculations predict the formation of a large number of oxygen vacancies when $h$-LuFeO$_{\mathrm{3}}$ is grown in the reducing conditions. We find that even when the concentration of oxygen vacancies is large, they do not create as much free charge as they form relatively deep localized defect states. Cation vacancies are predicted to have shallow transition levels and the large formation energy, which makes them unlikely in this compound. The electronic structure of all these defects and their effect on the magnetic and polarization properties of $h$-LuFeO$_{\mathrm{3\thinspace }}$are discussed. [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V6.00015: Magneto-electric control of toroidic moments in multiferroic LiCoPO4 Judit Romhányi, Vilmos Kocsis, István Kézsmárki, Karlo Penc In addition to the three widely known forms of ordering, elastic, electric and magnetic orders, a new so called ferrotoroidic phase has been recently observed.[1] The toroidic moment is asymmetric under both time reversal and space inversion symmetries allowing ferrotoroidic materials to exhibit intrinsic magneto-electric effect. Possibility to control magnetic properties using electric field makes such materials desirable for applications. We discuss the magneto-electric control of toroidic moments in the multiferroic material, LiCoPO$_4$. Based on symmetry arguments we derive microscopic model for induced polarization and 'toroidization'. Using multiboson approach we investigate the experimentally observed magnon absorption spectrum following different magneto-electric poling processes. We reproduce the mono-domain ferrotoroidic state established by magneto-electric poling, as well as unconventional optical properties, such as the unidirectional light transmission, emerging in magnon spectrum of LiCoPO$_4$. [1] Bas B. Van Aken et al, Nature 449, 702-705 (11 October 2007), Anne S. Zimmermann et al, Nature Communications 5, Article number: 4796 [Preview Abstract] |
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