Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session D23: Focus Session: Triangular Lattice and Spinels |
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Sponsoring Units: DMP GMAG Chair: Art Ramirez, Lucent Room: Morial Convention Center 215 |
Monday, March 10, 2008 2:30PM - 2:42PM |
D23.00001: Magnetic ordering and structural transition in layered Li$_{2}$RuO$_{3}$ Devina Pillay, Michelle Johannes Li$_{2}$RuO$_{3}$ is a layered, triangular-lattice metal oxide system much like Na$_{x}$CoO$_{2}$, NaNiO$_{2}$ and LiNiO$_{2}$ with the exception that one of every three transition metal ions (Ru) is replaced by a Li ion. This results in a honeycomb arrangement of spin-carrying ions and eliminates the magnetic frustration intrinsic to the triangular lattice. Here we investigate the electronic structure of Li$_{2}$RuO$_{3}$, especially in relation to its magnetic ordering both in-plane and between adjacent planes. We find that the dimerization of Ru atoms within the metal-oxide planes acts in conjunction with magnetic ordering to establish a gapped, magnetic ground state. The change in the energy level spectrum brought on by the formation of spin-polarized Ru-Ru molecular orbitals replaces the expected Jahn-Teller mechanism as a way of relieving a degeneracy at the Fermi energy. [Preview Abstract] |
Monday, March 10, 2008 2:42PM - 2:54PM |
D23.00002: Magnetic interactions in geometrically frustrated triangular lattice antiferromagnet CuFeO2 Jaime Fernandez-Baca, Feng Ye, Randy Fishman , H. J. Kang, J. W. Lynn, Tsuyoshi Kimura Geometrically frustrated magnetic systems have received considerable attention due to their extraordinary magnetic properties. The delafossite CuFeO2 is of particular interest because it exhibits multiferroic behavior with either the application of a magnetic field or introduction of nonmagnetic impurities. Our recent [1] on CuFeO2 shows that spin waves in this material can be explained by antiferromagnetic interactions up to third nearest neighbors within the hexagonal plane as well as out-of-plane coupling, indicating that the quasi-like Ising nature of this material results from the delicate balance between competing interactions. Two energy dips in the spin wave dispersion occur at the incommensurate wavevectors associated with multiferroic phase, and are dynamic precursors to the magnetoelectric behavior. In this talk we will present preliminary measurements of the field magnetic field dependence of these excitations as CuFeO2 approaches the multiferroic phase [1] Ye et al. \textit{ Phys. Rev. Lett}. \textbf{99}, 157201 (2007) [Preview Abstract] |
Monday, March 10, 2008 2:54PM - 3:06PM |
D23.00003: Neutron scattering investigation of Lithium based spinels. Wouter Montfrooij, Marcus Petrovic, Michael Kraus, Alexander Schmets Lithium based spinels Li$_{x}$M$_{2}$O$_{4}$, (with M a transition metal like Mn, V, Ti) offer a rich variety of ground states, depending on the transition metal in question. What makes these materials particularly attractive for both fundamental research as well as for applications is that the Li atoms can easily be extracted from the crystal without affecting the overall spinel structure. The oxidation state of the mixed valent transition metal M will change as a function of Li removal, with the result that the system can go from a disordered state to a long range ordered state. We present new neutron scattering results on a variety of Lithium spinels [Li$_{x}$Mn$_{2}$O$_{4}$, LiCoVO$_{4}$, LiCo$_{0.94}$Fe$_{0.06}$VO$_{4}$, LiNiVO$_{4}$, and LiNi$_{0.94}$Fe$_{0.06}$VO$_{4}$]. We discuss the changes in ground states as a function of Lithium content, and we review how the magnetic properties of the transition metal ions influence the electronic properties of the system. [Preview Abstract] |
Monday, March 10, 2008 3:06PM - 3:18PM |
D23.00004: Elastic properties of the vanadate spinel MnV$_{2}$O$_{4}$ V. Keppens, Y. Luan, V.O. Garlea, R. Jin, D. Mandrus Spinel vanadates AV$_{2}$O$_{4}$ are known to undergo a cubic-to-tetragonal structural phase transition (SPT) at temperature T$_{S}$ and order magnetically at lower temperature T$_{N}$. ZnV$_{2}$O$_{4}$ is characteristic of the entire series and has received extensive theoretical attention. When Mn occupies the A site there is an additional superexchange interaction between Mn and V. This superexchange interaction leads to ferrimagnetic order at about 56 K, involving a ferromagnetic configuration of the V spins. The current work focuses on the elastic properties of MnV$_{2}$O$_{4}$. Resonant Ultrasound Spectroscopy (RUS) has been used to measure the elastic response of the sample, as a function of temperature (5-300K) and magnetic field (0-7 Tesla). The temperature dependence of the frequencies is found to be quite unusual, displaying a softening over a large temperature range. Measurements in magnetic field reveal an additional feature near 50 K, which could represent a striking manifestation of direct spin-orbital coupling. [Preview Abstract] |
Monday, March 10, 2008 3:18PM - 3:30PM |
D23.00005: Neutron Scattering Study of Magnetic and Orbital Order in MnV$_{2}$O$_{4}$ Stephen Nagler, Ovidiu Garlea, Ronying Jin, David Mandrus, Doug Abernathy, Bertrand Roessli, Martha Miller, Arthur Schultz, Qingzhen Huang, Chris Frost Neutron diffraction and inelastic scattering has been used to study the spinel system MnV$_{2}$O$_{4}$. The results confirm the existence of two phase transitions. The first, near 56 K, is from a paramagnetic state to a collinear ferrimagnetic phase. There is a second simultaneous structural and magnetic transition at 53 K to a tetragonal structure with a non-collinear ferrimagnetic state. The low T magnetic structure has been definitively resolved, and is seen to be associated with a gap in the magnetic excitation spectrum. The magnetic structure is compatible with a staggered orbital ordering.Inelastic scattering has been carried out using both triple axis and time of flight techniques, and several branches of magnetic excitations are visible. These observations put tight constraints on theoretical models for MnV$_{2}$O$_{4}$. An initial account of some of the results of this work appears in arXiv:0711.1844v1. [Preview Abstract] |
Monday, March 10, 2008 3:30PM - 3:42PM |
D23.00006: Magnetic and magnetoelastic properties of Ge-substituted cobalt ferrite Naresh Ranvah, Ikenna Nlebedim, Yevgen Melikhov, John Snyder, David Jiles, Anthony Moses, Paul Williams Research in past has shown chemical substitution of Fe$^{3+}$ by trivalent ions (Cr$^{3+}$, Mn$^{3+}$ and Ga$^{3+})$ to alter the properties of these materials, notably reducing Curie temperature and magnetic anisotropy, which lead to increase in permeability. The change in properties has been understood in terms of site preference of the dopant cation. The current study investigated the temperature dependence of magnetic and magnetoelastic properties of germanium substituted cobalt ferrite, Co$_{1-x}$Ge$_{x}$Fe$_{2-2x}$O$_{4}$, over a temperature range of 10 -- 400 K. Both magnetic hysteresis loops and magnetostriction loops have been measured for several selected compositions with 0$\le $x$\le $0.8. At room temperature, saturation magnetization, magnetic anisotropy and magnetostriction were seen to decrease with increasing Ge-content. This new class of materials is therefore highly suitable for magnetic sensor and actuator applications in which the magnetic properties can be tailored to meet specific needs. [Preview Abstract] |
Monday, March 10, 2008 3:42PM - 4:18PM |
D23.00007: Charge order and frustrated magnetism in the orbitally-degenerate triangular metallic antiferromagnet AgNiO2 Invited Speaker: We explore the electronic ground state in the orbitally degenerate triangular metallic antiferromagnet AgNiO$_2$. In high-resolution neutron diffraction we observe a structural transition below 365 K to a tripled unit cell in the triangular layers with a periodic arrangement of expanded and contracted NiO$_6$ octahedra, naturally explained by a three-sublattice ($\sqrt{3} \times \sqrt{3}$) charge order pattern on the triangular lattice of Ni sites. Band-structure calculations suggest that charge order occurs in order to lift the orbital degeneracy and is favoured by the weak electron delocalization over local Jahn Teller distortions found in more insulating systems. An unusual magnetic order is observed at low temperatures with only one third of sites (the electron-rich Ni sites) carrying a magnetic moment arranged in an unexpected collinear stripe order pattern on an antiferromagnetic triangular lattice. Possible mechanisms stabilizing the observed ground state will be discussed. E. Wawrzy\'{n}ska, R. Coldea, E.M. Wheeler, I.I. Mazin, M.D. Johannes, T. S\"{o}rgel, M. Jansen, R.M. Ibberson, P.G. Radaelli, Phys. Rev. Lett. 99, 157204 (2007). We acknowledge support from EPSRC UK. [Preview Abstract] |
Monday, March 10, 2008 4:18PM - 4:30PM |
D23.00008: Electrical, Thermal, and Magnetic Properties of Single Crystal CaMn$_2$O$_4$ Marokite B.D. White, J.J. Neumeier, J.A. Souza, C. Chiorescu, J.L. Cohn CaMn$_2$O$_4$ was first described [1] in 1963 as a natural mineral called Marokite. Since its discovery, it has been studied as a minor structural impurity phase in CMR- related CaMnO$_3$ and for its structural similarities to high-pressure phases of spinel-oxide compounds. However, little attention has previously been paid to physical properties beyond its temperature-dependent magnetization. We will present a detailed physical properties study of CaMn$_2$O$_4$ single crystals grown by the optical floating zone method. [2] These measurements, several of which display anisotropy as a result of an orthorhombic crystal structure, include electrical transport, thermal transport, thermal expansion, heat capacity, and magnetization. \newline [1] C. Gaudefroy, G. Jouravsky, F. Permingeat, Bull. Soc. Fran\c c. Min\'er. Crist. {\bf86} (1963) 359. [2] B. D. White, C. A. M. dos Santos, J. A. Souza, K. J. McClellan, J. J. Neumeier submitted to J. Cryst. Growth. [Preview Abstract] |
Monday, March 10, 2008 4:30PM - 4:42PM |
D23.00009: X-ray diffraction and reciprocal space mapping in ZnMnGaO$_{4}$ films with checkerboard nanostructures A.A. Sirenko, S.M. O'Malley, P.L. Bonanno, A. Kazimirov, S. Park, S.-W. Cheong Reciprocal space maps (RSM) in ZnMnGaO$_{4}$ films with checkerboard nanostructures were measured with the energy of the x-ray beam of 10.53 keV at the A2 beamline at Cornell High Energy Synchrotron Source (CHESS) using a four-circle diffractometer. Structural properties of the checkerboards, such as elastic strain, relaxation effects, twists, and tilts of the nanodomains, were analyzed using H-K, H-L, and K-L cross sections of the RSM's measured around various symmetric and asymmetric reflections (022), (004), (044), (226), (222) of the spinel structure. Work at Rutgers was supported by the DE-FG02-07ER46382 and the NSF-DMR- 0706326. Work at NJIT was supported by the NSF-DMR-0546985. The Cornell High Energy Synchrotron Source is supported by the NSF and the NIH/NIGMS under Award No. DMR-0225180. [Preview Abstract] |
Monday, March 10, 2008 4:42PM - 4:54PM |
D23.00010: Magnetic coupling in CoCr$_2$O$_4$ and MnCr$_2$O$_4$: an LSDA+$U$ study Claude Ederer, Matej Komelj Competing magnetic interactions can lead to very interesting magnetic structures and pronounced effects due to spin-lattice coupling. Chromium spinels with additional magnetic $A$-site cations are prime examples for such systems, and have recently regained attention due to the discovery of a small electric polarization in CoCr$_2$O$_4$ [1]. The multiferroic character of CoCr$_2$O$_4$ is supposedly a result of its inversion symmetry-breaking magnetic structure, which has been classified as a ``ferrimagnetic spiral'' [2]. To achieve a better understanding of the complicated magnetic structure in CoCr$_2$O$_4$ and similar systems, an {\it ab initio} determination of the magnetic coupling constants is very desirable. Here, we present results of LSDA+$U$ calculations of the magnetic coupling constants in both CoCr$_2$O$_4$ and MnCr$_2$O$_4$ [3]. We carefully assess the predictive power of such calculations, and then give quantitative estimates for the strengths of the most prominent magnetic interactions. Our results highlight the possible importance of $AA$ interactions in spinel systems with magnetic ions on both $A$ and $B$ sites, and provide an important link between previous theory and experimental observations. \par \noindent [1] Y. Yamasaki {\it et al.}, Phys. Rev. Lett. 96, 207204 (2006). [2] D. H. Lyons {\it et al.}, Phys. Rev. 126, 540 (1962). [3] C. Ederer and M. Komelj, Phys. Rev. B 76, 064409 (2007). [Preview Abstract] |
Monday, March 10, 2008 4:54PM - 5:06PM |
D23.00011: Crystal and Magnetic Structure of FeV$_{2}$O$_{4}$ with e$_{g}$ and t$_{2g}$ orbital degeneracies Sungdae Ji, J.-H. Kim, S.-H. Lee, Q. Huang, T. Katsufuji We have performed neutron diffraction measurements on a powder and a single crystal of a spinel FeV$_{2}$O$_{4}$ that has e$_ {g}$ as well as t$_{2g}$ orbital degeneracy. Our data show that upon cooling this system undergoes three successive phase transitions: a cubic-to-tetragonal structural transition at 140 K, a tetragonal-to-orthorhombic transition at 110 K accompanied by a ferri-magnetic order and an orthorhombic-to-orthorhombic phase transition with another magnetic order at 80 K. The magnetic structures of the two magnetic phases were refined by the group theoretical analysis of our powder diffraction data. We will also discuss implication of the magnetic structures regarding to the orbital states of FeV$_{2}$O$_{4}$. [Preview Abstract] |
Monday, March 10, 2008 5:06PM - 5:18PM |
D23.00012: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 5:18PM - 5:30PM |
D23.00013: The Dzyaloshinskii-Moriya Anisotropy in Hyper-Kagome Lattice for $\mbox{Na}_4 \mbox{Ir}_3 \mbox{O}_8 $ Gang Chen, Leon Balents The effect of quantum correction on the classical spin order in the iridium antiferromagnet $\mbox{Na}_4 \mbox{Ir}_3 \mbox{O}_8 $ are considered. The spin-wave spectrum is calculated to the linear order. The nearest-neighbor exchange interactions between the $\mbox{Ir}$spins as well as the Dzyaloshinskii-Moriya interactions are taken into account. Numerical simulation by exact diagonalization and experimental results are used to compare with the theoretical prediction. [Preview Abstract] |
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