Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session W4: Recent Progress in Spin-Spiral Ferroelectricity |
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Sponsoring Units: DCMP GMAG Chair: Sang-Wook Cheong, Rutgers University Room: 306/307 |
Thursday, March 19, 2009 11:15AM - 11:51AM |
W4.00001: Magnetically driven spiral ferroelectrics with high transition temperature Invited Speaker: In the past few years, a new class of multiferroics have been discovered, wherein non-collinear spiral magnetic order induces ferroelectricity. In these multiferroics, it is not too much to say that the origin of the ferroelectricity is driven by magnetism and is completely different from that in conventional ferroelectrics. However, most of known magnetically driven spiral ferroelectrics operate only at low temperature [ferroelectric Curie temperature ($T_{C}) \quad <$ 40K]. To develop magnetically driven ferroelectrics with higher $T_{C}$, we combined studies of `high $T_{c}$ superconductivity in cuprates' and `multiferroism'. We propose that cuprates having large magnetic superexchange interactions can be good candidates for magnetically driven ferroelectrics with high $T_{C}$. In fact, we demonstrate ferroelectricity accompanied by a spiral magnetic ordering in an simple copper oxide, CuO, which is known as a starting material for the synthesis of high-$T_{c}$ cuprates. CuO shows a spiral magnetic ordering and multiferroics nature below 230K [1]. This result provides a new route to develop magnetically driven ferroelectrics with high $T_{C}$. This work is in collaboration with Y. Sekio, H. Nakamura, T. Siegrist, A. P. Ramirez, W. B Wu, and D. J. Huang. \\[4pt] [1] T. Kimura et al., Nature Mater. 7, 291 (2008). [Preview Abstract] |
Thursday, March 19, 2009 11:51AM - 12:27PM |
W4.00002: Static and dynamic magnetoelectric coupling in frustrated magnets Invited Speaker: The recently discovered multiferroic materials, where ferroelectricity is induced by spin orders breaking inversion symmetry, show strong sensitivity of electric polarization and dielectric constant to applied magnetic fields. Most of these multiferroics are frustrated magnets with incommensurate spiral spin structures, in which case a polar lattice distortion is driven by the Dzyaloshinskii-Moriya interaction between non-collinear spins. Since this interaction originates from the relatively weak spin-orbit coupling, the induced electric polarization in spiral multiferroics is small compared with that of proper ferroelectrics. Much larger polarizations were predicted for multiferroics where electric dipoles are induced by superexchange interactions between spins. This mechanism of magnetoelectric coupling works for spin structures commensurate with the crystal lattice and does not require non-collinear spins. In many frustrated magnets incommensurate spiral and commensurate collinear spin states compete. Furthermore, in materials such as orthorhombic rare earth manganites RMnO$_{3}$ and RMn$_{2}$O$_{5}$, both types of magnetic states are ferroelectric. This competition has important implications for the dynamic magnetoelectric coupling between spin waves and polar phonons resulting in mixed electromagnon excitations. I will discuss microscopic mechanisms of the single-magnon and bi-magnon excitation by an electric field of light in multiferroic and magnetoelectric materials, focusing in particular on the recently observed electromagnon peaks in orthorhombic manganites and Kagome magnets carrying monopole and toroidal magnetic moments. I will show that optical studies can provide useful information about competing multiferroic states in frustrated magnets. [Preview Abstract] |
Thursday, March 19, 2009 12:27PM - 1:03PM |
W4.00003: Multiferroic domain wall and its relevance to magnetoelectric phenomena in ferroelectric helimagnets Invited Speaker: Recently, magnetically induced ferroelectricity and the giant magnetoelectric (ME) effect in helimagnets (HMs) have attracted much attention. In the ferroelectric HMs, the ferroelectric domain walls (DWs) may be clamped with the DWs of the magnetization, the helical plane direction, and/or the wave vector $k$ of HM. In this talk, we show the role of the multiferroic domain wall motion in the giant magnetoelectric effect. We have observed the $P$ under $H$ unparallel to $k$ in a proper screw HM ZnCr$_2$Se$_4$. The origin of the $P$ can be ascribed to the rotation of the conical spin structure. In the high $H$ region, we observe the discontinuous change of the $P$ due to the $k$-flop in this material. The $k$-flop is driven by the DW of $k$. There are two types of the $k$-DW. The stability of the DWs determines the sign of the spin helicity after the $k$-flop. Another example of the ME phenomena related to DW is $P$-flop in DyMnO$_3$. In DyMnO$_3$, the magnetic field along $b$-axis induces the $P$-flop from $P||c$ to $P||a$. The dielectric constant shows a large enhancement in the course of the $P$-flop. We have investigated the dielectric dispersion of the giant magnetocapacitance (GMC) effect and found that the GMC is attributable to the motion of the DW between $bc$ plane spin cycloid ($P||c$) and $ab$ plane spin cycloid ($P||a$) domains. [Preview Abstract] |
Thursday, March 19, 2009 1:03PM - 1:39PM |
W4.00004: Electric field control of magnetism and ferroelectricity in single crystals of multiferroic BiFeO$_3$ Invited Speaker: BiFeO$_3$ is a room-temperature multiferroic combining large electric polarization (P) with long-wavelength spiral magnetic order. Significant efforts have been devoted to studies of thin- film BiFeO$_3$ model multiferroic devices, and local control of magnetization by an electric field has been demonstrated recently. However, the extant thin films consist of a poorly controlled patchwork of ferroelastic domains severely impeding experimental work. We report growth of mm-sized single crystals consisting of a single ferroelectric (FE) domain. Switching between two (out of 8) unique directions of P by an electric field is demonstrated. Magnetic moments are strongly coupled to the lattice, and rotate together with P when the field is applied. Electric field can be used to control the populations of the 3 equivalent magnetic domains with different directions of the spiral wave vector. In particular, a FE monodomain with a single-wave-vector magnetic spiral can be prepared. The spiral has the same helicity in the entire sample. All these effects are reversible. Thus, electric field can be used to control the ferroelectric and magnetic states, and even the magnetic helicity of the sample. This level of control, so far unachievable in thin films, makes single- crystal BiFeO$_3$ a promising object for investigation of physics of magnetoelectric coupling in multiferroics, as well as for model multiferroic device research. [Preview Abstract] |
Thursday, March 19, 2009 1:39PM - 2:15PM |
W4.00005: Study of Multiferroic Manganites using Double-Exchange Models Invited Speaker: The double exchange (DE) model, supplemented by lattice distortions and superexchange between the $t_{\rm 2g}$ spins, has been very successful in describing the physics of manganites, such as La$_{1-x}$Ca$_{x}$MnO$_3$, including the presence of colossal magnetoresistance in Monte Carlo simulations [1]. In this presentation, we describe the first steps toward the application of the theoretical framework previously used for CMR manganites now to the study of multiferroic manganites. An encouraging result was recently obtained when S. Dong {\it et al.} [2] showed that the addition to the DE model of a next-nearest-neighbor antiferromagnetic $t_{\rm 2g}$ coupling $J_2$ was found to produce a phase diagram that correctly predicts a transition from an A-type AF to a spiral phase and finally to an E-type AF state with increasing $J_2$, as in experiments. This result paves the way for a variety of investigations and theoretical predictions now varying both the hole doping $x$ and $J_2$. Other issues in the area of multiferroics will also be addressed in this presentation, including the prediction of ferroelectricity in the spin zigzag E-type AF state [3]. \vskip 0.2cm [1] C. \c{S}en {\it et al.}, Phys. Rev. Lett. {\bf 98}, 127202 (2007); R. Yu {\it et al.}, Phys. Rev. B {\bf 77}, 214434 (2008); and references therein. \vskip 0.1cm [2] S. Dong, R. Yu, S. Yunoki, J.-M. Liu, and E. Dagotto, Phys. Rev. B {\bf 78}, 155121 (2008). \vskip 0.1cm [3] I. A. Sergienko {\it et al.}, Phys. Rev. Lett. {\bf 97}, 227204 (2006); S. Picozzi {\it et al.}, Phys. Rev. Lett. {\bf 99}, 227201 (2007). [Preview Abstract] |
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