Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D20: Focus Session: Multiferroics I |
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Sponsoring Units: DMP GMAG Chair: Nicola Spaldin, University of California, Santa Barbara Room: Baltimore Convention Center 317 |
Monday, March 13, 2006 2:30PM - 2:42PM |
D20.00001: Local Structure of the Multiferroic System RMn$_2$O$_5$ Trevor Tyson, Qing Qian, Alexander Ignatov, Soonyong Park, Sang-Wook Cheong In order to understand the origin of the coupled magnetic and ferroelectric properties observed in the system RMn$_2$O$_5 $, detailed temperature dependent local structure measurements on powders and single crystals have been performed. The local structure about the rare earth (R) and the Mn sites have been examined. Comparisons with optical measurements and with various models of the temperature dependence of the pair correlations functions are made. [Preview Abstract] |
Monday, March 13, 2006 2:42PM - 2:54PM |
D20.00002: Ferroelectricity induced by acentric spin-density waves in YMn2O5 Graeme R. Blake, Laurent C. Chapon, Paolo G. Radaelli, S. Park, S-W. Cheong The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn$_{2}$O$_{5}$, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magneto-elastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature-dependence of the spontaneous polarization, in particular its sign reversal at the commensurate-incommensurate transition. [Preview Abstract] |
Monday, March 13, 2006 2:54PM - 3:06PM |
D20.00003: X-ray and neutron scattering studies of the magnetoelectric DyMn$_2$O$_5$ V. Kiryukhin, A. Borissov, S-W. Cheong, W. Ratcliff II, S.-H. Lee Magnetic phase diagram of magnetoelectric DyMn$_2$O$_5$ is established using neutron and magnetic x-ray scattering. The magnetic origin of the complex dielectric anomalies in this material is revealed. New diffraction peaks arising due to structural distortion at low temperatures are found and analyzed in the framework of the existing models of the ferroelectric state. Effects of an applied electric field on the magnetic structure are discussed. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:18PM |
D20.00004: Structural anomalies at the magnetic and ferroelectric transitions in RMn$_{2}$O$_{5}$ B. Lorenz, C.R. dela Cruz, F. Yen, Y.Y. Sun, C.W. Chu, S. Park, S-W. Cheong Multiferroic RMn$_{2}$O$_{5}$ (R=rare earth, Y), have attracted significant attention because of their magneto-electric properties giving rise to complex phase diagrams and novel phenomena such as magnetic control of ferroelectric polarization and giant magneto-dielectric effects. In understanding their ferroelectricity and magneto-electric properties the magneto-elastic lattice distortions at the phase transitions are assumed to play a key role. Such distortions are difficult to detect by x-ray or neutron scattering experiments due to the limited resolution. Employing high-precision capacitance dilatometry, we show the existence of distinct, anisotropic lattice anomalies in RMn$_{2}$O$_{5}$ (R=Ho, Tb, Dy) at all magnetic and ferroelectric phase transitions as function of temperature and magnetic fields. These data provide unambiguous evidence for strong magneto-elastic coupling in multiferroic RMn$_{2}$O$_{5}$. [Preview Abstract] |
Monday, March 13, 2006 3:18PM - 3:30PM |
D20.00005: The Pressure Effect on the Ferroelectricity in Multiferroic $R$Mn$_{2}$O$_{5}$ ($R$=Tb, Dy, Ho) Clarina dela Cruz, Bernd Lorenz, Ching-Wu Chu, Soon Yong Park, Sang-Wook Cheong, Marin Gospodinov The effect of isotropic pressure (\textbf{P}) up to 1.7 GPa on the magnetic and ferroelectric phase diagram of $R$Mn$_{2}$O$_{5}$ ($R$=Tb, Dy, Ho) is investigated. Distinctive anomalies of the $b$-axis dielectric constant identifying the critical temperatures for the various magnetic and ferroelectric transitions are monitored as a function of \textbf{P} and the temperature-pressure phase diagram of multiferroic $R$Mn$_{2}$O$_{5}$ was constructed. The magnetic and ferroelectric orders are stabilized under pressure and their respective onset temperatures increase with \textbf{P}. Most notably, the step-like change of the dielectric constant at lower temperatures (T$_{C2})$ that is associated with a drop in the ferroelectric polarization is suddenly quenched upon passing a critical pressure. These results suggest that above the critical pressure the ferroelectric polarization is restored below T$_{C2}$ and the ferroelectric phase in RMn$_{2}$O$_{5}$ is stabilized and extends to the lowest temperatures. $^{\ast }$also at LBNL, Berkeley and HKUST, Hong Kong [Preview Abstract] |
Monday, March 13, 2006 3:30PM - 3:42PM |
D20.00006: High Magnetic Field Investigations of Multiferroic RMn2O5 (R=Tb, Dy, Bi, and Y) Kee Hoon Kim, S. Y. Haam, J. W. Kim, Y. S. Oh, N. Hur, S. Park, S.-W. Cheong, P. A. Sharma, N. Harrison, A. Migliori We have systematically investigated electric/magnetic phase diagram of a series of multiferroic crystals RMn2O5 (R=Tb, Dy, Bi, and Y) from dielectric constant, electric polarization, and magnetization measurements under intense magnetic fields up to 45 tesla by use of various static and pulsed magnets. Comparative studies of the phase diagram reveal several unprecedented findings: (1) phase evolution of the incommensurate antiferromagnetic Mn d spin ordering and related magneto-dielectric effects up to 33 T, (2) a new high field phase coupled to an incommensurate Mn d spin ordering appearing around 20 T, (3) phase evolution coupled to rare earth f-spin ordering and resultant polarization changes, and (4) significant modification of phase boundaries due to strong d-f spin interaction. Based on the determined electric/magnetic phase diagram, we also discuss how dielectric constant and polarization can be dependent on the commensurate and incommensurate magnetic phases under magnetic field to result in the observed large magneto-dielectric effects. [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 3:54PM |
D20.00007: Phonon anisotropy in ZnCr$_2$O$_4$ from first principles Craig J. Fennie, Karin M. Rabe The geometrically frustrated spinel ZnCr$_2$O$_4$ undergoes a cubic-to-tetragonal structural transition simultaneously as it enters a Neel state at T$_c$=12.5K. A recent experiment\footnote{A.B. Sushkov et al., Phys. Rev. Letters 94 (2005) 137202.} using infrared spectroscopy measured a large splitting of an optical phonon frequency at T$_c$; it was argued that this splitting was due to a direct spin-phonon coupling. Here, we describe a first-principles study of the zone-center optical phonons in metrically cubic ZnCr$_2$O$_4$, with the LSDA+U as implemented in VASP. For selected collinear magnetically ordered structures, the influence of magnetic order on the phonon frequencies is identified, confirming the recent experimental results. The origin of this large phonon anisotropy will be discussed. [Preview Abstract] |
Monday, March 13, 2006 3:54PM - 4:06PM |
D20.00008: IR phonons induced by the helical magnetic order in multiferroic TbMn$_2$O$_5$ Rolando Valdes Aguilar, A. Sushkov, H.D. Drew, S.W. Cheong The interplay between magnetic order and the lattice in multiferroic crystals has produced such interesting phenomena as polarization reversal and change of dielectric properties with magnetic fields \footnote{Hur, N et al. Nature \textbf{429} (2004) 392.}. Ferroelectricity in the multiferroic materials REMn$_{2}$O$_{5}$ (RE = rare earth) is thought to originate from a helical antiferromagnetic order. In order to study this possiblity we have made an infrared study of TbMn$_{2}$O$_{5}$. We find that several IR phonons show correlations with the distinct magnetic and dielectric phase transitions. Of special interest is the phonon spectrum for light polarization along the b axis where a mode at $\sim$ 706 cm$^{-1}$ exists only in the commensurate magnetic phase with \textbf{k} = (1/2,0,1/4) in the temperature range of 24-33 K. Possible scenarios for this phonon are: (1) the appearance of zone-folded modes; (2) the activation of previously silent modes due to the reduction of crystal symmetry. These scenarios are discussed in terms of the spin-lattice coupling in this class of materials. [Preview Abstract] |
Monday, March 13, 2006 4:06PM - 4:18PM |
D20.00009: Microwave dielectric constant measurements of multiferroic TbMn$_{2}$O$_{5}$ Samuel Lofland, A Sushkov, H.D. Drew, S.W. Cheong We have measured the temperature dependence of the dielectric constant of TbMn$_{2}$O$_{5}$ by a cavity perturbation technique at frequencies between 2 and 13 GHz. There are three anomalous features seen in the quasistatic dielectric constant as a function of termperature below the onset of magnetic ordering ($\sim $ 38 K); however at microwave frequencies, the enhancement in the dielectric constant is significantly reduced. In fact, by 13 GHz, the anomalies have nearly disappeared. Each anomaly has its own characteristic relaxation frequency, with the ones at higher and lower temperature being in the GHz range and the intermediate one being in the MHz. We discuss these results in terms of magnetoelectric domains. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:30PM |
D20.00010: Phonon coupling to the Tb spin in multiferroic TbMn2O5 Young-noh Yoon, A.B. Sushkov, R. Valdes Aguilar, H.D. Drew, N. Hur, S.W. Cheong TbMn2O5 is a representative of a group of multiferroic materials where ferroelectricity is induced by spiral magnetic ordering at low temperatures. These materials demonstrate several weak structural and magnetic transitions reflecting complex interplay between magnetic order and the lattice. Phonons serve as a probe of lattice changes and, via spin- phonon coupling, of magnetic ordering. We measured the reflectivity spectra in a- and b-polarizations of an orthorhombic single crystal. The Tb-dominating phonons are active in both polarizations but only b-polarization shows a magnetic shift below 24 K where Tb moments start to order. We made lattice dynamics calculations using popular GULP program to understand why a particular infrared phonon is strongly coupled to spin ordering. [Preview Abstract] |
Monday, March 13, 2006 4:30PM - 4:42PM |
D20.00011: Temperature depending studies of multiferroic TbMnO$_3$ by spectral ellipsometry and Raman scattering Ilka Mahns, M. Bastjan, R. Rauer, G. Neuber, B. Schulz, S. Mueller, A. Rusydi, M. Ruebhausen, D. N. Argyriou, M. Kim, H. Barath, S.L. Cooper TbMnO$_3$ has been studied in order to understand the orbital, structural, and magnetic structure as a function of temperature by Raman scattering and spectral ellipsometry. We present optical spectra, in which anomalies can be observed above and below the N\'eel temperature of 41 K. Below T$_N$, TbMnO$_3$ develops a complex magnetic structural phase. Another characteristic temperature of this material is T$_F$ = 28 K. Below this temperature, TbMnO$_3$ develops a multiferroic state. From the ellipsometry data, we find a rearrangement of the spectral weight at 41 K. Raman scattering with an excitation energy of 1.91 eV shows Jahn-Teller mode changes below T$_N$. Below T$_F$ we also detected a new ferroelectric mode at 128 cm$^{-1}$. In the ellipsometry data as well as in the Raman spectra we identify another critical temperature at T$^*$ $\sim$ 150 K. From our observations we can conclude that there is a strong coupling between the electronic and lattice degrees of freedom, which influence both the formation of the multiferroic state and the Jahn-Teller-distortions. [Preview Abstract] |
Monday, March 13, 2006 4:42PM - 4:54PM |
D20.00012: Investigation of coupling between antiferromagnetic and ferroelectric orders in TbMnO$_{3}$ using magnetic field dependent Raman scattering Harini Barath, Minjung Kim, S.L. Cooper, I. Mahns, M. Ruebhausen, D.N. Argyriou TbMnO$_{3}$ is an antiferromagnetic insulator with a distorted orthorhombic perovskite structure and a N\'{e}el temperature at T$_{N}$ = 41K. TbMnO$_{3}$ is of particular interest because it is one of the few materials that exhibits not just a co-existence, but a strong coupling between antiferromagnetic and ferroelectric order, as evidenced by a para- to ferro- electric transition below T = 28K. In this talk, we discuss magnetic field dependent Raman scattering measurements of TbMnO$_{3}$ -- both for magnetic fields oriented along various crystallographic directions and as a function of temperature through the ferroelectric and N\'{e}el transitions - the goal of which is to investigate the magnetic-ferroelectric coupling mechanism in this interesting material. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:06PM |
D20.00013: Dielectric properties of TbMnO$_{3}$ and La(Sr,Ca)$_{0.3}$MnO$_{3 }$-- evidence for spectral weight changes up to 20 eV. S. M\"{u}ller, M. Rubhausen, R. Rauer, A. Rusydi, M. Bastjan, G. Neuber, S. Dastjani-Faharani, B. Schulz, S. Singer, A. Lichtenstein, D. Argyriou, K. D\"{o}rr In order to analyze changes in the kinetic energy that occur at the transition into the magnetic state we have performed ellipsometry measurements (0.5-5.5 eV) and reflectance measurements (4 -- 20 eV) covering in total a spectral range between 05 and 20 eV. First, we evaluate thermal difference reflectance spectra and find changes at the transition into the magnetic state for both undoped TbMnO3 and doped La(SrCa)MnO3 exceeding energies up to 20 eV. We find distinct changes at the magnetic transitions for energies around 2 eV, 4 eV, 8 eV, 12 eV, and 18 eV. We attribute the first two transitions to Mn d-d high-spin and Op-Mnd charge transfer transitions. The transition at 18 eV is most likely connected to the O2s to O2p transition, whereas the origin of the transitions at 8 and 12 eV remain to be discussed. [Preview Abstract] |
Monday, March 13, 2006 5:06PM - 5:18PM |
D20.00014: The role of lattice distortions in magnetoelectrics Turab Lookman, Avadh Saxena The orthorhombic magnetoelectrics, such as RMnO3, R=Tb,Dy,Gd, exhibit lattice modulations as measured by diffraction. Symmetry analysis predicts a shuffle based transition associated with the magnetoelectric state. We include coupling of the shuffle modes to polarization and incoommensurate magnetization within a Landau free energy framework. We explore the possible role of strain in recently observed pressure effects in some of the materials. In addition, using strain we model the orthorhombic to monoclinic structural transition in certain doped materials. We attempt to connect the free energy coefficients to avaliable structural and thermodynamic data. ~ [Preview Abstract] |
Monday, March 13, 2006 5:18PM - 5:30PM |
D20.00015: Field induced linear magneto-elastic coupling in multiferroic TbMnO$_{3}$ Nadir Aliouane, Dimitri Argyriou, J. Strempfer, I. Zegkinoglou, M. v. Zimmerman We have used in-field neutron and X-ray single crystal diffraction to measure the incommensurability $\delta$ of the crystal and magnetic structure of multiferroic TbMnO$_{3}$. We show that the flop in the electric polarization at the critical field $H_{C}$, for field $H$ along the $a-$ and $b-$axis coincides with a 1st order transition to a commensurate phase with propagation vector $\kappa=(0,\frac{1}{4},0)$. In-field X-ray diffraction measurements show that the quadratic magneto-elastic coupling breaks down with applied field as shown by the observation of the 1st harmonic lattice reflections above and below $H_{C}$. This indicates that magnetic field induces a linear magneto-elastic coupling. We argue that the commensurate phase can be described by an ordering of Mn-O-Mn bond angles. [Preview Abstract] |
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