Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L30: Focus Session: Multiferroics I |
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Sponsoring Units: DMP GMAG Chair: Craig Fennie, Cornell University Room: 334 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L30.00001: Ferroelectric magnets: a Conical Spiral and an Ising Chain Invited Speaker: Multiferroics, where magnetic order with broken inversion symmetry accompanies the occurrence of ferroelectric polarization, can show remarkable tunability of dielectric properties by applied magnetic fields, such as reversibly flipping of ferroelectric polarization or a drastic change of dielectric constant with fields. It turns out that the origin of ferroelectricity in most of new multiferroics is spiral magnetism that tends to produce uniform lattice distortions, i.e., ferroelectric distortions, through antisymmetric exchange coupling. Among such materials, spinel CoCr$_{2}$O$_{4}$, exhibiting a conical-spiral spin order, is unique in that it exhibits spontaneous magnetization as well as electric polarization. We have studied the detailed switching behavior of magnetoelectric domains, characterized by polarization, magnetization, and magnetic wave vector, under variation of temperature and applied magnetic fields. New aspects that we have discovered from this study include [1] polarization reversal at the magnetic lock-in transition temperature (T$_{L})$ with thermal variation or with isothermal variation of magnetic fields (without changing their direction) at a temperature near T$_{L}$, [2] surprisingly, this polarization reversal happens without change in spin rotation direction in spiral spins. We also present newly-discovered multiferroicity in a ``collinear'' chain magnet of Ca$_{3}$(Co,Mn)$_{2}$O$_{6}$. In the Ising chain magnet, ``symmetric'' exchange coupling seems responsible for the multiferroicity. We show that in Ca$_{3}$Co$_{2-x}$Mn$_{x}$O$_{6}$ (x$\approx $1), the ferroelectricity originates from the coexistence of the alternating order of magnetic ions with different charges and the collinear up-up-down-down spin order in the frustrated spin chain. The system also exhibits magnetic freezing transitions that affect the temperature dependence of the electric polarization magnitude. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L30.00002: Magnetic-filed-induced polarization in the square-lattice antiferromagnetic Ba$_{2}$CoGe$_{2}$O$_{7}$ Hee Taek Yi, Young Jai Choi, Seongsu Lee, Sang-Wook Cheong We have discovered the appearance of ferroelectricity below the Neel temperature of 6.7 K in the square-lattice antiferromagnetic Ba$_{2}$CoGe$_{2}$O$_{7}$ single crystal, grown by using a floating zone technique. The ferroelectric polarization aligns along the tetragonal $c$ axis, but is very small in magnitude. However, the magnitude of polarization increases remarkably and the polarization direction smoothly rotates away from the $c$ axis with increasing magnetic fields along the $c$ axis. This change of polarization and the associated change of dielectric constant with fields are monotonic without going through any phase transition. [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L30.00003: Electric polarization reversal under high magnetic field in square lattice antiferromagnet Ba$_{2}$CoGe$_{2}$O$_{7}$ Jae Wook Kim, S.H. Chun, S.H. Kim, Kee Hoon Kim, Y. Jo, L. Balicas, Y.J. Choi, S.-W. Cheong, F. Balakirev, N. Harrison Recently, Ba$_{2}$CoGe$_{2}$O$_{7}$ was found to develop electric polarization ($P)$ below $T_{N}$=6.7 K [1]. Interestingly, $P$ along the $a$-axis increases linearly, crossing zero at $H$=0 when magnetic field ($H)$ is applied along the $c$-axis. To investigate the linear $H$-dependence of $P$ further, we measured $P$ dielectric constant (\textit{$\varepsilon $}), and magnetization ($M)$ under high $H$ up to 45 T. On application of high $H$, $P$ increases linearly up to $H\sim $15 T but suddenly decreases to a constant negative value. A peak in \textit{$\varepsilon $} is found at the $P$-reversal point which is suppressed with increasing $H$ to lower temperature with a concomitant sharpening up to $H\sim $36 T at $T$=0.6 K Furthermore, $M(H)$ curves below $T_{N}$ show saturation above the $P$-reversal magnetic field, indicating that the negative $P$ state is due to the fully ordered spin configuration. This phenomenon is similar to the case of multiferroic BiMn$_{2}$O$_{5}$, in which $P$reversal is driven by a spin-flop crossover [2]. However, in Ba$_{2}$CoGe$_{2}$O$_{7}$, $P$-reversal does not accompany a $H$ induced magnetic phase transition. We discuss possible mechanisms for this unique magnetoelectric behavior and suggest possible quantum phase transition behavior. [1] H. Yi \textit{et al.}, Appl. Phys. Lett. 91, 212904 (2008). [2] Jae Wook Kim \textit{et al.}, arXiv:0810.1907. [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L30.00004: Magnetic Frustration and Magnetoelectric Coupling in CoSe$_2$O$_5$ Brent Melot, Ram Seshadri, Ambesh Dixit, Gavin Lawes, Emmanuelle Suard We present structural and magnetic measurements on CoSe$_2$O$_5$, a compound with one dimensional chains of irregular edge-shared CoO$_6$ octahedra, separated by Se$_2$O$_5$ units. Below 8.5\,K low-field magnetic susceptibility and heat capacity measurements show long range antiferromagnetic order develops. The magnetic structure of this ordered state has been determined by neutron diffraction to consist of moments aligned antiparallel along the length of the chain as well as antiparallel to neighboring chains. The magnetic ordering becomes more complex when the compound is cooled under strong fields, with highly non-linear $M-H$ behavior below the ordering temperature. Measurement of the dielectric constant and pyrocurrent has also shown exposure to fields larger than 3\,T results in a spontaneous electric polarization of the order of 1.5\,$\mu$C m$^{-2}$ which develops below the magnetic transition temperature. [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L30.00005: Ferroelectricity from magnetic order Invited Speaker: Magnetic insulators with competing exchange interactions can give rise to strong fluctuations and qualitatively new ground states. The proximity of such systems to quantum critical points can lead to strong cross-coupling between magnetic long-range order and the chemical lattice. Case in point is a new class of multiferroic materials in which the magnetic and ferroelectric order parameters are directly coupled, and a magnetic field can suppress or switch the electric polarization [1]. Our neutron measurements reveal that ferroelectricity is induced by magnetic order and emerges only if the magnetic structure creates a polar axis [2-5]. Our experiments prove that the onset of ferroelectricity is described by a magneto-electric Landau theory that seems to apply for a wide range of multiferroic materials [6]. The spin dynamics and the field-temperature phase diagram of the ordered phases provide evidence that competing ground states are essential for ferroelectricity. The magneto-electric coupling, however, arises from relatively small interactions that are currently under intense investigation. \\[4pt] [1] T. Kimura et al, Nature 426, 6962 (2003).\\[0pt] [2] G. Lawes et al, Phys. Rev. Lett. 95, 087205 (2005).\\[0pt] [3] M. Kenzelmann et al, Phys. Rev. Lett. 95, 087206 (2005).\\[0pt] [4] M. Kenzelmann et al, Phys. Rev. B 74, 014429 (2006).\\[0pt] [5] M. Kenzelmann et al, Phys. Rev. Lett. 98, 267205 (2007).\\[0pt] [6] A.B. Harris, Phys. Rev. B 76, 054447 (2007). [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L30.00006: Q-domains in Multiferroic CoCr$_2$O$_4$ Thomas A. Kaplan In spinel CoCr$_2$O$_4$, the observed spin state at low temperature is, approximately, a ``ferrimagnetic spiral"\footnote{N. Menyuk et al., J. de Physique \textbf{25}, 528 (1964)}, given by\footnote {D. H. Lyons et al., Phys. Rev. \textbf{126},540 (1962)} $\mathbf{S}_{n\nu} =\sin\theta_\nu[\hat{x}\cos(\mathbf{Q} \cdot\mathbf{R}_{n\nu}+\gamma_\nu)+ \hat{y}\sin(\mathbf{Q} \cdot\mathbf{R}_{n\nu}+\gamma_\nu)] +\cos\theta_\nu\hat{z}$ . $\nu=1\cdots6$ goes over the six magnetic fcc sublattices, $\mathbf{R}_{n\nu}$ are the positions of the magnetic ions, $\hat{z}$ = [001] crystallographic direction, $\theta_\nu$ are the cone half-angles on which the spins lie, and $\gamma_\nu$ are the phases of the 6 conical spirals, all with wave vector $\mathbf{Q}$ in the [110] direction. This yields magnetization $\mathbf{M},^1$ and, via the Katsura et al mechanism\footnote{H. Katsura et al., Phys. Rev. Lett. \textbf {95}, 057205 (2005)}, electric polarization~$\mathbf{P} $.\footnote{Y. Yamasaki et al., Phys. Rev. Lett. \textbf{96}, 207204 (2006)} Equivalent $\mathbf{Q}$'s, e.g. $\pm\mathbf{Q} $, with associated $\mathbf {M}$'s and $\mathbf{P}$'s, are expected to give degenerate states, ``$\mathbf{Q-M-P}$ domains"; poling in electric and magnetic fields selects a single such domain. Reversal of magnetic field then leads to $\mathbf{P}$ reversal$^{4,}$\footnote{Y. J. Choi et al, submitted for publication} and $\mathbf{Q}$ reversal$^5$. But $\mathbf{Q}\rightarrow -\mathbf{Q}$ in the equation above does not appear to give a degenerate state. I show, via the Heisenberg model and the Generalized Luttinger-Tisza method used in the prediction of the spin state,$^2$ that $\gamma_\nu\rightarrow- \gamma_\nu$ on $\mathbf{Q}$ reversal, making manifest the $\mathbf{Q} \rightarrow-\mathbf {Q}$ degeneracy. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L30.00007: Ginzburg-Landau theory for the conical cycloid state in multiferroics: Applications to CoCr$_{2}$O$_{4}$ Chuanwei Zhang, Sumanta Tewari, John Toner, Sankar Das Sarma We show that the cycloidal magnetic order of a multiferroic can arise in the absence of spin and lattice anisotropies, e.g., in a cubic material, and this explains the occurrence of such a state in CoCr$_{2}$O$_{4}$. We discuss the case when this order coexists with ferromagnetism in a so-called ``conical cycloid'' state and show that a direct transition to this state from the ferromagnet is necessarily first order. On quite general grounds, the reversal of the direction of the uniform magnetization in this state can lead to the reversal of the electric polarization as well without the need to invoke ``toroidal moment'' as the order parameter. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L30.00008: Spin-driven ferroelectricity in triangular lattice antiferromagnets $A$CrO$_2$ ($A$ = Cu, Ag, Li, or Na) Shinichiro Seki, Yoshinori Onose, Yoshinori Tokura The correlation between the dielectric and magnetic properties is investigated on the triangular-lattice antiferromagnets $A$CrO$_2$ ($A=$ Cu, Ag, Li, or Na) with the 120-degree spiral spin structure. For the $A=$ Cu and Ag compounds with delafossite structure, the ferroelectric polarization emerges with the spiral spin order, implying the strong coupling between the ferroelectricity and the spiral spin structure. For the $A=$ Li and Na compounds with ordered rock salt structure, on the other hand, no spontaneous polarization is discerned, while the clear anomaly in dielectric constant can be observed upon the transition to the spiral-spin ordered state. This feature can be ascribed to the possible antiferroelectric state induced by the alternate stacking of the Cr-spin sheet with opposite vector spin chirality. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L30.00009: Magnetoelectric coupling in a triangular lattice antiferromagnet CuCrO$_{2}$ Kenta Kimura, Hiroyuki Nakamura, Kenya Ohgushi, Tsuyoshi Kimura A triangular lattice antiferromagnet CuCrO$_{2}$ shows an out- of-plane 120\r{ } spin structure. Recently, ferroelectricity in the 120\r{ } phase has been reported for polycrystalline samples. However, no anisotropic information (e.g. direction of polarization) has been provided for lack of single-crystal measurement [1]. Therefore, we grew single crystals of CuCrO$_{2}$ and investigated their magnetic and ferroelectric properties [2]. We found that CuCrO$_{2}$ undergoes two successive magnetic phase transitions ($T_{N2} \quad \approx $ 24.2 K and $T_{N1} \quad \approx $ 23.6 K), probably into a collinear antiferromagnetic phase and then the out-of-plane 120$^{\circ}$ phase. Ferroelectric polarization within the triangular lattice plane appears only in the 120$^{\circ}$ phase. In the talk, we also discuss the magnetoelectric properties. [1] S. Seki et al., \textit{Phys. Rev. Lett.} \textbf{101}, 067204 (2008). [2] K. Kimura et al., \textit{Phys. Rev. B }\textbf{78}, 140401(R) (2008). [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L30.00010: Synthesis and properties of PbTi$_{1-x}$Ni$_{x}$O$_{3}$. Larry Buroker, Somaditya Sen, Marija Gajdardziska-Josifovska, Ying Zou, Shishir Ray, Mark Wiliamsen, Prasenjit Guptasarma Magnetoelectrics are a class of multiferroic materials with magnetic and ferroelectric properties in the same phase. These have been a subject of intense investigation due to their fascinating physical properties, and the potential for new devices. We examine here the question of whether the successful substitution of a magnetic ion into a traditional ferroelectric lattice can result in a new magnetoelectric phase. Using a sol gel technique employing metal-ion chelate complexes, we have synthesized phase pure nanoparticulate samples of PbTi$_{1-x}$Ni$_{x}$O$_{3}$ for 0$<$x$<$0.3. We report our studies of crystal structure refinement, magnetic and dielectric properties 0.3$<$T$<$300 Kelvin, microstructure studies using High Resolution TEM, optical properties and vibrational spectroscopy in this new system. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L30.00011: Impurity effects in multiferroic componds Trinanjan Datta We investigate the effect of impurities in multiferroic systems. Using an equation of motion approach for the spin dynamics of the host multiferroic compound we find that the amplitude of the spin components of the material are affected by the impurities. We model the impurities as a two-level system and focus on the regime where the impurity spins relax slowly. When the impurity strength is weak the host spins oscillate with no decay and the electric polarization survives. However as the impurity strength is increased the host spin components get damped. This in turn causes the ferroelectricity to be destroyed. Since polarization in multiferroic materials is driven by magnetic ordering we conclude that the presence of impurities is detrimental to multiferroicity. [Preview Abstract] |
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