Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y4: New Symmetries and Excitations in Multiferroics |
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Sponsoring Units: DCMP Chair: Dennis Drew, University of Maryland Room: Morial Convention Center 206 |
Friday, March 14, 2008 11:15AM - 11:51AM |
Y4.00001: Observation of ferrotoroidic order in LiCoPO$_4$ Invited Speaker: Domains are an essential property of any ferroic material. Three forms of ferroic order (ferromagnetism, ferroelectricity, ferroelasticity) are widely known. It is currently debated whether to include an ordered arrangement of magnetic vortices as fourth form of ferroic order termed ferrotoroidicity [1]. Although there are reasons to do this from the point of view of thermodynamics a crucial hallmark of the ferroic state, i.e., a ferrotoroidic domain structure, has not been observed before. Here ferrotoroidic domains are spatially resolved by optical second harmonic generation in LiCoPO$_4$ where they coexist with independent antiferromagnetic domains [2]. The origin of ferrotoroidicity in LiCoPO$_4$ is discussed and the general relation between ferrotoroidicity and antiferromagnetism or spin-spiral magnetism will be highlighted. Their space- and time asymmetric nature relates ferrotoroidics to multiferroics with magnetoelectric phase control and other systems in which space and time asymmetry leads to exciting possibilities for future application.\newline $[1]$ C. Ederer, N.A. Spaldin, arXiv:0706.1974v1 [cond-mat.str-el], Phys.\ Rev.\ B, in press (2007) \newline $[2]$ B.B. Van Aken, J.P. Rivera, H. Schmid, M. Fiebig, Nature {\bf 449}, 702 (2007) [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:27PM |
Y4.00002: Symmetry in Multiferroics Invited Speaker: Symmetries govern Nature ubiquitously from the beauty of human faces to the local gauge invariance of quantum field theory. Magnetic order in frustrated magnets can occur without space inversion symmetry. When it relaxes to the magnetically-ordered configuration through exchange-striction, lattice can also loose inversion symmetry, leading to the presence of ferroelectric polarization. In these magnetically-driven ferroelectrics, dielectric properties turn out to be highly susceptible to applied magnetic fields. Both symmetric and antisymmetric exchange coupling can be involved in the exchange-striction. One form of symmetry often broken in Nature is the symmetry between left- and right-handedness. For example, the manner in which light propagates naturally selects one handedness, and is customarily described by a right-handed rule, depicting the relationship among the oscillating electric field, magnetic field and propagation vector of light. Chiral molecules also have a definite handedness, and given the preponderance of chiral molecules, it is not surprising that most complex proteins as well as their constituent amino acids are chiral. What is remarkable however, is that most of naturally occurring amino acids share the same chirality; only left-handedness. Such handedness, or chirality, appears to be a characteristic signature of life. In the multiferroic spinel CoCr$_{2}$O$_{4}$, conical magnetic order accompanies ferroelectric polarization as well as ferromagnetic moment. The relevant handedness and chirality in the multiferroic state will be also discussed. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 1:03PM |
Y4.00003: Towards a microscopic theory of toroidal moments in periodic crystals Invited Speaker: The recent resurgence of interest in magnetoelectric multiferroics has prompted discussion of the relevance of the concept of magnetic toroidal moments in such systems. In particular, the toroidal moment has the same symmetry as the antisymmetric part of the linear magnetoelectric tensor, suggesting a role in mediating coupling between magnetization and electric polarization in multiferroics. In addition, materials in which the toroidal moments are aligned cooperatively -- so-called ferrotoroidics -- have been proposed to complete the group of primary ferroics\footnote{B.B. Van Aken, J.P. Rivera, H. Schmid and M. Fiebig, Nature 449, 702 (2007).}. Here we review the basic microscopic and macroscopic definitions of toroidal moments and illustrate the difficulties in evaluating the toroidal moment of an infinite periodic system. We show that periodic boundary conditions give rise to a multivaluedness of the toroidal moment per unit cell, in close analogy to the case of the electric polarization in bulk periodic crystals. We then evaluate the toroidal moments of several multiferroic and magnetoelectric materials (BaNiF$_4$, LiCoPO$_4$, GaFeO$_3$ and BiFeO$_3$) in the ``localized dipole limit'', where the toroidal moment is caused by a time- and space-reversal symmetry-breaking arrangement of localized magnetic moments\footnote{C. Ederer and N.A. Spaldin, arXiv:0706.1974v1 [cond-mat.str.el], Phys. Rev. B in press.}. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:39PM |
Y4.00004: Role of Spin current in multiferroic behavior Invited Speaker: |
Friday, March 14, 2008 1:39PM - 2:15PM |
Y4.00005: Electromagnons in multiferroics Invited Speaker: Multiferroic materials with simultaneous magnetic and ferroelectric order exhibit strong cross coupling between electric and magnetic phenomena. One important new effect is the strong coupling between the low lying magnetic and lattice excitations to produce spin waves that interact strongly with light by acquiring electric dipole activity from the phonons. As a result, these excitations, which are called electromagnons, produce contributions to the static dielectric constant which appear in the ordered phases and that can be manipulated with an applied magnetic field. This appears to be the origin of the giant magneto-capacitance effect observed in these multiferroics. Predicted more than three decades ago, electromagnons were reliably observed only recently. In my talk, I will discuss electromagnons in two classes of multiferroic materials: RMnO3 and RMn2O5 (R = Y, Rare Earth) in which the multiferroicity derives from different mechanisms. Correspondingly the electromagnons in these two materials systems have characteristically different spectra and selection rules. The electromagnon H-T phase diagrams for Eu0.75Y0.25MnO3, TbMnO3, TbMn2O5 will be presented. I will also discuss the outstanding problems in understanding these novel excitations and the prospects for electromagnons in other materials. [Preview Abstract] |
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