Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A13: Focus Session: Multiferroics I |
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Sponsoring Units: DMP GMAG Chair: SangWook Cheong, Rutgers University Room: Colorado Convention Center Korbel 4C |
Monday, March 5, 2007 8:00AM - 8:12AM |
A13.00001: Magnetic Symmetry of Two-Dimensional Multiferroics Avadh Saxena, Turab Lookman Hexagonal rare earth manganites are multiferroic materials which exhibit triangular antiferromagnetic ordering in the basal plane which can be characterized by two-dimensional (2D) magnetic symmetry. Reduced dimensionality is also desirable for achieving large (usually nonlinear) magnetoelectric coupling at higher temperatures. Indeed, the magnetization in BaMnF$_4$ orders two dimensionally below the transition temperature with a change in the b-axis dielectric constant. Moreover, there can be phase transitions between different 2D magnetic phases. From this perspective we study two dimensional magnetic (or color) symmetry, enumerate 2D magnetic space groups and illustrate their role in multiferroic phase transitions. [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A13.00002: Rotation of orbital stripes and the consequent charge-polarized state in Pr(Sr$_{,}$Ca)$_{2}$Mn$_{2}$O$_{7}$ Yusuke Tokunaga, Thomas Lottermoser, Yunsang Lee, Reiji Kumai, Masaya Uchida, Takahisa Arima, Yoshinori Tokura Nano-scale self-organization of electrons is ubiquitously observed in correlated-electron systems such as complex oxides of transition metals. The phenomenon of charge ordering (CO) or the formation of charge stripes, as observed for layered-structure cuprates and nickelates, is one such example. Among them, the CO in the manganites is closely tied to the orbital degree of freedom of $3d$ electrons, leading to the staggered orbital ordering (OO) or the formation of orbital stripes in the layered structure. Here, we present the phenomena of thermally-induced rotation of the orbital stripes by 90 degrees for bilayered manganite Pr(Sr$_{1-x}$Ca$_{x})_{2}$Mn$_{2}$O$_{7}$(x=0.9) with half hole-doping, i.e., a 1:1 ratio of Mn$^{3+}$/Mn$^{4+}$ [1]. The rotation of orbital stripes and the consequent CO coupled with the underlying lattice distortion were found to produce the charge-polarized state, as also evidenced by its activity of optical second harmonic generation. [1] Y. Tokunaga \textit{et al}., Nature Materials, doi:10.1038/nmat1773 (2006). [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A13.00003: Structural changes related to dielectric anomalies in RFe$_{2}$O$_{4}$ (R=Lu and Y). Y. Horibe, S. Mori, Y. Matsuo, S. Shinohara, N. Ikeda, S-W. Cheong RFe$_{2}$O$_{4}$ (R=Lu and Y) have a characteristic rhombohedral structure ith the space group R-3m, in which the hexagonal double-layers of Fe-O ions are sandwiched by Lu-O layers. In addition, the average valence of Fe ions is Fe$^{2.5+}$, which implies that Fe$^{2+}$ and Fe$^{3+}$ ions occupy the equivalent site on the hexagonal layers with equal density. Recently, a regular arrangement of Fe$^{2+}$ and Fe$^{3+}$ in the hexagonal plane (charge ordering) is suggested on the basis of the anomalous dielectric behavior in YFe$_{2}$O$_{4.}$ Thus, we investigated structural change due to the charge ordering in RFe$_{2}$O$_{4}$ (R=Lu and Y) mainly by transmission electron microscopy. We found characteristic superlattice reflections at (1/3 1/3 1/2)-type positions at room temperature in YFe$_{2}$O$_{4}$. It is suggested that the diffuse streaks are due to the charge ordering in the three-dimensional hexagonal plane. We examined structural change by obtaining the electron diffraction (ED) patterns in the warming process and found that successive structural phase transition takes place around 220K. It is considered that these transitions should be characterized as the change of the charge ordering pattern in the hexagonal plane and are strongly correlated to the anomalous dielectric properties found in YFe$_{2}$O$_{4}$. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A13.00004: X-ray Magnetic Circular Dichroism Investigation of Fe Valence Ordering in Multiferroic LuFe$_2$O$_4$ Vemuru Krishnamurthy, Jonathan Lang, Daniel Haskel, George Srajer, Brian Sales, Mas Subramanian, David Singh, Lee Robertson, Manuel Angst, David Mandrus A new mechanism of ferroelctricity that is based on the iron valence ordering in a charge frustrated lattice has been reported for LuFe$_2$O$_4$. In this compound, a ferroelectric transition occurs at 330 K and ferrimagnetic order develops below 250 K. The ferroelectric polarization shows a sharp increase at the ferrimagnetic ordering temperature suggesting that the two order parameters are coupled. X-ray magnetic circular dichroism (XMCD) at the Fe K edge and at Lu L$_{2,3}$ edges has been measured in LuFe$_2$O$_4$ using 4-ID-D beamline at Advanced Photon Source. Two clear peaks are seen in the Fe K-edge XMCD suggesting that the magnetism of Fe is associated with two types of Fe sites. Fe K edge XMCD probes the 4p shell, thus it is sensitive to different charge states and gives an indirect measure of the Fe magnetism through 3d-4p hybridization. The observed double peak structure in the XMCD is an indication of charge ordering of Fe$^{2+}$ and Fe$^{3+}$ in the ferrimagnetic state. XMCD is also observed at Lu L$_{2,3} $ edges suggesting a small induced Lu 5d moment. Funded by US Dept. of Energy. [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A13.00005: Spin-Charge-Orbital States and Electric Polarization in Multiferroic RFe$_2$O$_4$ Sumio Ishihara, Makoto Naka, Jyoji Nasu, Aya Nagano Layered iron oxides RFe$_2$O$_4$ (R: rare-earth ion) is recognized to be an electronic ferroelectric and multiferroic compounds. Crystal structure of this compound consists of stacked FeO triangle layers. Charge and spin states have been studied by the electron and neutron diffraction experiments. Long range charge and spin orders characterized by the momentum (1/3, 1/3) appear around 320K and 250K, respectively, in LuFe$_2$O$_4$. Electric polarization is induced around the charge ordering temperature of Fe$^{2+}$ and Fe$^{3+}$, and is enhanced around the magnetic ordering temperature. We examine theoretically spin-charge-orbital structures and electric polarization in RFe$_2$O$_4$. We suggest that Fe$^{2+}$ ion has the doubly degenerate orbital degree of freedom. Effective Hamiltonian for spin, charge and orbital degrees of freedom is derived. Numerical analyses with the multi-canonical Monte-Carlo simulation and the mean-field approximation show that the electric polarization is attributed to the charge order with momentum (1/3, 1/3). A magnitude of the polarization is enhanced around the magnetic ordering temperature due to the coupling between spin and charge. Conventional orbital order is not expected from the numerical calculation, and possible orbital states at low temperatures are discussed. [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A13.00006: Charge ordering and ferroelectricity in magnetite Daniel Khomskii Magnetite Fe3O4 is one of the most fascinating material in solid state physics. Besides being the first magnetic material known to the mankind, it is also the first example of an insulator-metal transition in transition metal oxides -- the famous Verwey transition [1]. One usually connects this transition with the charge ordering of Fe$^{2+}$ and Fe$^{3+}$. However the detailed pattern of CO in Fe$_{3}$O$_{4 }$is still a matter of debate. Another aspect, which is not so widely known and which did not yet receive sufficient attention, is that below T$_{V}$, besides being completely spin polarised, magnetite apparently is also \textit{ferroelectric} (FE) [2]. Thus it seems that magnetite, besides being the first magnetic material and the first transition metal oxide with an insulator-metal transition, is also the first \textit{multiferroic} material. Using the idea of a coexistence of site-centred and bond-centred charge ordering [3], I suggest a novel type of ordering in magnetite which explains the observed FE in Fe$_{3}$O$_{4}$ and which agrees with the structural data. \newline [1] Verwey E.J.W., Nature \textbf{144}, 327 (1939) \newline [2] Rado G.T. and Ferrari J.M., Phys.Rev.B \textbf{12}, 5166 (1975); Kato K. and Iida S., J.Phys.Soc.Japan \textbf{50}, 2844 (1981) \newline [3] Efremov D.V., van den Brink J. and Khomskii D.I., Nature Mater. \textbf{3}, 853 (2004) [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A13.00007: Lattice and Magnetic Effects on Multiferroic Transitions in Garnets Despina Louca, K. Kamazawa, T. Proffen The possible presence of ferroelectricity in a magnetically ordered state has attracted considerable attention particularly in ABO$_{3}$ and AB$_{2}$O$_{5}$ systems with B = Mn. Evidence for strong coupling of the two order parameters has been provided in the so-called multiferroics, where the field-induced polarization leads to a giant magnetoelectric effect and a magneto-dielectric effect. It was recently shown that the ferrimagnetic garnet crystal of Tb$_{3}$Fe$_{5}$O$_{12}$ exhibits a large magnetodielectric response as well when a very small magnetic field is applied (1). To understand the origin of the high sensitivity of the dielectric effect in garnets, we investigated the crystal and magnetic structures of Tb$_{3}$(Fe/Ga)$_{5}$O$_{12}$ using pulsed neutron diffraction. The garnet crystal appears to be very close to a lattice instability and high-resolution diffraction showed that the lattice gradually changes symmetry from cubic to rhombohedral with cooling over a wide temperature range. At the same time, magnetic diffuse scattering is observed that goes away by 15 K. The role of the lattice and of local distortions in the magnetic polarization and the coupling of the magnetostriction to the dielectric effect will be discussed. (1) N. Hur \textit{et al, }Appl. Phys. Lett. \textbf{87}, 042901 (2005). [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 10:00AM |
A13.00008: Magnetoelectric coupling in multiferroic materials from first principles Invited Speaker: The combination of magnetic and ferroelectric properties in a single material is very appealing both because of the interesting coupling effects that emerge as well as due to a variety of technological applications that can be envisaged. Computational methods based on density functional theory have made invaluable contributions to the present understanding of such magnetoelectric multiferroics. In this talk I will show how we use these methods to understand the intriguing properties of presently known multiferroics and to design new multiferroic materials with more desirable properties. In particular, I will focus on the coupling between structural distortions and so-called ``weak'' magnetic order that is mediated by the Dzyaloshinskii-Moriya interaction, and I will discuss the possibility of electric-field induced magnetization switching in prototypical multiferroic systems such as BiFeO$_3$ and BaNiF$_4$. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A13.00009: First-principles Investigation of Ground State Magnetic Structure and Effective Exchange Interaction in GaFeO$_3$ Myung Joon Han, Taisuke Ozaki, Jaejun Yu Among many multiferroic materials, GaFeO$_3$ has attracted much attention due to its large magnetoelectric effect and the unique magneto-electric, magneto-optic, and piezoelectric properties. We report our first-principles calculations on the electronic and magnetic structures of multiferroic GaFeO$_3$. Based on the LDA+U density-functional theory by employing a linear-combination-of-localized-pseudo-atomic orbitals (LCPAO) method, GaFeO$_3$ in its ideal structure is shown to be antiferromagnetic. Through calculations of effective exchange interactions among Fe atoms at either Ga or Fe sites in GaFeO$_3 $, it is concluded that net magnetic moments observed in experiments may arise from the Fe substitution at the Ga sites. Total energy calculations show that the site disorder among Fe and Ga sites is quite feasible, which is consistent with experiments. Unquenched orbital magnetic moment of Fe is found to exist possibly due to the broken inversion symmetry at the Fe site with the distortion of surrounding oxygens. The calculated orbital magnetic moments are discussed in comparison with the results of recent X-ray magnetic circular dichroism measurement. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A13.00010: Systematic investigation of rare-earth doped BiFeO$_{3}$ thin films using composition spreads S. Fujino(1), V. Nagarajan(2), M. Murakami(1), S.-H. Lim(1), A. Vartharajan(2), L. Salamanca-Riba(1), M. Wuttig(1), I. Takeuchi(1)(3) (1) Department of Materials Science and Engineering, University of Maryland, USA (2) University of New South Wales, AU (3) Center for Superconductivity Research, University of Maryland, We have systematically investigated compositionally varied rare-earth (RE) doped BiFeO$_{3}$ thin films using the combinatorial approach. Epitaxially grown (Bi$_{1-x}$RE$_{x})$FeO$_{3}$ composition spread thin films were fabricated by laser molecular beam epitaxy on SrTiO$_{3}$ (001) substrates with an SrRuO$_{3}$ buffer layer. Transmission electron microscopy of the films showed that homogeneous epitaxial films were obtained throughput the composition range. Structural properties of (Bi$_{1-x}$RE$_{x})$FeO$_{3}$ was mapped using scanning x-ray diffraction, and structural transitions were observed at various compositions. In some compositions, substantial enhancement in ferroelectric properties was observed at the structural transition: increase in the dielectric constant, increase in the piezoelectric response, and decrease in the coercive field were observed, while high polarization is maintained. Detailed dependence of various properties on composition variation will be discussed. Work supported by NSF DMR 0094265, DMR 0231291, MRSEC DMR-00-0520471 and the W. M. Keck Foundation. [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A13.00011: Exchange bias between ferromagnetic metals and multiferroic BiFeO$_{3}$, LuMnO$_{3}$, and TbMnO$_{3}$ Makoto Murakami, S. Fujino, J. Hattrick-Simpers, S.-H. Lim, L. Salamanca-Riba, D. Kundaliya, S. Ogale, T. Venkatesan, J. Higgins , M. Wuttig, I. Takeuchi, S. Lofland, S.-W. Cheong We are using exchange bias at ferromagnet layer/multiferroic interfaces to study the nature of magnetism in multiferroic materials. Co 5 nm layers have been deposited by sputtering on surfaces of epitaxial BiFeO$_{3}$ and TbMnO$_{3}$ thin films and on LuMnO$_{3}$ single crystals. Epitaxial BiFeO$_{3}$ and TbMnO$_{3}$ films were prepared by PLD. Magnetic properties of the Co/multiferroic bilayers are measured using SQUID, VSM, MOKE and XMCD. In BiFeO$_{3}$, we find that the bilayers exhibit exchange bias even at room temperature. In the TbMnO$_{3}$ system, increasing of coercive field and exchange bias was also clearly observed below the N\'eel temperature. In LuMnO$_{3}$, we observe positive exchange bias as well as switching of the sign of the exchange bias depending on the cooling procedure. This behavior may be related to the frustration in Mn spins. Difference in the exchange bias behavior between different multiferroic materials will be discussed. The effect of electric field on exchange bias is currently under investigation. Supported by ONR N000140110761, ONR N000140410085, NSF DMR 0094265, DMR 0231291, MRSEC DMR-00-0520471, and the W. M. Keck Foundation. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A13.00012: Charge ordering as alternative to Jahn-Teller distortion Igor Mazin, Daniel Khomskii It was pointed out in the seminal paper of Jahn and Teller that a partially occupied degenerate molecular level, often a doubly degenerate $E_{g}$ level in a cubic ligand field, is unstable against a distortion that splits the level and lowers the total energy of the occupied states. Since then, this effect has been commonly found in solids where it takes a form of a \textit{cooperative} Jahn-Teller (JT) effect (orbital ordering), when the crystal lattice distorts coherently so as to lift orbital degeneracy at each site or, in band language, to split an entire band ($e.g.$, $e_{g}$) and thus open a gap at the Fermi level. Upon the gradual delocalization of degenerate electrons, the JT distortion and corresponding orbital ordering becomes less and less favorable, but, as we show, below a crossover region exists with the possibility of lifting degeneracy not by an \textit{orbital ordering}, but by a \textit {charge ordering} (CO): an electron can be transferred from one ion to another, so that, say, the doubly degenerate $e_{g}$ level on one site will be fully occupied, and on the other site empty. In this paper we demonstrate, experimentally and by first principles calculations, that just such a ``JTCO'' effect actually occurs in the rare earth nickelates such as YNiO$_{3}$ and LuNiO$_{3}$. Apparently this novel phenomenon can also take place in other similar systems. [Preview Abstract] |
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