Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X6: Rahman Prize Lecture and Theory of Multiferroics |
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Sponsoring Units: DCOMP Chair: Craig Fennie, Cornell University Room: 406 |
Thursday, March 19, 2009 2:30PM - 3:06PM |
X6.00001: Aneesur Rahman Prize for Computational Physics Talk: Numerical Simulations of Spin Glasses and Related Systems Invited Speaker: Systems with disorder and ``frustration'' occur in many branches of science. There has been considerable effort to understand one such type of system, known as the ``spin glass'', because it can be probed in fine detail experimentally by applying a magnetic field, and because it can be modeled by simple-looking Hamiltonians which are amenable to numerical simulation. Analytical work is very difficult and has been carried out mainly on models with unphysical features such as infinite-range interactions. Hence, much of what we know about spin glasses and related systems comes from numerical simulations on simplified models. In this talk I will describe some of the difficulties in performing \textit{reliable} spin glass simulations. Then I will discuss several questions concerning phase transitions in spin glasses and related systems that have been addressed by simulations in recent years including (i) whether there is universality, (ii) whether there is a ``vortex glass'' transition in a disordered type-II superconductor in a magnetic field, (iii) whether ``chiralities'' play a crucial role in Heisenberg spin glasses, and (iv) whether there is a line of transitions (AT line) in a magnetic field. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:42PM |
X6.00002: Superexchange-driven Magnetoelectricity in Magnetic Vortices Invited Speaker: We demonstrate that spins in topologically frustrated antiferromagnetic systems can form periodic arrays of magnetic vortices with symmetry allowing for a linear magnetoelectric response. Realization of this magnetic structure can be provided by transition-metal oxides with a layered Kagom\'e lattice. In such systems, an appropriately structured lattice leads to a microscopic coupling between spins and polar lattice distortions via Anderson superexchange, which has the potential to provide a large magnetoelectric response. In order to quantitatively probe the strength of the magnetoelectric coupling, we have performed density functional theory calculations in the presence of an applied electric field (using linear response) for hexagonal manganites. We demonstrate that the coupling is large and summarize the challenges for achieving such a response in real materials. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 4:18PM |
X6.00003: First-Principles Approach to Lattice-Mediated Magnetoelectric Effects Invited Speaker: I will present a microscopic theory of the magnetoelectric response of an insulator, and derive from it an analytical expression for the lattice-mediated part of the effect. As I will show, such a result provides us with distinct hints at strategies to increase the magnitude of the response, as well as with a convenient method for performing first-principles calculations. I will illustrate the usefulness of the proposed approach with applications to Cr$_{2}$O$_{3}$, a model magnetoelectric crystal, and BiFeO$_{3}$ and related compounds, the best studied, and arguably most technologically promising, family of multiferroics. \newline \newline Ref.: J. I\~{n}iguez, Phys. Rev. Lett. 101, 117201 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X6.00004: First-principles Study of Improper Ferroelectricity in TbMnO$_3$ Invited Speaker: Perovskite TbMnO$_3$ at room temperature forms an orthorhombically distorted lattice with the {\it Pbnm} space group. Below $\sim$27~K the magnetic moments on the Mn atoms adopt an incommensurate cycloidal wave order, and simultaneously a polarization appears along the $c$ direction. We present the results of our first-principles theoretical study of the magnetically induced polarization in TbMnO$_3$ with a {\it commensurate} cycloidal wave of Mn$^{3+}$ moments with a wave-vector close to the experimental value.\footnote{A.~Malashevich and D.~Vanderbilt, Phys.~Rev.~Lett.~\textbf{101}, 037210 (2008).} The calculations are based on density-functional theory in the local-density approximation with the on-site Coulomb correction (LDA+U). The polarization is computed using the Berry-phase technique. We show, in particular, that the spin-orbit interaction is essential for the magnetoelectric coupling. We compute both the electronic and the lattice-mediated contributions to the polarization, and find that the latter is strongly dominant. We analyze the spin-orbit induced forces and lattice displacements from both atomic and mode-decomposition viewpoints, and show that a simple model based on nearest Mn-Mn neighbor Dzyaloshinskii-Moriya interactions is not able to account fully for the results. The direction and magnitude of our computed polarization are in good agreement with experiment. If time permits, calculations on other magnetically induced improper ferroelectrics will be discussed. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:30PM |
X6.00005: Density functional study of the spin exchange interactions, magnetic structures and ferroelectric polarizations of multiferroics driven by magnetic order Invited Speaker: The electronic structures of magnetic insulators LiCuVO$_{4}$, LiCu$_{2}$O$_{2}$, TbMnO$_{3}$, Ca$_{3}$CoMnO$_{6}$, MnWO$_{4}$, CuFeO$_{2}$, Ba$_{2}$CoGe$_{2}$O$_{7}$ and CuBr were examined on the basis of first principles DFT+U+SOC calculations to evaluate their spin exchange parameters and account for their ordered magnetic structures. We then explored how the electric polarizations of these compounds are related to the magnetic ordering and spin-orbit coupling. In this talk results of our studies will be presented. [Preview Abstract] |
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