Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W20: Focus Session: Multiferroics IV |
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Sponsoring Units: DMP GMAG Chair: Sang-Wook Cheong, Rutgers University Room: Baltimore Convention Center 317 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W20.00001: Electronic Mechanism for the Coexistence of Ferroelectricity and Ferromagnetism James Gubernatis, Cristian Batista, Wei-Guo Yin We study the strong coupling limit of a two-band Hubbard Hamiltonian that also includes an inter-orbital on-site repulsive interaction $U_{ab}$. When the two bands have opposite parity and are quarter filled, we prove that the ground state is simultaneously ferromagnetic and ferroelectric for infinite intra-orbital Coulomb interactions $U_{aa}$ and $U_ {bb}$. We also show that this coexistence leads to a singular magnetoelectric effect. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W20.00002: ``Lorentz force" acting on a ray of light in multiferroics Kei Sawada, Naoto Nagaosa We theoretically propose that optical analogue of a Lorentz force acting on a ray of light is realized in multiferroic materials showing an optical magneto-electric effect. The toroidal moment ${\vec T} = \sum_j {\vec r}_j \times {\vec S}_j$ plays a role of ``vector potential" while its rotation corresponds to a ``magnetic field" for photons. Hence the light is subject to the Lorentz force when propagating through the domain wall region of the ferromagnetic or ferroelectric order. Realistic estimate on the magnitude of this effect is given. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W20.00003: Enhanced optical magnetoelectric effect in a patterned polar ferrimagnet N. Kida, Y. Kaneko, J.P. He, M. Matsubara, H. Sato, T. Arima, H. Akoh, Y. Tokura A simple method to dramatically enhance the optical magnetoelectric (ME) effect, i.e., nonreciprocal directional birefringence, is proposed and demonstrated for a polar ferrimagnet GaFeO$_3$ as a typical example. We patterned a simple grating with a pitch of 4 $\mu$m on a surface of GaFeO$_3 $ crystal and used the diffracted light as a probe. Optical ME modulation signal for Bragg spot of the order $n=1$ becomes gigantic in the photon energy 1--4 eV and reaches 1--2\% of the bare diffracted light intensity in a magnetic field of 500 Oe. This is amplified by more than three orders of magnitude compared to that for the reflection of bulk GaFeO$_3$. Fabricating a photonic crystal will make it possible to lead a new route for the practical use of the optical ME effect. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:42PM |
W20.00004: Magnetoelectric effects in multiferroics Invited Speaker: Magneto-electric phenomena were investigated in two different multiferroic systems: The strong coupling of dielectric and magnetic properties and the simultaneous occurrence of long-range magnetic and ferroelectric order are discussed for rare earth manganites and sulfo spinels. A phase diagram of Eu$_{1-x}$Y$_{x}$MnO$_{3}$ is established, which recovers the main features of the well-known magneto-electric phase diagram for the pure rare earth manganites RMnO$_{3}$. Here a variety of magnetic and electric phases emerge with varying rare earth ions R. As function of temperature and external magnetic field, also Y doped EuMnO$_{3 }$compounds undergo a sequence of different magnetic and polar phase transitions for varying effective ionic radii of the rare earth ions. Special attention is paid to the occurrence of fundamentally new hybrid spin-electromagnetic excitations, which we name electromagnons and are characterized as spin waves that can be excited by an ac electric field. These excitations are identified in Eu$_{1-x}$Y$_{x}$MnO$_{3 }$with x = 0.2, in GdMnO$_{3}$, and in TbMnO$_{3}$. Specifically in GdMnO$_{3}$ the electromagnons can easily be suppressed by external magnetic fields and allow tuning the index of refraction by moderate fields. In the second part we discuss the simultaneous appearance of colossal magneto-resistance (CMR) and colossal magneto-capacitance (CMC) effects in chromium sulfo spinels. In CdCr$_{2}$S$_{4}$ ferromagnetism of localized Cr spins evolves at 85 K, while polar order is established below 130 K. The onset of ferroelectric order is neither accompanied by the occurrence of soft modes nor by structural changes which break the inversion symmetry of the high-temperature cubic phase. HgCr$_{2}$S$_{4}$ becomes ferroelectric close to 70 K while a complex antiferromagnetic order is found below 25 K. CMR and CMC effects are specifically strong in the mercury compound, as moderate magnetic fields of only 0.1 T induce ferromagnetism at much higher temperatures. We speculate that the occurrence of ferroelectricity in these multiferroic compounds is rather of electronic than of ionic origin. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W20.00005: A neutron scattering study on a ferrimagnetic magnetocapacitance system Mn$_{3}$O$_{4}$ Jae-Ho Chung, Jung Hwa Kim, Seung-Hun Lee, Takuro Katsufuji The low-temperature phase of the tetragonal Mn$_{3}$O$_{4}$ has long been known as a ferrimagnet with the Yafet-Kittel structure. The long-range ferrimagnetic order first develops at 41.2 K, upon cooling, and a commensurate cell-doubling magnetic order occurs along the $b$-axis at 32.7 K. Recently magnetocapacitance behaviors were observed in Mn$_{3}$O$_{4}$. We present our high-resolution neutron scattering data to show that Mn$_{3}$O$_{4}$ undergoes an additional lattice distortion around 25 K. Peak broadening of selected reflections suggests that the crystal structure becomes pseudo-orthorhombic at low temperatures. Relation between the lattice distortion and magnetism will also be discussed. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W20.00006: Magnetodielectric consequences of phase separation in the colossal magnetoresistance manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ R.S. Freitas, J.F. Mitchell, P. Schiffer We have studied the low-frequency dielectric properties of the phase-separated manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ as a function of applied magnetic field in the low temperature phase-separated state. The dielectric constant is strongly field dependent and also depends on the magnetic field history of the sample. The dielectric behavior appears to be associated with the hopping of polaronic charge carriers, and we can derive the field dependent hopping energy barrier from the frequency dependence of the dielectric constant. This analysis allows us to associate the metal-insulator transition observed in this material with the field-induced suppression of the polaron activation energy. Reference: \textit{Phys. Rev. B} \textbf{72}, 144429 (2005). Research was supported by the NSF and DOE. [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W20.00007: Dielectric anomalies in CoCr$_2$O$_4$ G. Lawes, B. Melot, K. Page, C. Ederer, M.A. Hayward, Th. Proffen, R. Seshadri We investigate the magnetic,dielectric, and thermodynamic properties of CoCr$_2$O$_4$ polycrystalline samples. AC susceptibility and specific heat measurements show the existence of two distinct magnetic transitions in this material. Neutron scattering experiments confirm a ferrimagnetic ordering transition at T$_c$=95 K and a transition to a spiral magnetic phase below T$_N$~25 K. We observe a significant dielectric anomaly coincident with the onset to long-range spiral magnetic order, and a separate feature with significant thermal hysteresis above T=50 K. We associate this higher temperature dielectric anomaly with short-range spiral magnetic order, and discuss these results in the context of utilizing magnetodielectric couplings to capacitively probe short range magnetic structures. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W20.00008: Mossbauer Spectroscopy Investigation of Substituted Cobalt Ferrites (CoM$_{x}$Fe$_{2-x}$O$_{4}$, where M = Mn or Cr, and x = 0.0-0.8) John Snyder, Chester Lo, Yevgen Melikhov, Paul Matlage, Kelly Krieble In order to enable applications of substituted cobalt ferrites for strain sensing, magnetostrictive actuating, and ``multiferroic'' composites, more basic knowledge is needed concerning how cation substitution affects the atomic level environments, distributions, and interactions of the cations. In this study, the local environments of the Fe atoms in two series of substituted cobalt ferrites (CoM$_{x}$Fe$_{2-x}$O$_{4}$, where M = Mn or Cr, and x = 0.0-0.8) have been investigated using Mossbauer spectroscopy. Results of both series show two distinct six-line hyperfine patterns, indicating Fe in A (tetrahedral) and B (octahedral) sites. They can be identified by isomer shift and hyperfine distribution width. Both series show some similar behavior: with increasing substitution, magnetic hyperfine field decreases and hyperfine field distribution width increases for both A and B sites. B-site hyperfine fields and distribution widths are more affected than A. All of these effects are more pronounced for Cr-substitution than for Mn. Results are consistent with a model of Mn or Cr ions substituting into B-sites and displacing Co ions onto A sites. It would appear that Cr has an even stronger B-site preference than Mn, and displaces more of the Co to the A sites. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W20.00009: Preparation and properties of single phase PbTi$_{1-x}$Mn$_{x}$O$_{3}$ perovskites at high Mn concentrations. Stanislav Stoupin, Carlo Segre, Soma Chattopadhyay Recent observation of multiferroic properties in the PbTi$_{0.5}$Fe$_{0.5}$O$_{3}$ perovskite material raises questions about the electronic and structural driving forces causing the coupling between ferroelectric and ferromagnetic properties. It is known that the Jahn-Teller distortion of oxygen octahedra due to d orbital occupancy inhibits formation of the ferroelectric double well potential in ABO$_{3}$ perovskites. Thus, the presence of a ferroelectric distortion in d$^{n}$ magnetic transition metal perovskite oxide is an unexpected result. We report observation of tetragonal structure (XRD) in a similar PbTi$_{1-x}$Mn$_{x}$O$_{3}$ system. The material is prepared using the sol-gel method with various Mn concentrations. In this study, we access a range of concentrations starting from a relatively high value of x=0.1 in order to introduce considerable amount of magnetic sites into the system. Reduction in the tetragonal ratio (c/a) is observed with increase in x as expected. Further characterization of the material involves magnetoelectric measurements and X-ray Absorption Spectroscopy. Preliminary results are discussed. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W20.00010: Colossal magnetocapacitance and scale-invariant dielectric response in mixed phase manganites Ryan Rairigh, Amlan Biswas, Arthur Hebard We are studying thin-film capacitors utilizing (La$_{0.5}$Pr$_{0.5})_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LPCMO) as the base electrode, AlOx as the dielectric and Al as the counter-electrode. The LPCMO films exhibit \textit{colossal magnetoresistance} (CMR). Likewise, the capacitance changes by three orders of magnitude in the region of the resistance drop. These \textit{colossal magnetocapacitance} (CMC) effects are related to magnetic field induced changes in the relative extent of coexisting ferromagnetic metal and charge ordered insulating phases. The widths of the hysteresis loops, in capacitance and resistance, are about the same, but the center of the capacitance loop is shifted 20~K below the center of the resistance loop. When the LPCMO resistance is at a maximum (low capacitance) the electrode comprises filamentary conductors threading an insulating medium. In this region log-log Cole-Cole plots reveal an intrinsic dielectric response in which the data plotted as a function of frequency ($\omega )$ collapse onto single straight lines, implying scale-invariance over a wide range of $\omega $, magnetic field and temperature. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W20.00011: Epitaxial thin films of novel multiferroic double perovskites. A. Venimadhav, Qi Li Recently multiferroic materials have attracted great interest. However, relatively a few pure multiferroic compounds are currently known. Here we show the exploration of design of multiferroic properties in double perovskites by combining the ferrolectrisity driven by the Bi lone pairs and selectively choosing the 3d transition metals following Goodenough-Kanamori's rules to bring in ferromagnetism. We present growth issues in stabilizing the single phase, epitaxial thin films of new double perovskite multiferroic systems such as Bi$_{2}$NiMnO$_{6}$, Bi$_{2}$FeCrO$_{6 }$and La$_{2}$NiMnO$_{6 }$by pulsed laser deposition. Targets of these compositions were synthesized by solid state method with 15{\%} of excess Bi in the composition to compensate the volatility of Bi during the deposition. We also present the synthesis of Bi$_{2}$FeCrO$_{6 }$by growing a superlattice structures from individual targets of Bi FeO$_{3}$ and BiCrO$_{3}$. In the cubic double perovskites, cations show rock salt kind of ordering in the (111) direction and hence growing these films on STO (111) substrates has an advantage. We present the growth, structural and multiferroic properties in these double perovskite thin films.. [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W20.00012: First-principles exploration of multiferroic oxides with double-perovskite structure Tamio Oguchi, Tatsuya Shishidou, Yoshitaka Uratani Multiferroics have attracted much attention recently because of their novel properties. There are a few known as ferromagnetic and ferroelectric materials, particularly with perovskite-type crystal structure. Ferroelectrics should be insulating and likely ionic. Furthermore, it is widely recognized that covalent bonds between the cation and anion orbitals are crucial to realize atomic displacements to a noncentrosymmetric structure. As for magnetism, most of magnetic perovskite oxides usually have an antiferromagnetic order (mostly frustrating) due to a superexchange coupling. According to the Kanamori-Goodenough rule for the superexchange coupling, certain combinations of the transition-metals ions ($d^3$-$d^5$ and $d^3$-$d^8$ configurations) may possibly give a ferromagnetic coupling by the 180$^{\circ}$ superexchange mechanism. In this study, we explore possible co-existence of spontaneous electric polarization and ferromagnetic ordering from first principles, by focusing bismuth double-perovskite oxides Bi$_2$$BB'$O$_6$ ($B, B'$ = 3$d$ ions) as target materials. Ferromagnetic and ferrimagnetic solutions are obtained for cubic Bi$_2$MnNiO$_6$, Bi$_2$CrFeO$_6$ and Bi$_2$CrCuO$_6$ with nearly gapped electronic structure. Quite recently, Bi$_2$MnNiO$_6$ has been successfully synthesized by a high-pressure technique and revealed multiferroic properties. Possible multiferroic properties of Bi$_2$MnNiO$_6$ with the observed monoclinic structure are investigated in detail. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W20.00013: Magnetoelectric gyrator Dwight Viehland, Junyi Zhai, Jiefang Li, M.I. Bichurin As well-known [1], an ideal gyrator would be an unusual device with respect to other network elements. It would have the unique properties of (i) anti-reciprocity, and (ii) being capable of acting like an impedance inverter. Here, for the first time, we report the design and study of such an ideal gyrator. Our ideal gyrator consists of a trilayer composite of Terfenol-D/PZT/Terfenol-D operated in a L-L mode at its electromechanical resonance (f$\approx $80kHz). Measurements have shown that magnetoelectric (ME) susceptibility of our composite is comparable with its permeability and permittivity, and that the gyration coefficient achieves a value 0.9. In addition, we have observed a 180o phase-shift between an input current and an output voltage, or vice versa, and proved that our ME laminate behaves as an impedance inverter. We believe that our gyrator may enable resolutions to numerous important and complex network problems. [1] B.D.H. Tellegen, Phillips Research Reports 3, 81 (1948). The work was supported by grants from the Office of Naval Research. [Preview Abstract] |
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