Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D6: Focus Session: Emergent Properties in Bulk Complex Oxides: Orthochromates and Other Oxides |
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Sponsoring Units: GMAG DMP Chair: Brian Kirby, National Institute of Standards and Technology Room: 108 |
Monday, March 3, 2014 2:30PM - 3:06PM |
D6.00001: Breathing Pyrochlore Lattice Realized in the A-Site Ordered Spinel Oxides LiGaCr$_{4}$O$_{8}$ and LiInCr$_{4}$O$_{8}$ Invited Speaker: Yoshihiko Okamoto A Cr spinel oxide ACr$_{2}$O$_{4}$ with a nonmagnetic A$^{2+}$ ion at the tetrahedral site provides an interesting playground for studying magnetic frustration in a pyrochlore lattice made of Cr$^{3+}$ ions with an $S =$ 3/2 spin. We found a novel type of frustrated lattice called ``breathing'' pyrochlore lattice, which is made of Cr$^{3+}$ ions in two A-site ordered spinel oxides, LiGaCr$_{4}$O$_{8}$ and LiInCr$_{4}$O$_{8}$ [1]. Because of the large size mismatch between Li$^{+}$ and Ga$^{3+}$/In$^{3+}$ ions, they alternately occupy the tetrahedral sites so as to form a Zinc Blende lattice. This transforms the conventional pyrochlore lattice into an alternating array of small and large tetrahedra, while keeping their shapes regular. LiGaCr$_{4}$O$_{8}$, with a lesser degree of alternation, shows similar magnetic properties to the conventional Cr spinel oxides such as ZnCr$_{2}$O$_{4}$. In contrast, LiInCr$_{4}$O$_{8}$ shows a spin-gap behavior in its magnetic susceptibility caused by a large alternation of magnetic interaction in the more breathing pyrochlore lattice. This suggests that LiInCr$_{4}$O$_{8}$ exists in a proximity to an exotic singlet ground state based on a tetramer singlet formed in the smaller tetrahedron, although it finally goes to a magnetically ordered state below 13 K, which may be triggered by a structural transition. We will also present NMR and neutron scattering measurements carried out to elucidate the nature of these compounds, and our recent results on solid solutions between the two compounds.\\[4pt]The work has been done in collaboration with T. Nakazono, Z. Hiroi, Y. Tanaka, M. Yoshida, M. Takigawa, T. Masuda (ISSP, Univ. of Tokyo), G. J. Nilsen, H. Mutka, T. Hansen (ILL), and J. P. Attfield (Univ. of Edinburgh). \\[4pt] [1] Y. Okamoto, G. J. Nilsen, J. P. Attfield, and Z. Hiroi, Phys. Rev. Lett. 110, 097203 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D6.00002: Magnetic Behavior of Rare-Earth Substituted Orthochromites Austin McDannald, Lukasz Kuna, Menka Jain Rare-earth orthochromites (RCrO$_3$) with ABO$_3$ perovskite structure have recently attracted attention due their potential as magnetoelectric multiferroics (ME MFs). Materials with both magnetic and ferroelectric ordering are considered magnetoelectric mulitferroics. As compared to the rare earth manganite based ME MFs, such as TbMnO$_3$ (with Neel temperature ~ 40 K), the rare-earth orthochromites have Neel temperatures close to 150 K that could result in higher temperature ferroelectric transition than that of TbMnO$_3$. While the mechanism of ferroelectricity and ferroelectric transition in the orthochromites is still unclear, these materials are known to show canted antiferromagnetic ordering. However, there is little understanding of the R-Cr interaction that may result in interesting magnetic behavior. In this work, several rare-earth (such as Er, Nd, etc.) substituted at the A-site of DyCrO$_3$ have been studied. The bulk powder samples were prepared by a citrate route and the phase purity was examined by the X-ray diffraction measurements. In the Nd substituted samples trends in the coercive field and the emergence of exchange bias were observed. A better understanding of the magnetic properties of these orthochromites may lead to development of high temperature ME MFs. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D6.00003: Half Metallicity in Trigonal MnPO$_{4}$ and CrPO$_{4}$ Crystals Boris Kiefer The search for half-metallic compounds continues to be an active area even after several decades of intense research. Half-metals are prime candidates with applications as spin injection materials. Yet, the corresponding material set remains comparatively limited. Here we report a new structural template with Mn and Cr in tetrahedral oxygen coordination. The tetrahedral MnO$_{4}$ and CrO$_{4}$ groups share corners with intermittent PO$_{4}$ groups to form a 3d bond topology. All present computations are based on spin-polarized DFT computations at the GGA-PBE level using all-electron like PAW interaction potentials. The preliminary results show a spin-gap in the minority spin channel for both compounds with magnetic moments of 3 $\mu_{\mathrm{B}}$/fu and 4 $\mu _{\mathrm{B}}$/fu for the Cr and Mn compound, respectively. Furthermore, in both compounds the half-metallic state is energetically more favorable as compared to the competing antiferromagnetic state. Therefore, these compounds which are isomorphic to the previously synthesized Fe analog may provide a new structural class of half-metallic compounds. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D6.00004: Direct Observation of Localized Spin Antiferromagnetic Transition in PdCrO$_2$ by Angle-Resolved Photoemission Spectroscopy Han-Jin Noh, Jinwon Jeong, Bin Chang, Dahee Jeong, Hyun Sook Moon, En-Jin Cho, Jong Mok Ok, Jun Sung Kim, Kyoo Kim, B.I. Min, Han-Koo Lee, Jae-Young Kim, Byeong-Gyu Park, Hyeong-Do Kim, Seongsu Lee We report the first case of the successful measurements of a localized spin antiferromagnetic transition in delafossite-type PdCrO$_2$ by angle-resolved photoemission spectroscopy (ARPES). This demonstrates how to circumvent the shortcomings of ARPES for investigation of magnetism involved with localized spins in limited size of two-dimensional crystals or multi-layer thin films that neutron scattering can hardly study due to lack of bulk compared to surface. Also, our observations give direct evidence for the spin ordering pattern of Cr$^{3+}$ ions in PdCrO$_2$ suggested by neutron diffraction and quantum oscillation measurements, and provide a strong constraint that has to be satisfied by a microscopic mechanism for the unconventional anomalous Hall effect recently reported in this system. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D6.00005: Phase diagram of the spinel system Fe$_{1+x}$Cr$_{2-x}$O$_{4}$ (0 $\leq x \leq$ 1) Andhika Kiswandhi, James Brooks, Haidong Zhou Here, we report the resistivity, specific heat, and the susceptibility of the series of polycrystalline spinel Fe$_{1+x}$Cr$_{2-x}$O$_{4}$ with 0 $\leq x \leq$ 1. The study shows that as the degree of inversion (x) increases, the magnetic transition temperature increases while the resistivity decreases in general. This demonstrates the intricate relationship between the magnetic interaction and the transport properties. Comparison to the previously studied normal vanadate spinel AV$_{2}$O$_{4}$ is also presented. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D6.00006: NMR studies of anisotropy and metal-insulator crossover in quasi-one-dimensional metal Li$_{0.9}$Mo$_{6}$O$_{17}$ Guoqing Wu, W. Gilbert Clark, Stuart Brown, John J. Neumeier, C.A.M. dos Santos, J. Marcus, C. Berthier, M. Horvatic The quasi-1D metal Li$_{0.9}$Mo$_{6}$O$_{17}$ is thought to exhibit transport properties associated with a Luttinger liquid at high temperatures, and otherwise many of its properties have long been mysterious. Among these is an unusual increase in resistivity at low temperatures, for which a robust explanation remains elusive. We present the $^{7}$Li-NMR/$^{95}$Mo-NMR measurements over a wide range of temperature and angle of alignment of applied magnetic field (B0) from 6 T to 14.8 T on single crystal of Li$_{0.9}$Mo$_{6}$O$_{17}$. We find a Korringa relation at high temperatures which indicates electron correlations are unimportant for $T > T_{m}$ (resistivity minimum temperature), and apparent deviations from Korringa for $T$ $\leq$ 24 K. Further, in the single crystal studied, inequivalent magnetic environments are detected at the Li sites in the same low-temperature regime, but only for fields applied near to $B_{0}$ $\parallel$ $c$. Only one $^{95}$Mo site (out of 6 different crystallographic sites) was detected, and 1/($T_{1}T$) at this site is decreasing at low temperatures. We discuss these observations in relation to possible mechanisms for the low temperature resistivity increase. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D6.00007: Magnetic field-induced spontaneous polarization reversal in multiferroic Mn$_{0.85}$Co$_{0.15}$WO$_{4}$ N. Poudel, K.-C Liang, Y.Q. Wang, Y.Y. Sun, B. Lorenz, F. Ye, J.A. Fernandez-Baca, C.W. Chu In this work, we report the effect of c-axis magnetic field in magnetic and ferroelectric properties of multiferroic Mn$_{1-x}$Co$_{x}$WO$_{4}$ for $x$=0.15,0.135, and 0.17. For $x$=0.15, which is the critical doping that separates ground state AF5 and AF2/4 magnetic phases, the positive b-axis polarization($P_{b}$) is reversed spontaneously at $\sim$ 7K, when the magnetic field along c-axis is $\geq20 kOe$ even for the positive poling voltage. From the polarization measurement for $x$=0.135 and 0.17, we found that $P_{b}$ originates from both AF5 and AF2/4 phases, however, c-axis magnetic field of $\geq 20 kOe$ is needed for former case which is the effect of spin flop transition. Magnetic data for $x$=0.135 clearly show the existence of spin flop transition in a c-axis magnetic field. By comparing similar data for $x$=0.15 we conclude that the spin flop also happens in the AF5 phase which coexists with AF2/4 magnetic structure. The polarization reversal is explained by a coupling of different domains preserving the chirality of the spiral spin structure. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D6.00008: Predicting Nanocheckerboards in $ZnMnGaO_4$ Spinel from First Principles Mordechai Kornbluth, Chris A. Marianetti Self-organizing nanocheckerboards raise fascinating questions and present exciting possibilities for ultrahigh-density memory devices. A family of checkerboards fabricated from Mn-based spinels consist of phase-separated Mn-rich (tetragonal) and Mn-poor (cubic) phases. We analyze the earliest example of this family, $ZnMnGaO_4$. Density functional theory (DFT) confirms that the phase separation originates in the Jahn-Teller effect present when Mn ions occupy octahedrally-coordinated sites. DFT calculations demonstrate a strongly preferred (011) interface, which generates checkerboards by geometric considerations. We further investigate both kinetic and thermodynamic limitations in nanocheckerboard composition and size. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D6.00009: Colossal magnetoelectric effect in Co$_3$TeO$_6$ family of compounds Sergey Artyukhin, Yoon Seok Oh, Jun Jie Yang, Vivien Zapf, Jae Wook Kim, Sang-Wook Cheong, David Vanderbilt Multiferroic Co$_3$TeO$_6$ and related materials attracted much attention recently due to their rich phase diagrams, magnetic field -- driven electric polarization and incommensurate spin structures. We model the interacting magnetic and ferroelectric degrees of freedom in these compounds with Landau-type theory and calculate the phase diagram. Comparison of our results with experiment reveals that a particular magnetic anisotropy in some of the compounds results in a second-order spin-flop transition, associated with a large change of polarization. In the vicinity of the transition the spin-flopped phase can be stabilized by a small external magnetic field, which gives rise to a colossal magnetoelectric effect, recently demostrated experimentally. Furthermore, we analyze the types of domain walls that can occur in these materials, and study their interactions. The clamping of domain walls of different types enables the cross-control of ferroic orderings, although they may not be coupled in the bulk. We corraborate our results with ab-initio computations of the polarization, piezoelectric response and optical properties. Our results could pave the way to the design of a new generation of magnetoelectric devices. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D6.00010: Directional dichroism of THz radiation in Sr$_2$CoSi$_2$O$_7$ Toomas R\~o\~om, U. Nagel, V. Kocsis, D. Szaller, I. K{\'e}zsm{\'a}rki, Y. Tokunaga, Y. Taguchi, Y. Tokura The microscopic mechanism of magnetoelectric coupling in akermanite-like Co-oxide multiferroics is unique because the local electric polarization mainly arises from the hybridization of Co ion and its ligands orbitals and is less affected by the details of the actual magnetic order of Co spins. As a consequence of this magnetoelectric effect, the spin waves located in the THz range exhibit giant directional dichroism in Ba$_2$CoGe$_2$O$_7$ [S. Bordacs et al., Nature Physics {\bf 8}, 734 (2012)]. Here we studied spin excitations in a sister compound Sr$_2$CoSi$_2$O$_7$ in magnetic fields up to 17 T. We found that the giant directional dichroism at THz frequencies is present below the Neel temperature (T$_N$) where the spins are ordered antiferromagnetically and persists as well above T$_N$ due to the large uniform magnetization and electric polarization induced by the external magnetic field. The relation of the observed ac magnetoelectric effect to the dc magnetoelectric effect studied by Akaki et al. [Phys. Rev. B {\bf 86}, 060413(R) (2012)] is also discussed. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D6.00011: A new bonding insight on Ba$_{2}$NaOsO$_{6}$ based on orbital-quenching-induced magnetism Shruba Gangopadhyay, Kwan-Woo Lee, Kyo-Hoon Ahn, Warren Pickett Double perovskite Ba$_{2}$NaOsO$_{6}$ (BNOO) is an exotic example of a heptavalent osmium compound, and also uncommon by being a ferromagnetic insulator. Although the single 5d t$_{\mathrm{2g}}$ electron from Os orders magnetically, there is no evidence of orbital order that would destroy its cubic symmetry. Local density approximation with Hubbard U (LDA$+$U) calculation revealed very strong Os d -- O p hybridization into weakly overlapping cluster orbitals, but was unable to obtain the observed Mott insulating behavior. The gap can obtained using DFT$+$U$+$ spin-orbit coupling (SOC) with unreasonably high value of U. Building from the basic understanding from LDA$+$U calculations, we have performed hybrid DFT studies, including SOC, implemented in Wien2k. This method obtains a narrow gap, and an orbital moment of -0.42 muB that strongly compensates the $+$0.52 muB spin moment. The effects of SOC on the spin density will be presented and discussed, as will the change in the electronic and magnetic properties under pressure. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D6.00012: Successive magnetic field-induced phase transition in a multiferroic hexagonal system up to 92 T J.W. Kim, E. Mun, M. Jaime, N. Harrison, V. Zapf, Y. Oh, J. Yang, S.-W. Cheong, S. Artyukhin, D. Vanderbilt We report the observation of successive magnetic field-induced phase transitions in a multiferroic hexagonal system up to 92 T. We find unusually strong magnetoelectric coupling at a hysteresis-free phase transition at low fields in which magnetization can be switched by electric fields and electric polarization can be switched by magnetic field. This transition is accompanied by a large magnetoelectric response that is due to the very small energy barrier between the low and high field phase. We explore this compound to high magnetic fields and observe another phase transition at $\sim$ 50 T in magnetization ($M$), electric polarization ($P$), and magnetostriction measurements. The high field transition displays a relatively small jump in $M$ but much larger change in $P$ compared to the low field one. Measurements to very high magnetic field in combination with modeling reveal the hierarchy of exchange and dipole interactions that is relevant to the successive magnetic transitions in this compound and suggests possible spin structures at each phases. Both field-induced transitions in this material shows a sharp and large jump in magnetostriction which, in combination with the non-centrosymmetric structure, allow for significant changes in the electric polarization. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D6.00013: A ferroelectric-like structural transition in a metallic 5$d$ oxide LiOsO$_{3}$ Kazunari Yamaura, Youguo Shi, Masao Arai, Kenji Tsuda, Yanfeng Guo, Andrew Princep, Andrew Boothroyd, Dmitry Khalyavin, Pascal Manuel Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ``ferroelectric'' metal, in which a ferroelectric-like transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric ($R$-3$c)$ to non-centrosymmetric ($R$3$c)$ transition in metallic LiOsO$_{3}$ that is structurally equivalent to the ferroelectric transition of LiNbO$_{3}$. The transition involves a continuous shift in the mean position of Li$^{+}$ ions on cooling below 140 K. Its discovery realizes the scenario described by Anderson and Blount, and establishes a new class of materials whose properties may differ from those of normal metals. This research was supported in part by a Grant-in-Aid for Scientific Research (22246083, 25289233) from JSPS, Japan; the Funding Program for World-Leading Innovative R{\&}D on Science and Technology (FIRST Program) from JSPS, Japan; and the United Kingdom Engineering and Physical Sciences Research Council (EPSRC). [Preview Abstract] |
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