Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M17: Focus Session: Frustrated Multiferroics |
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Sponsoring Units: DMP GMAG Chair: Jaime Fermandez-Baca, Oak Ridge National Laboratory Room: 319 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M17.00001: Magnetic Order and Spin Correlations in the Multiferroic Sr$_{0.56}$Ba$_{0.44}$MnO$_{3}$ Jeffrey Lynn, James Mais, Omar Chmaissem, Bogdan Dabrowski Neutron diffraction and inelastic scattering measurements have been carried out on a polycrystalline sample of Ferroelectric Sr$_{0.56}$Ba$_{0.44}$MnO$_{3}$ (T$_{\mathrm{F}}=$400 K) using the BT-7 and SPINS triple-axis spectrometers. The system orders antiferromagnetically at 190 K with an order parameter that varies smoothly with temperature. Inelastic measurements at base temperature reveal an energy gap of 1.7 meV, with a continuous distribution of magnetic scattering above the gap that exhibits a weak peak at 7.5 meV. The inelastic scattering is strongly peaked at the magnetic reciprocal lattice position up to the highest energy of 15 meV measured so far, indicating strong spin correlations. With increasing temperature the magnetic scattering increases in intensity as expected according to the Bose-Einstein thermal population factor for spin waves. Above Tn strong correlations persist, but the scattering does noticeably broaden. [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M17.00002: Magnetic field effects on the multiferroic phases and the ferroelectric polarization of Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_4$ B. Lorenz, K.-C. Liang, Y.Q. Wang, Y.Y. Sun, F. Ye, J.A. Fernandez-Baca, C.W. Chu MnWO$_4$ is a classical multiferroic where ferroelectricity is induced by an inversion symmetry breaking helical spin order. The origin of the helical order is found in competing magnetic exchange interactions with strong uniaxial anisotropy, resulting in magnetic frustration. The extreme sensitivity of the multiferroic state with respect to chemical substitution of Fe, Zn, or Co for Mn was recently shown and Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_4$ (0 \textless\ x \textless\ 0.3) has the most complex phase diagram with multiple polarization flops upon increasing Co content. We report the effects of external magnetic fields on the multiferroic phases in Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$WO$_4$ and show that, depending on the Co content, magnitude and orientation of the ferroelectric polarization can be continuously controlled and even complete reversals of the polarization as function of temperature or field are observed. The experimental results are discussed in terms of the external field tuning of the helical or conical spin structures giving rise to the multiferroic state. [Preview Abstract] |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M17.00003: Magnetic properties of multiferroic hexagonal LuFeO$_3$ thin film Wenbin Wang, Xiaoshan Xu, Jun Zhao, Zheng Gai, Wei Tian, Jian Shen We present magnetic properties of multiferroic hexagonal LuFeO$_{3}$ single crystalline thin films grown on Al$_{2}$O$_{3}$(0001) substrates using~ pulsed laser deposition(PLD) technique. Neutron diffraction and superconducting quantum interference device (SQUID) measurements suggest that the hexagonal LuFeO$_{3}$ thin film displays an antiferromagnetic order above room temperature and a second magnetic phase transition at lower temperature. The possible magnetic structures of this system are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 9:12AM |
M17.00004: Origin of ferroelectricity and exotic magnetism in frustrated LiCuVO$_4$ Invited Speaker: Martin Mourigal The spin-1/2 Heisenberg chain with competing ferromagnetic nearest-neighbor ($J_1$) and antiferromagnetic next-nearest neighbor ($J_2$) interactions is probably one the simplest, yet richest model in frustrated magnetism. It is experimentally realized in a diversity of Mott insulators, in particular in copper-oxide materials built-up from edge-sharing CuO$_6$ octahedra. The quasi-1D compound LiCuVO$_4$ stands out for the diverse emergent magnetic and multiferroic phenomena it displays, its simple crystal structure and its availability as high-quality single crystals. I will review recent elastic neutron scattering works [1,2] on LiCuVO$_4$ which elucidate the nature of its ground-state as a function of applied electric field and magnetic field up to 14~T. Below 3.5~T [1], a model long-range ordered ferroelectric spin-cycloid is unveiled, its chirality fully controlled by an applied electric field, and the corresponding magnetoelectric coupling in excellent agreement with the predictions of a purely electronic mechanism based on spin currents. Above 8~T [2], a transition to a new quantum state is observed. This new phase resembles the longitudinal density-wave of magnon-pairs ($p$=2 SDW) predicted in the purely 1D case but is characterized by the intriguing absence of long-ranged dipolar correlations.\\[4pt] [1] M. Mourigal {\it et al.}, PRB {\bf 83}, 100409R (2011).\\[0pt] [2] M. Mourigal {\it et al.}, PRL {\bf 109}, 027203 (2012) [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M17.00005: Magnetism and Ferroelectricity in the frustrated spin chain compound Ca$_{3}$CoMnO$_{6}$ Eundeok Mun, J. Kim, M. Jaime, N. Harrison, V. Zapf, Y. Kamiya, C. Batista, H. Yi, Y. Oh, S. Cheong In many multiferroics, there is little or no net magnetism coupled to electric polarization. Ca$_{3}$CoMnO$_{6}$ is unusual among multiferroics since it has a net, hysteretic magnetization coupled to electric polarization, which is important for many applications. Thus, understanding the origin of the magnetic behavior and its coupling to the electric polarization is important. Up to now the arrangement of magnetic exchange interactions, the size of the Co spin, and the origin of magnetic hysteresis were not completely understood. We show magnetization, magnetostriction, electric polarization, and magnetocaloric effect data up to 100 T, including notably a 1/2 and a 2/3 plateau in the magnetization and non-monotonic magnetostriction behavior. We determine that the spin state of Co is definitely \textbf{S} $=$ 3/2 at both high fields and low fields. We show that this behavior is consistent with an ANNNI-like model with antiferromagnetic interactions in the hexagonal \textbf{ab-}plane, and ferromagnetic interactions along \textbf{c-}axis. The model takes into account Ising-like Co spins and Heisenberg-like anisotropic Mn spins. The evolution of the Ising-like Co spins accounts for the hysteresis and steps in the physical properties up to 20 T, and also produces a positive magnetostriction, whereas alignment of the Heisenberg-like Mn spins produce non-hysteretic behavior up to saturation at $\sim$85 T, as well as negative magnetostriction. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M17.00006: Magnetic field switching of ferroelectricity in spiral magnet CuCrO$_{2}$ E.-D. Mun, V. Zapf, A. Podlesnyak, G. Ehlers, R. Fishman, S. Shiryaev, S. Barilo, M. Frontzek The triangular lattice antiferromagnet CuCrO$_{2}$ show ferroelectricity induced by a proper-screw spiral magnetic structure, where spins in form 120$^{\circ}$ angles with neighboring spins due to frustration. CuCrO$_{2}$ is thought to be a rare example of the Arima mechanism for multiferroic behavior. In addition, it has been shown that the magnetoelectric coupling can be tuned by both an electric and a magnetic field along \textbf{ab-}plane. We test a prediction for the magnetic field-evolution of the physical properties of CuCrO$_{2}$ via magnetization and electric polarization measurements up to 65 T. We explore the complicated $H-T$ phase diagram along different crystalline directions. In zero field, a spontaneous electric polarization in CuCrO$_{2}$ is coupled to antiferromagnetic ordering below 24 K without an accompanying structural phase transition. In high fields, we observe electric polarization flops for magnetic fields applied along both the \textbf{ab-}plane and the \textbf{c}-axis, although at different magnetic fields than predicted. By contrast no noticeable anomaly is detected in magnetization isotherms, which are linear in fields up to 65 T. The electric polarization reversal is highly sensitive to the external magnetic field for both the \textbf{ab}-plane and \textbf{c}-axis due to a 3-dimensional proper-screw structure. We find that additional interactions may be necessary to explain our observed results. [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M17.00007: The Effect of Electric Field on Multiferroic Ba$_{0.5}$Sr$_{1.5}$Zn$_2$(Fe$_{0.92}$Al$_{0.08}$)$_{12}$O$_{22}$ Investigated by NMR Sangil Kwon, Soonchil Lee, Yi Sheng Chai, Sae Hwan Chun, Kee Hoon Kim, Byeongki Kang, Changsoo Kim, Euna Jo Multiferroic helimagnet Ba$_{0.5}$Sr$_{1.5}$Zn$_2$(Fe$_{0.92}$Al$_{0.08}$)$_{12}$O$_{22}$ (Al-BSZFO) shows extremely high magnetoelectric susceptibility so that the critical field for switching electric polarization is less than 1 mT below 90 K [1]. Recently, a large macroscopic magnetization was successfully induced by the electric field ($\pm$2 $\mu_B$/f.u. by $\pm$2 MV/m) in properly annealed Al-BSZFO [2]. To reveal the microscopic origin, a study on the magnetic domain structure is needed. In the magnetic material, NMR intensity is enhanced by the coupling between the electron magnetic susceptibility and the nuclear magnetic susceptibility. Hence if we trace out the amount of NMR intensity enhancement, we would get the information of the magnetic domain configuration. By measuring both the magnetic field and the electric field dependence of NMR intensity enhancement, we found the area of the magnetic domains is actually tuned by the electric field. [1] S. H. Chun et al., Phys. Rev. Lett. 104, 037204 (2010). [2] K. H. Kim, The 19th International Conference on Magnetism (2012); Y. S. Chai et al., unpublished. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M17.00008: Gigantic ferroelectric polarization and magnetoelectric coupling in a ferrimagnetic oxide CaBaCo$_{4}$O$_{7}$ John Mitchell, Vincent Caignert, Antoine Maignan, Kiran Singh, Charles Simon, Bernard Raveau, Valerie Pralong, Laurent Chapon From both fundamental and applications points of view, improper ferroelectrics that exhibiting a strong coupling between polarization and magnetic structure are challenging the scientific community. Several multiferroics belonging to that category have been reported; however, they exhibit rather small values of electric polarization combined with low magnetic ordering temperatures. Only the CuO (tenorite), the ordered perovskites LBaCuFeO$_{5}$ and the Z-type hexaferrites display magnetic ordering temperatures near room temperature, but they all suffer from polarization much smaller than that of proper ferroelectrics. Here, we report a ferrimagnetic cobaltite, CaBaCo$_{4}$O$_{7}$, crystallizing in a polar space group, which enters an improper ferroelectric phase below T$_{\mathrm{C}}=$ 64 K. Single crystals of CaBaCo$_{4}$O$_{7}$ demonstrate the highest polarization value reported among improper ferroelectrics to date, reaching 10 mC/m$^{2}$ at T$_{\mathrm{C}}$ and approaching 16 mC/m$^{2}$ at 8 K. Moreover a large magnetoelectric coupling coefficient is also evidenced near T$_{\mathrm{C}}$. This result points to routes for exploring new multiferroics among ferrimagnetic phases. [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M17.00009: Spin Wave Excitations in the Multiferroic Ba$_2$CoGe$_2$O$_7$ Toomas Room, Karlo Penc, Judit Romhanyi, Urmas Nagel, Agnes Antal, Titus Feher, Andras Janossy, Hans Engelkamp, H. Murakawa, Yoshi Tokura, David Szaller, Sandor Bordacs, Istvan Kezsmarki Ba$_2$CoGe$_2$O$_7$ is a multiferroic material where spin waves exhibit giant directional dichroism and natural optical activity at THz frequencies due to the large ac magnetoelectric effect [S. Bordacs et al., Nature Physics {\bf 8}, 734 (2012)]. We studied spin excitations in the magnetically ordered phase of the noncentrosymmetric Ba$_2$CoGe$_2$O$_7$ in high magnetic fields up to 33 T [Penc et al., Phys. Rev. Lett. {\bf 108}, 257203 (2012)]. In the ESR and THz absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. A multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode to become electric dipole active. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M17.00010: Muller matrix ellipsometry of dynamic magnetoelectric effects in multiferroics T.N. Stanislavchuk, R. Basistyy, T.D. Kang, M. Kotelyanskii, G.L. Carr, S-W. Cheong, A.A. Sirenko Far-IR spectra of magneto-electric (ME) and multiferroic materials are in the focus of modern experimental and theoretical studies. Bi-anisotropic optical properties of these materials require consideration of not only dielectric susceptibility tensor $\hat{{\varepsilon }}(\omega )$ but also magnetic permeability $\hat{{\mu }}(\omega )$ and ME $\hat{{\alpha }}(\omega)$ tensors that cannot be distinguished from a single transmission or reflection spectrum. We report on the application of Mueller matrix spectroscopic ellipsometry (MM-SE) for studies of elementary excitations in multiferroic materials such as TbMnO$_{3}$, TbMn$_{2}$O$_{5}$, and TbFe$_{3}$(BO$_{3})_{4}$ single crystals. We show that magnetic, electric, and ME dipole excitations, such as magnons, phonons, and electromagnons can be distinguished from each other using a single MM measurement without introducing any modeling arguments. The fit of MM spectra based on the Berreman's $4\times 4$ propagation matrix formalism allowed us to determine parameters of electromagnon excitations separating the electric $\hat{{\varepsilon }}(\omega )$ and ME $\hat{{\alpha }}(\omega )$ tensors components. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M17.00011: Reinvestigation of the linear magnetoelectric effect in Cr$_{2}$O$_{3}$ single crystals Ayato Iyama, Tsuyoshi Kimura Cr$_{2}$O$_{3}$ is not only the first experimentally confirmed magnetoelectric compound but also a rare example compound in which the magnetoelectric effect occurs at room temperature. It is worthwhile to revisit this compound from the standpoint of recently developed ``multiferroic'' where electric and magnetic orders coexist. Thus, we grew single crystals of Cr$_{2}$O$_{3}$ and measured their magnetodielectric and magnetoelectric effects. We found that the temperature dependence of the dielectric constant measured in a magnetic field shows a sharp peak around Neel temperature 307 K. Furthermore, we observed that the electric polarization induced by a magnetic field is reversed by sweeping an electric field at room temperature. In this talk, we present our experimental results on electric and magnetic properties in Cr$_{2}$O$_{3}$, and discuss the origins from current point of view. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M17.00012: Temperature-dependent electrical and electro-optical properties of LuFe$_{2}$O$_{4}$ thin films Ram Rai, B.S. Franks, B. Cai, M.L. Nakarmi We present temperature-dependent electrical properties of LuFe$_{2}$O$_{4}$ (LFO) thin films deposited on (001) sapphire substrates. The Hall-effect measurements of LFO thin films showed the p-type conductivity at temperatures above 440 K, which is the 2D charge-ordered (CO) state of LFO. In the 3D CO stated below 340 K, we observed complex electrical properties of LFO thin films: dc voltage-current measurements displayed a hysteresis behavior and transient response of voltage-under-current pulses showed a nonlinear voltage-current relationship. We also present the electro-optical effects of LFO in the photon energy range of 0.5 - 6 eV. At 170 K, LFO thin films show the electro-optical effects of size up to 8{\%} near Fe$^{2+}$ d to d on-site electronic transitions. The electrical and electro-optical properties of LFO thin films could be associated with the changes of the ferroelectric polarization in applied electric fields through the interplay of the spin, charge, and lattice degrees of freedom in the multiferroic state of LFO. We will discuss the measured data in the view of the Maxwell-Wagner effects at the contacts, and demonstrate that LFO does show the ferroelectric state below 330 K. [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M17.00013: Crystal field splitting and optical band gap of hexagonal LuFeO$_3$ films Xiaoshan Xu, Wenbin Wang, Hongwei Wang, Xiaoying Xu, Leyi Zhu, Lixin He, Elizabeth Wills, Xuemei Cheng, David Keavney, Jian Shen, Xifan Wu In order to study the electronic structures, we have characterized the hexagonal LuFeO$_3$ films (grown by pulsed laser deposition) using x-ray absorption and optical spectroscopy. The crystal splitting of Fe$^{3+}$ is extracted as $E_{e'}-E_{e''}$=0.7 eV and $E_{a_1'}-E_{e'}$=0.9 eV and a 2.0 eV optical band gap is determined assuming a direct gap. First-principles calculations confirm the experiments that the relative energies of crystal field splitting states do follow $E_{a_1'}>E_{e'}>E_{e''}$ with slightly underestimated values and a band gap of 1.35 eV. [Preview Abstract] |
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