Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session J13: Focus Session: Multiferroics III |
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Sponsoring Units: DMP GMAG Chair: Valery Kiryuhkin, Rutgers University Room: Colorado Convention Center Korbel 4C |
Tuesday, March 6, 2007 11:15AM - 11:27AM |
J13.00001: Interatomic spin-orbit coupling: mechanism for spin-spiral-caused ferroelectricity T.A. Kaplan, S.D. Mahanti There are two general classes of mechanisms that have been proposed for spin-spiral caused ferroelectricity, one based on ionic displacements as primary cause, the other on charge distortion without ionic displacements. Here we discuss the latter$^{1,2}$. The mechanism proposed here is illustrated by a model where a pair of ions a and b each have low-lying s- and excited p- states with a prescribed spin state $\chi_a$ for the a-site states, similarly for the b-site, and there are 2 electrons; interatomic spin orbit coupling resides in inter-ion hopping due to s-p matrix elements of the spin-orbit coupling operator $\propto \nabla V \times \mathbf{p}\cdot\mathbf{s}$, $V, \mathbf{p}, \mathbf{s}$ are 1- electron potential, momentum, spin, respectively. Assuming the symmetry of a nearest-neighbor pair of cubic- spinel B-sites (there's no center of inversion (coi)) we find an electric dipole moment in the direction $\mathbf{r}_ {ab}\times(\mathbf{S}_a\times\mathbf{S}_b)$, as was found when there is a coi$^{1,2}$. For the spins in a chain parallel to the spiral wave vector in CoCr$_2$O$_{4}$, direction [110]$^3$, this results in ferroelectricity, as observed$^{4}$; the spins in a [1,-1,0]-directed chain, give an antiferroelectric component. Extension to a pair of Cr$^{3+}$ ions will be discussed.\\ 1. H. Katsura et al,Phys. Rev. Lett.\textbf{95},057205 (2005)\\ 2. T. A. Kaplan and S. D. Mahanti, cond-mat/0608227, 10 Aug 2006 \\ 3. N. Menyuk et al, J. Phys. (Paris) \textbf{25}, 528 (1964)\\ 4. Y. Yamasaki et al, Phys. Rev. Lett. \textbf{96}, 207204 (2006) [Preview Abstract] |
Tuesday, March 6, 2007 11:27AM - 11:39AM |
J13.00002: Magnetoelectric properties of cobalt oxides with low dimensional structures H. Kuwahara, M. Akaki, K. Noda, F. Nakamura, D. Akahoshi Since the discovery of novel ferroelectric transition due to spiral spin structures in TbMnO$_3$, materials with spin frustration or nontrivial spin structures have attracted renewed interest as a promising candidate for new magnetoelectrics. In this context, we have focused on compounds with low dimensional structures because they often possess geometrical frustration and resultantly exhibit nontrivial spin structures. In this work, we have investigated the magnetic and dielectric properties of cobalt oxides with low dimensional structures. The subject compound, BaCo$_2$Si$_2$O$_7$ single crystal, is a derivative from Ba$_2$CuGe$_2$O$_7$ in which the spiral spin structure is reported below 3.26K\@. We have substituted Co$^{2+} $ for Cu$^{2+}$ to increase the transition temperature. The crystallographic symmetry of the obtained crystal at RT was confirmed to be $C2/c$ which does not break the inversion symmetry. The Weiss temperatures estimated in paramagnetic region are $-20$K ($H$$\| $$c$) and $-74$K ($H$$\bot $$c$), indicating the large magnetic anisotropy. The weak ferromagnetic magnetization rises up at 21K, where the dielectric constant perpendicular to the $c$ axis ($\varepsilon_{\bot}^c$) decreases concomitantly. In addition, we have observed the magnetocapacitance effect below 21K: $\Delta \varepsilon_{\bot} ^c (\mu_0H_{\bot}^c$=8T$)/ \varepsilon_{\bot}^c (0)$ reaches 0.2\% at 5.5K\@. This result suggests that there exists a coupling between magnetism and dielectricity. Results of Ba$_2$CoSi$_2$O$_7$ will also be presented. [Preview Abstract] |
Tuesday, March 6, 2007 11:39AM - 11:51AM |
J13.00003: Ferroelectricity in a quantum chain magnet. S. Park, Y.J. Choi, C.L. Zhang, S-W. Cheong Multiferroics with enhanced cross-coupling effects exhibit magnetic orders with broken centrosymmetry. It turns out that the lattice relaxation through exchange striction associated with the magnetic orders with non-centrocymetry is the origin of magnetism-induced ferroelectricity. Among the exchange strictions, the Dzyaloshinskii-Moriya type interaction becomes active when ferroelectricty is induced by spiral magnetic orders. Herein, we report our suprising discovery that a quantum chain magnet exhibits ferroelectricity when the spiral magnetic order sets in. [Preview Abstract] |
Tuesday, March 6, 2007 11:51AM - 12:03PM |
J13.00004: Direct transition from a disordered to a multiferroic phase Michel Kenzelmann, Gavin Lawes, Brooks Harris, Gasparovic Goran, Broholm Collin, Art Ramirez, G.A. Jorge, Marcelo Jaime, Sungil Park, A. Ya. Shapiro, L. A. Demianets We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in a triangular lattice antiferromagnet. Ferroelectricity is only observed when the magnetic structure has chirality and breaks inversion symmetry. A magnetic field extinguishes the electric polarization through a change in the magnetic symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that the chiral magnetic order of the triangular lattice antiferromagnet gives rise to a pseudoelectric field, whose temperature dependence agrees with that of the observed electric polarization. The observed multiferroic behavior provides a theoretically tractable example of ferroelectricity from competing spin interactions. [Preview Abstract] |
Tuesday, March 6, 2007 12:03PM - 12:15PM |
J13.00005: Mapping Structural Phase Separation in Eu$_{0.5}$Y$_{0.5}$MnO$_{3}$ using 3D X-ray Microdiffraction J.D. Budai, J.Z. Tischler, W. Liu, D.D. Sarma, D. Topwal, G. Shenoy, W. Yang, B.C. Larson, S-W. Cheong, A.A. Mukhin Phase coexistence in multicomponent manganite systems is known to occur over a wide range of length scales and strongly influences the magnetic and electronic properties. We have used 3D synchrotron x-ray Laue microdiffraction to investigate domain formation and local lattice structure in bulk Eu$_{0.5}$Y$_{0.5}$MnO$_{3}$ single-crystals. X-ray microdiffraction yields 3D spatially-resolved maps of the crystal structure, orientation and lattice parameter, while microfluorescence yields depth-integrated composition maps. The x-ray measurements reveal alternating lamella of orthorhombic Eu-rich and hexagonal Y-rich phases with a self-organized periodicity of $\sim $15 microns. Both phases maintain a well-defined long-range ($\sim $mm) average crystal orientation with respect to the growth direction and to each other. However, small local variations in both orientation (i.e. mosaic) and lattice parameter (strain and composition) are observed, and the possible origins and implications of these inhomogeneities will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 12:15PM - 12:27PM |
J13.00006: Resonant magnetic scattering of multiferroic HoMnO$_{3}$ S. Nandi, A. Kreyssig, L. Tan, J.-W. Kim , J.Q. Yan, R.J. McQueeney, P.C. Canfield , A.I. Goldman , A. Barcza The multiferroic material HoMnO$_{3 }$displays electrical polarization below the Curie temperature $T_{C}$ = 875 K and antiferromagnetic Mn$^{3+}$ ordering at N\'{e}el temperature, $T_{N}$~= 75 K. The ferroelectric phase possesses hexagonal P6$_{3}$cm symmetry with polarization $P_{c}$~=~5.6~$\mu $C~cm$^{-2}$ along the hexagonal $c$ axis. In order to shed further light on the magnetic order in this compound, element specific X-ray resonant magnetic scattering was performed at the Ho $L_{III}$ absorption edge. Resonance enhancement was observed in both quadrupole and dipole channels below 40 K. Measurement of ($0 0 l)$ and $(h 0 l) $( $l$ odd) reflections have resolved contradictions present in the literature regarding the magnetic order of Ho$^{3+}$ moments. From 40 K down to 6 K, Ho$^{3+ }$ moments order according to magnetic space group P6$_{3}^{\mbox{'}}$cm$^{\mbox{'}}$ with different values for the ordered moment on the Wyckoff sites 2a and 4b. According to this space group, Ho moments are ferromagnetically correlated in the \textbf{a-b} plane and antiferromagnetically correlated in the \textbf{c}-direction. The coincidence of the transition temperature (40 K) for Ho$^{3+ }$moment ordering, spin rotation for Mn$^{3+}$ moments, and sharp anomaly in dielectric constant indicates that the interplay between ferroelectricity and magnetism is strongly related to the Ho magnetic order. [Preview Abstract] |
Tuesday, March 6, 2007 12:27PM - 12:39PM |
J13.00007: Crystal Field Excitations in Multiferroic HoMnO$_3$ Owen Vajk, Michel Kenzelmann, Jeff Lynn, Sung-Baek Kim, Sang-Wook Cheong Antiferromagnetic and ferroelectric order coexist in hexagonal HoMnO$_3$, and strong coupling between these two order parameters has been previously observed. Neutron scattering measurements of the low-energy excitations in HoMnO$_3$ reveal a complex spectra of Ho$^{3+}$ crystal-field excitations which depend upon both temperature and applied magnetic field. These crystal-field changes are correlated with changes in the magnetic symmetry of the Mn$^{3+}$ magnetic sublattice. Measurements of the magnon excitations near these crystal field levels indicates strong coupling between the Mn$^{3+}$ moments and the Ho$^{3+}$ crystal field levels. This coupling may play a critical role in explaining the interaction of ferroelectricity and antiferromagnetism in HoMnO$_3$. [Preview Abstract] |
Tuesday, March 6, 2007 12:39PM - 12:51PM |
J13.00008: Ferroelectricity driven by Y $d^0$-ness with re-hybridization in YMnO$_3$ J.-H. Park, D.-Y. Cho, J.-Y. Kim, B.-G. Park, K.-J. Rho, H.-J. Noh, B.-J. Kim, S.-J. Oh, H.-M. Park, J.-S. Ahn, H. Ishibashi, S-W. Cheong, J. H. Lee, P. Murugavel, T. W. Noh, A. Tanaka, T. Jo Recently multiferroicity, in which magnetism and ferroelectricity co-exist, takes much attention due to its exotic magnetoelecric (ME) phenomena. Hexagonal $R$MnO$_3$ ($R$ = Ho, Er, Tm, Yb, Lu, Y, Sc) exhibits multiferroicity with high ferroelectric and low antiferromagnetic transition temperature ($T_E >$ 600K, $T_M\sim$ 90K). The hexagonal structure ($P6_3cm$) brings out soft mode phonons required for ferroelectricity, but its driving force has been puzzled. We investigated electronic structure of hexagonal multiferroic YMnO$_3$ using the polarization dependent x-ray absorption spectroscopy (PXAS) at O $K$- and Mn $L_{2,3}$-edges. PXAS exhibits strong polarization dependence at both edges, reflecting anisotropic Mn 3$d$ orbital occupation. Moreover, the O $K$-edge spectra show that Y $4d$ states are strongly hybridized with O $2p$ ones, resulting in large anomalies in Born effective charges on off-centering Y- and O-ions. These results manifest that the Y $d^0$-ness with re-hybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites. [Preview Abstract] |
Tuesday, March 6, 2007 12:51PM - 1:03PM |
J13.00009: Spin-lattice coupling in RMnO3 (R=rare earth and Y) perovskites J.-Q. Yan, R. J. McQueeney, Y. Ren, J.-S. Zhou, J. B. Goodenough The magnetic order in RMnO3(R=rare earth and Y) perovskites is quite sensitive to the R3+ ionic radius. Type A magnetic order has been observed for R=Gd?La. For R=Dy and Tb, no classic magnetic order was observed down to the lowest temperature. The rest members of the family show a type E magnetic order. As far as we know, the lattice response to the magnetic order has not been systematically studied. Here we will discuss the lattice response to the magnetic order studied by synchrotron x-ray powder diffraction and thermal conductivity. [Preview Abstract] |
Tuesday, March 6, 2007 1:03PM - 1:15PM |
J13.00010: Epitaxially stabilized hexagonal TbMnO$_{3}$ thin films Daesu Lee, Jung-Hyuk Lee, Pattukkannu Murugavel, Hyejin Ryu, Tae Won Noh, Jae Wook Kim, Hyung Jin Kim, Kee Hoon Kim, Younghun Jo, Myung-Hwa Jung, Jong-Gul Yoon, Jin-Seok Chung We observed that hexagonal TbMnO$_{3}$ thin films showed multiferroic properties with enhanced ferroelectricity. The hexagonal TbMnO$_{3}$ thin film shows 1.6 \textit{$\mu $}C/cm$^{2}$ as the remnant polarization value, which is 20 times larger than that of its orthorhombic bulk phase. In addition, the ferroelectric Curie temperature is shifted to 60 K compared to the low temperature value (27 K) of its bulk orthorhombic phase. Interestingly, the hexagonal TbMnO$_{3}$ film displayed the emergence of a new antiferroelectric-like phase just above 65 K, which is the first of its kind in the family of multiferroic hexagonal manganites. A clear anomaly in dielectric constant near the antiferromagnetic Neel temperature ($T_{N}\sim $70 K) shows the possible coupling between the spin and charge degrees of freedom. This is indeed confirmed by the observed second-order magnetoelectric effect below $T_{N}$. [Preview Abstract] |
Tuesday, March 6, 2007 1:15PM - 1:27PM |
J13.00011: Spin structures of magnetic phases in magnetic ferroelectrics, RMn2O5(R = Y and Tb) J.H. Kim, S.-H. Lee, J-H. Chung, M. Kenzelmann, J. Schefer, C.F. Majkrzak, S. Park, S-W. Cheong We report polarized and unpolarized neutron diffraction data obtained from single crystals of magnetic ferroelectrics, RMn2O5(R=Y and Tb). Each system undergoes, upon cooling, more than one magnetic and ferroelectric phase transitions. By using the group representation theory and fitting the data, we have determined the spin structures of the different phases to elucidate the microscopic mechanism of the static spin-charge coupling in the multiferroics. Our results show that the magnetic and ferroelectric phases of the two systems have spin structures that can only be constructed by a linear combination of the basis functions of two two-dimensional representations of the magnetic space group. Implication of the spin structures to the electric polarization of the systems and to theoretical models will also be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 1:27PM - 1:39PM |
J13.00012: Magnetic excitations in magnetic ferroelectrics, RMn$_2$O$_5$ (R = Y and Tb) Seung-Hun Lee, Jung Hwa Kim, Jae-Ho Chung, Y. Qiu, M. Kenzelmann, T.J. Sato, S. Park, S-W. Cheong We report inelastic neutron scattering data obtained from powder and single crystal samples of magnetic ferroelectrics, RMn$_2$O$_5$ (R=Y and Tb). In these systems, magnetic moments are lying on the crystalline ab-plane, and the spontaneous electric polarization occurs along the b-axis. Our data shows that there are several different magnetic excitation modes of Mn spin waves upto 15 meV. The lowest energy excitation is the sliding mode of the moments in the ab-plane, called phason. We have performed linear spin wave calculations to reproduce our data including dispersions as well as polarizations of the spin wave excitations. Relation of the spin waves to electric polarization will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 1:39PM - 1:51PM |
J13.00013: Resonant and Non-resonant x-ray Scattering Studies on Multiferroic TbMn$_{2}$O$_{5}$ J. Koo, C. Song, S. Ji, J.-S. Lee, T.H. Chang, C.-H. Yang, J.-H. Park, Y.H. Jeong, K.-B. Lee, T.Y. Koo, Y.J. Park, J.-Y. Kim, D. Wemeille, A.I. Goldman, G. Srajer, S. Park, S.-W. Cheong Comprehensive x-ray scattering studies on single crystal TbMn$_{2}$O$_{5}$, including resonant scattering Mn $L$ edge, Tb $L$ edge and $M$ edges, have been carried out. Non-resonant x-ray scattering data provide the first crystallographic evidence of symmetry lowering. The x-ray intensities were observed at a forbidden Bragg position in ferroelectric phase. Their temperature dependence as well as q-dependence of CDW peaks are consistent with exchange striction mechanism for multiferroicity in the sample. Intensities of incommensurate CDW peaks show anomalous temperature dependences, which are attributed to existence of magnetic ordering having different temperature dependences. Resonant scattering data confirmed that magnetic moments of Mn$^{3+}$, Mn$^{4+}$ and Tb have different temperature dependences below T$_{N} \quad \sim $ 41K. [Preview Abstract] |
Tuesday, March 6, 2007 1:51PM - 2:03PM |
J13.00014: Elucidaion on the effects of hydrostatic pressure on multiferroic, HoMn2O5 William Ratcliff, C.R. dela Cruz, B. Lorenz, Q. Huang, S. Park, S-W. Cheong HoMn$_{2}$O$_{5}$ has been the subject of intense study as a mutltiferroic material in which both ferroelectricy and magnetic ordering coexist. Earlier work has shown that the ferrolelectric polarization and the dielectric constant are strongly effected by the application of a magnetic field. At low temperatures, as the system's magnetic structure lowers from a commensurate to an incommensurate magnetic phase, ferrolectricity is suppressed and the system becomes paralelectric. It has recently been shown that the applications of hydrostatic pressures of 6 Kbar can stabilize the ferroelectric phase of HoMn$_{2}$O$_{5}$ at low temperatures. During this talk, we discuss the results of neutron diffraction experiments performed on the BT1 powder diffractometer at the NCNR that correlate this preservation of ferroelectricty with changes in the magnetic structure. [Preview Abstract] |
Tuesday, March 6, 2007 2:03PM - 2:15PM |
J13.00015: Pressure Induced Stabilization of the Ferroelectricity and Commensurate Magnetic structure in $R$Mn$_{2}$O$_{5}$ ($R$=Tb,Dy,Ho) C.R. dela Cruz, B. Lorenz, C.W. Chu, W. Ratcliff, J. Lynn, S. Park, S.-W. Cheong, M. Gospodinov Measurements of ferroelectric (FE) polarization were done on $R$Mn$_{2}$O$_{5}$ ($R$=Tb,Dy,Ho) with applied isotropic pressures up to 17kbar. At low temperatures, the high polarization commensurate magnetic state destabilizes into a low polarization incommensurate magnetic state. This is shown by a sharp drop in the ferroelectric polarization with an associated increase in the dielectric constant. This feature was found to be pressure dependent and is suddenly quenched upon passing an $R$-dependent critical pressure, implying the stabilization of the high ferroelectric polarization state which is coincident with the commensurate magnetic structure. The direct correlation between the commensurability of the magnetic structure and the high polarization state is further revealed by high pressure neutron powder diffraction measurements on HoMn$_{2}$O$_{5 }$showing the pressure induced phase transition from the incommensurate to the commensurate magnetic state. [Preview Abstract] |
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