Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides IV |
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Sponsoring Units: DMP GMAG Chair: Sang-Wook Cheong, Rutgers University Room: LACC 150C |
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S43.00001: Density Functional Theory of Multiferroics Invited Speaker: The electronic and structural properties of complex magnetic oxides are notoriously difficult to calculate accurately because of the strong correlations between their localized, magnetic electrons. Indeed with the traditional local density approximation to density functional theory, ferromagnetic metallic behavior is often predicted for materials which are known experimentally to be antiferromagnetic insulators. This is particularly problematic in the modeling of multiferroic materials, because such spurious metallicity is fundamentally incompatible with the occurrence of ferroelectricity. Fortunately methodological advances such as the LDA+U method, and the use of self-interaction corrections, now allow the reliable calculation of multiferroic phenomena without significant increase in computational cost. Here we review the utility of such modern density functional methods in explaining and predicting multiferroic behavior. We describe the elucidation of ferroelectric mechanisms that are compatible with magnetism, the successful prediction and subsequent synthesis of new multiferroics, and some recent computational explorations of magneto-electric switching. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S43.00002: High magnetic field phase diagram of multiferroic DyMn2O5 up to 45 T K. H. Kim, T. H. Kim, S. Y. Haam, N. Hur, S. Park, S.-W. Cheong, Y. H. Jo, J.-G. Park, A. Migliori Strong magnetoelectric coupling in multiferroic crystals such as ReMn$_{2}$O$_{5}$ (Re=rare earth) has provided unprecedented opportunity to manipulate ferroelectric (FE) polarization using magnetic fields. Most of investigations to dates have yet been limited to a rather low magnetic field region. Herein, we present the first high magnetic field (B) versus temperature phase diagram of DyMn2O5 in magnetic fields up to 45 T and temperatures below 50 K, determined from the dielectric constant, pyroelectric, and magnetoelectric current measurements using various magnets; superconducting magnets up to 17 T, a dc resistive magnet up to 33 T, and a mid-pulse magnet up to 45 T. Our phase diagram reveals that at least 4 different kinds of FE domains, which show strong temperature-and field-history-dependence, develop at low temperatures below 40 K, and exhibit dramatic evolution under B. For example, as B increases at 4 K, FE polarization shows successive flopping at B~2, 7 T and 22 T, producing large dielectric constant changes, $\Delta$$\epsilon$(B)/$\epsilon$(0T) $\sim$5, 70, 20 \%, respectively. We discuss the complex phase diagram in the context of strong spin- lattice coupling that is linked to the exchange interaction between Dy f- and Mn d-spins. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S43.00003: Magnetic Phase Diagram of the Giant Magnetoelectric, DyMn2O5 W. Ratcliff II, V. Kiryukhin, M.A. Kenzelmann, S.-H. Lee, Ross Erwin, N. Hur, S. Park, S.-W. Cheong It has been recently found that DyMn$_{2}$O$_{5}$ develops a spontaneous electric polarization below the Neel ordering temperature. Furthermore, this spontaneous polarization can be switched through the application of a magnetic field. ~Several anomalies were also observed in the dielectric constant. ~We have performed neutron diffraction measurements on a single crystal of this material, and have found that anomalies in the dielectric constant and the polarization are correlated with magnetic transitions induced by field or temperature. ~During this talk, I will present the magnetic phase diagram of this system and show how it corresponds to the spontaneous polarization and anomalies in the dielectric constant. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S43.00004: Evolution of ferroelectric and antiferromagnetic phases of TbMn$_{2}$O$_{5}$ under high magnetic fields up to 45 T S.Y. Haam, T.H. Kim, K.H. Kim, S. Park, N. Hur, S.-W. Cheong, A. Migliori Recent discovery of ferroelectric (FE) polarization reversal/imprint actuated by an external magnetic field in multiferroic TbMn$_{2}$O$_{5}$ has opened up promising device application potentials such as magnetically-recorded ferroelectric memory [1]. For better understanding of the interplay between magnetism and ferroelectricity in the multiferroic, we determined high field vs temperature phase diagram of TbMn$_{2}$O$_{5}$ from dielectric constant, pyroelectric current and magnetoelectric current measurements (along b axis) under static or pulsed magnetic field (B) (along a axis) up to 45 T. Our results reveal that (1) as B increases, negative FE polarization phase coined with the Mn d-spin reorientation transition below T=25 K at B=0 T expands its region in temperature to merge into the main FE phase boundary below T$\sim$38 K and B$\sim$20 T (2) low temperature positive FE polarization phase stabilized with the ferromagnetic order of Tb f-spin survive up to 25 K under B$\sim$4T. [1] N. Hur et al., Nature 429, 392 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S43.00005: Magnetic Order and Spin Dynamics in Multiferroic HoMnO$_{3}$ and Related Systems Invited Speaker: Hexagonal HoMnO$_{3}$ is a c-axis ferroelectric (T$_{C}$=875 K) that couples to the antiferromagnetism at 72 K (1). The S=2 Mn$^{3+}$ ions occupy a frustrated triangular lattice, with the spins forming a non-collinear 120$^{\circ}$ magnetic structure from 72 to 40 K, and then undergoing a spin-flop transition to another 120$^{\circ}$ Mn spin structure below. The spin wave dispersion relations are well described by a two-dimensional nearest-neighbor Heisenberg model with exchange J=2.44 meV, and an anisotropy D that is 0.093 meV above the spin reorientation transition at 40 K, increasing to 0.126 meV below. For H$\Vert$c the phase diagram has been determined, and reveals a re-entrant phase boundary for the structure below 40 K, and additional hysteretic transitions below the magnetic ordering temperature of 8 K for the holmium spins. The effects of an applied electric field in the magnetically ordered phases will be discussed. We also briefly describe the effects of A-site chemical disorder on the ferromagnetic phase transition and spin dynamics of the La$_{1-x}$Ba$_{x}$MnO$_{3}$ perovskite (2). The dramatically reduced Curie temperature of the disordered system primarily originates from the enhanced polaron formation that truncates the ferromagnetic state, rather than a reduction in the exchange. The overall behavior observed in the CMR regime of the manganites is quite similar to that observed in the relaxor ferroelectrics as well as the spin and charge stripes found cuprate oxides, demonstrating a commonality of many of the underlying physical concepts of these perovskite oxides. \\ (1) Work in collaboration with O. P Vajk, M. Kenzelmann, S. B. Kim and S.-W. Cheong\\ (2) Work in collaboration with T. Sato and B. Dabrowski\\ Work at UM supported by NSF-MRSEC, DMR 00-80008. [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S43.00006: Ferroelectricity and magnetism in the hexagonal manganite YMnO$_3$ from first principles Craig Fennie, Karin Rabe The hexagonal manganites are a class of multiferroic materials that are simultaneously ferroelectric and antiferromagnetic. Here, we describe a first-principles study of the structural energetics and polarization in magnetic YMnO$_3$, with the LSDA+U as implemented in VASP. For selected collinear magnetically ordered structures, the lowest symmetry-allowed terms in the Taylor expansion of the energy as a function of zone-center and zone-boundary distortions are identified and computed. The implications for the phase transitions in YMnO$_3$ will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S43.00007: Large magneto-dielectric coupling in orthorhombic YMnO$_3$ and HoMnO$_3$ Bernd Lorenz, Y. Q. Wang, Y. Y. Sun, C. W. Chu We have found a remarkable increase (up to 60 {\%}) of the dielectric constant with the onset of magnetic order at 42 K in the metastable orthorhombic structures of YMnO$_{3}$ and HoMnO$_{3}$ that proves the existence of a strong magneto-dielectric coupling in the compounds. Magnetic, dielectric, and thermodynamic properties show distinct anomalies at the onset of the incommensurate magnetic order and thermal hysteresis effects are observed around the lock-in transition temperature at which the incommensurate magnetic order locks into a temperature independent wave vector. The orders of Mn$^{3+}$ spins and Ho$^{3+}$ moments both contribute to the magneto-dielectric coupling. A large magneto-dielectric effect was observed in HoMnO$_{3}$ at low temperature where the dielectric constant can be tuned by an external magnetic field resulting in a decrease of up to 8 {\%} at 7 Tesla. By comparing data for YMnO$_{3}$ and HoMnO$_{3}$ the contributions to the coupling between the dielectric response and Mn and Ho magnetic orders is separated. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S43.00008: Giant spin-lattice coupling in magnetic oxides with rare earths Sang-Wook Cheong, N. Hur, S. Park, S. Guha, A. Borissov, V. Kiryukhin Recently, the astonishing interplay between magnetic and lattice properties has been discovered in magnetic oxides with rare earths, including multiferroic Tb(Dy)MnO$_{3}$, Tb(Dy)Mn$_{2}$O$_{5}$, HoMnO$_{3} $. The discovered effects include reversible flipping of polarization (magnetization) by applied magnetic (electric) field, and the giant change of dielectric constant in applied magnetic field. We have investigated other magnetic oxides with various rare earths in order to reveal new phenomena of spin-lattice coupling as well as to find out the exact role of rare earths in the coupling. The results of our extensive investigation will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S43.00009: Spectral origin of the $c$-axis dielectric constant anomalies in hexagonal HoMnO$_3$ Andrei Sushkov, H. Dennis Drew, Sang-Wook Cheong The coupling between ferroelectric (P) and magnetic (M) order parameters in multiferroics is one of the most important problems both for basic understanding and applications. If this coupling in hexa-manganites occurs within a unit cell there should be a phonon, infrared-active in $E||c$ polarization, which modulates simultaneously $P_c$ and Mn--Mn superexchange in the $ab$-plane. The dielectric constant anomalies, observed at magnetic ordering/re-ordering temperatures, indicate existence of a spin-coupled phonon(s). To find the linking phonon we have measured temperature dependence of the $c$-axis infrared phonon spectrum of a hexagonal HoMnO$_3$ single crystal. These optical results and the dielectric anomalies will be discussed in terms of the coupling between the two order parameters in the multiferroic hexa-manganites. [Preview Abstract] |
Wednesday, March 23, 2005 5:06PM - 5:18PM |
S43.00010: Search for the soft mode phonons in TbMnO$_3$, TbMn$_2$O$_5$, and DyMn$_2$O$_5$ multiferroics using Raman and Far Infrared Transmission Spectroscopy A.A. Sirenko, S. Park, N. Hur, S-W. Cheong, C. Ulrich, L. Machtoub, B. Keimer, G.L. Carr One of the possible explanations of the anomalies in the dielectric properties of TbMnO$_3$ and TbMn$_2$O$_5$ multiferroics [Nature 426, 55 (2003) and Nature 429, 392 (2004)] is the phonon softening. Raman spectra of the optical phonons in TbMnO$_3$, TbMn$_2$O$_5$, and DyMn$_2$O$_5$ single crystals have been measured in the temperature range between 4 and 300 K and in the magnetic field up to 13 T. Different scattering configurations and orientations of magnetic field with respect to the crystallographic directions have been investigated. Structural phase transition in both TbMn$_2$O$_5$ and DyMn$_2$O$_5$ was found at T=150 K. FT-IR transmission spectra in TbMnO$_3$ have been measured in the frequency range between 20 and 250 cm$^{-1}$ and in the temperature range between 10 and 300 K in magnetic field up to 11 T oriented along the c- axis. TbMnO$_3$ single crystal is transparent below 120 cm$^{-1} $. No clear indication for the phonon softening has been found so far. [Preview Abstract] |
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S43.00011: Lamellar phase separation and dynamic competition in La0.23Ca0.77MnO3 Jing Tao, Dario Niebieskikwiat, Myron Salamon, Jian-Min (Jim) Zuo Lamellar charge ordered (CO) and charge disordered (CD) phase separation at low temperatures in La0.23Ca0.77MnO3 was firstly revealed using transmission electron microscopy (TEM). The in-situ TEM observations showed the evolution of the CO-CD phase separation with changed temperatures. Electron diffraction found the different crystal structures of CO and CD phases. Magnetism measurements in La0.23Ca0.77MnO3 indicate two magnetic phase transitions that interestingly coincide with two transition temperatures found in the resistivity relaxation curves. The TEM dark field images observed the dynamic competition between CO and CD phases and the observation is consistent with that in transport properties. The finding here suggests that the CO-CD phase coexistence in this manganite at certain temperature range is driven by magnetoelastic effects rather than being dictated by disorder. [Preview Abstract] |
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