Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A2: Magnetic-Ferroelectric Coupling in Multiferroics |
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Sponsoring Units: DCMP Chair: Jeffrey Lynn, National Institute of Standards and Technology Room: Baltimore Convention Center Ballroom III |
Monday, March 13, 2006 8:00AM - 8:36AM |
A2.00001: Multifunctional Multiferroics Invited Speaker: Discovery of intriguing phenomena due to~significant cross-coupling in multiferroics where magnetic order and ferroelectric structural~distortion coexist has drawn enormous attention in materials science and condensed mater physics community for the last about two years, partially due to the possible realization of new multifunctional devices. I will overview the recent progress and discuss the key issues in understanding and optimizing the cross-coupling effects. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 9:12AM |
A2.00002: Magnetic Order and Spin Dynamics in Ferroelectric HoMnO$_3$ and TbMnO$_3$ Invited Speaker: Although there are many ferroelectric and magnetic materials, very few have simultaneous magnetic and ferroelectric order. Only a select few of these materials, known as multiferroics, exhibit significant coupling between the magnetic and ferroelectric properties, and the nature of these interactions is not well understood. We have performed neutron scattering studies of two multiferroics which exhibit significant magnetic-ferroelectric coupling, hexagonal HoMnO$_3$ [1] and perovskite TbMnO$_3$ [2]. In HoMnO$_3$, inelastic neutron scattering measurements of the Mn$^{3+}$ spin-wave spectrum and Ho$^{3+}$ crystal field excitations reveal a significant coupling between magnons and the crystal field levels which may contribute to the complex phase diagram and strong magnetic-ferroelectric coupling. In TbMnO$_3$, extensive neutron diffraction studies show that the magnetic transition from a spin density wave to an incommensurate spiral order phase breaks the inversion symmetry of the lattice and induces the onset of ferroelectric distortion. Our results should help lead to a fuller understanding of the interaction between magnetism and ferroelectricity in multiferroics. [1] O.P. Vajk et al., Phys. Rev. Lett. 94, 087601 (2005) [2] M. Kenzelmann et al., Phys. Rev. Lett. 95, 087206 (2005) [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:48AM |
A2.00003: Ferroelectricity in Incommensurate Magnets Invited Speaker: In several systems (e. g. Ni$_3$V$_2$O$_8$ and TbMnO$_3$) there occurs a phase transition in which a new incommensurate magnetic order parameter appears simultaneously with the development of a uniform spontaneous electric polarization, $P$. Here I review the Landau theory which gives a phenomenological explanation (independent of the microscopic details) of this phenomenon. The key point is that although the quadratic term in $P$ in the free energy is always stable (positive), a nonzero value of $P$ is induced by a trilinear coupling of the $\sum_{kmn} a_{kmn}P_kM_m(q)M_n(-q)$, where subscripts label Cartesian components and $M(q)$ is the $q$-Fourier component of the magnetization. Using representation theory to identify the symmetry of the magnetically ordered phases, I identify the symmetry of this term which is consistent with crystal symmetry. This analysis shows that in the highest temperature magnetically incommensurate phase a nonzero value of $P$ is not allowed (as observed) and also that in the lower temperature magnetically incomensurate phase a nonzero $P$ can only appear in the direction which is observed by experiment. This theory indicates that a microscopic model must involve a trilinear interaction between a phonon displacement and two spin operators. A microscopic analysis of phonon distortions of the spin Hamiltonian reproduces these symmetries, as expected and indicates which gradients of the most general anisotropic exchange tensor can possibly come into play. Inelastic neutron scattering data on a Ni$_3$V$_2$O$_8$ powder sample and a symmetry analysis of the phonon spectrum enables us to speculate as to which phonon modes are crucial for this phenomenon. See PRL {\bf 95}, 087205 and cond-mats: 0508617, 0508730, 0510386, and 0510807 whose coauthors I hereby acknowledge. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:24AM |
A2.00004: (H,T,P) Phase Diagrams in Multiferroics: RMnO$_{3}$, RMn$_{2}$O$_{5}$ (R = Rare Earth) and Ni$_{3}$V$_{2}$O$_{8}$ Invited Speaker: Recently, great interest has been generated in the class of materials known as multiferroics (MF), which exhibit the simultaneous occurrence of two or more of the following properties, namely, ferro(anti) magnetic, ferro(anti) electric, ferroelastic, ferroacoustic, and ferroplastic. A subset of MF, known as magnetoelectrics (ME), in which the magnetic and ferroelectric orders coexist, can display the ME-effect where their magnetic and dielectric properties can be affected by an electric and a magnetic field, respectively. The attention first generated in the 60's waned due to the smallness of the effect. However, the situation has drastically changed in recent years due to the observation of a giant ME-effect [1]. Complex diagrams have been induced by magnetic field in the hexagonal single-crystalline rare-earth manganites RMnO$_{3}$ [2] and the Kagome-staircase compound Ni$_{3}$V$_{2}$O$_{8}$ [3], as evidenced by anomalies in their dielectric constant, specific heat, magnetic susceptibility, and thermal expansion coefficients at the phase boundaries. Some of these anomalies occur simultaneously. In this talk, our results of the dielectric, magnetic, calorimetric, and dilatometric properties of RMnO$_{3}$ (R = Ho, Y, Dy, Er, Tm, and Tb), HoMn$_{2}$O$_{5}$ and Ni$_{3}$V$_{2}$O$_{8}$ at ambient pressure in different magnetic fields up to 7 T and some under high pressures up to 2 GPa will be presented. Analysis of the anomalies and examination of the pressure effects on the phase boundaries demonstrate the critical role of strong spin-phonon coupling in the ME-effect and the richness of physics in the study of ME phase diagrams. Possible avenues to further enhance the ME-effect will be discussed. \newline \newline [1] see for example N. Hur et al., PRL 93, 107207 (2004). \newline [2] B. Lorenz et al., PRL 92, 087204 (2004); B. Lorenz et al., PRB 71, 014438 (2005); C. dela Cruz et al., PRB 71, 060407(R) (2005); F. Yen et al., PRB 71, 180407(R) (2005). \newline [3] G. Lawes et al., cond-mat/0503385. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 11:00AM |
A2.00005: Ferroelectricity in frustrated magnets Invited Speaker: The coupling between ferroelectricity and magnetism in multiferroics is a complex many-body phenomenon resulting from the interplay between charge, spin, and lattice degrees of freedom, and interesting both for its fundamental physics and possible technological applications. An exceptionally strong sensitivity of ferroelectric ordering to an applied magnetic field was recently found in a number of frustrated magnets with periodic non-collinear spin structures. In this talk I will present a phenomenological approach to ferroelectric magnets, which explains the relation between electric polarization and magnetic ordering, anomalies in dielectric constant at magnetic transitions, and sudden flops of electric polarization in magnetic field, observed in these materials. I will also discuss microscopic mechanisms that can give rise to ferroelectricity in magnetically ordered states and possible roads to high- temperature multiferroics. [Preview Abstract] |
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