Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session D17: Focus Session: Bulk Properties of Complex Oxides - Cobaltites |
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Sponsoring Units: DMP GMAG Chair: John Mitchell, Argonne National Laboratory Room: D174 |
Monday, March 21, 2011 2:30PM - 3:06PM |
D17.00001: Possible link of a structurally driven spin flip transition and the insulator-metal transition in the perovskite La$_{1-x}$Ba$_{x}$CoO$_{3}$ Invited Speaker: The intricate nature of the magnetic ground state near the insulator-metal transition (IMT) in La$_{1-x}$Ba$_{x}$CoO3 was investigated via neutron scattering. For $x$ less than the critical concentration, x$_{c}\sim $0.22, a commensurate antiferromagnetic (AFM) phase initially appears. As $x$ approaches x$_{c}$, the AFM component continuously weakens while ferromagnetic (FM) order sets in the rhombohedral lattice. The two magnetic phases appear to be growing in different domains and have different ordering temperatures, with the FM order parameter setting in first at higher temperatures while the AFM order parameter occurs at lower temperatures. At x$_{c}$, a spin flip to a new FM state occurs while the crystal transforms to an orthorhombic (Pnma) symmetry. The magnetic Pnma phase coincides with the minimum saturation reached in the resistivity. It is proposed that the orbital overlap in the Pnma phase is the most conducive to charge hopping. [Preview Abstract] |
Monday, March 21, 2011 3:06PM - 3:18PM |
D17.00002: Phase Control of Magnetic Order in (Y,Lu)BaCo$_{4}$O$_{7}$ John Mitchell, Hong Zheng, Sevda Avci, Laurent Chapon, Dmitry Khalyavin, Omar Chmaissem, Ashfia Huq The RBaCo$_{4}$O$_{7}$ (R=Ca, Y, Tb-Lu) provides a novel topology for studying geometric frustration, in which face-sharing tetrahedra of magnetic ions link to form trigonal bipyramids on a Kagom\'{e} lattice. Here we describe the structural and magnetic behavior of the Lu member and the solid solution joining Lu to Y as a chemical means to tune between magnetically ordered and disordered ground states. Mean-field models of the generic magnetic phase diagram of RBaCo$_{4}$O$_{7}$ determined recently by our group (D. D. Khalyavin et al. Physical Review B 82, 094401 (2010)) show a variety of magnetic states as a function of two exchange parameters: J$_{ab}$ and J$_{c}$, where J$_{ab}$ links Co ions in the Kagom\'{e} planes and J$_{c}$ links Co ions from the Kagome plane to the interleaving triangular layer. Experimentally, we find that YBaCo$_{4}$O$_{7}$ has a long-range ordered antiferromagnetic ground state, while LuBaCo$_{4}$O$_{7}$ appears to be disordered above 2 K. We use the solid solution to interpolate between these endpoints and discuss these results with respect to the mean-field phase diagram. [Preview Abstract] |
Monday, March 21, 2011 3:18PM - 3:30PM |
D17.00003: Determination of magnetic moments and orbital occupancy in the spin chain compound Ca$_3$Co$_2$O$_6$ Jonathan Duffy, Matthew Butchers, Jonathan Taylor, Stephen Dugdale, Tom Haynes, Stefano Agrestini, Martin Lees The one-dimensional cobaltate Ca$_3$Co$_2$O$_6$ exhibits a number of intriguing phenomena, including several metamagnetic steps as a function of applied magnetic field. Although it has attracted a considerable amount of research, the origin of the magnetism has not yet been fully determined. We report a measurement of the spin density in Ca$_3$Co$_2$O$_6$ using magnetic Compton scattering. The bulk spin moment was determined to be 3.78 $\pm$ 0.05$\mu_B$ at 7 T, confirming the existence of a large unquenched Co orbital moment of 1.4 $\pm$ 0.1 $\mu_B$. In combination with molecular orbital calculations, the results reveal that double occupation of the d$_{x^2-y^2,xy}$ orbital is responsible for the observed large unquenched orbital moment. Fitting the model to the experimental data shows that there is an induced oxygen moment of 0.8 $\pm$ 0.1 $\mu_B$. Unexpectedly, further comparison with KKR-SPA electronic structure calculations strongly indicates the existence of a Fermi surface. [Preview Abstract] |
Monday, March 21, 2011 3:30PM - 3:42PM |
D17.00004: Theory of the carrier concentration-dependent behavior in layered cobaltates Hongtao Li, R. Torsten Clay, Sumit Mazumdar Layered cobaltates -- anhydrous Na$_x$CoO$_2$, Li$_x$CoO$_2$ and the ``misfit'' cobaltates [Bi$_2$A$_2$O$_4$] $\cdot$ [CoO$_2$]$_m$, where A = Ba, Sr or Ca -- have attracted wide attention for their 2D layered structure and metallicity (both reminescent of 2D cuprates), and the tunability of the carrier concentration over a wide range. The Co ions form a 2D triangular lattice, and their formal charge in Na$_x$CoO$_2$ and Li$_x$CoO$_2$ can be tuned from Co$^{3+}$ at $x=1$ to Co$^{4+}$ at $x=0$. Charge carriers in all cases are holes, with the carrier concentration given by the fraction of Co-ions that are in the S = 1/2 Co$^{4+}$ state. Experiments have indicated remarkable carrier concentration dependent magnetic susceptibility and thermoelectric power that remains unexplained to date. Specifically, all three systems show weakly correlated behavior at small nonzero $x$ (large carrier concentration), and strongly correlated behavior at large $x$ (small carrier concentration). In this talk we give clear theoretical explanation of the observed carrier concentration dependence within an $a_{1g}$-only one-band extended Hubbard Hamiltonian. The key to understanding the $x$-dependence is to have realistic finite on-site correlation $U$ and significant intersite Coulomb interaction $V$. We present exact numerical results for triangular lattices upto 20 sites, and make detailed comparisons to experiments. [Preview Abstract] |
Monday, March 21, 2011 3:42PM - 3:54PM |
D17.00005: Probing the Na atomic order in Na$_{x}$CoO$_{2}$, x=0.67 and 0.71 by NMR spectroscopy Ben-Li Young, P.-Y. Chu, J.Y. Juang, G.J. Shu, F.-T. Huang, M.W. Chu, F.C. Chou The sodium cobaltate Na$_{x}$CoO$_{2}$ has a layered structure, consisting of alternating triangular CoO$_{2}$ and Na planes. Evidences of Na atomic ordering have been reported at certain Na contents by different diffraction experiments. The Co magnetism, strongly influenced by the Na ordering, gives a unique phase diagram in Na$_{x}$CoO$_{2}$. In order to investigate the Na ordering and the Co magnetism, we conducted $^{23}$Na and $^{59}$Co NMR experiments in single crystals Na$_{x}$CoO$_{2}$ for x=0.67 and 0.71. We found that Na$_{0.67}$CoO$_{2}$ does not have well-defined Na structural order. However, the oxygen slightly-deficient sample Na$_{0.67}$CoO$_{1.98}$ shows a superstructure, as evidenced by the narrow and well-resolved NMR spectrum. As for Na$_{0.71}$CoO$_{2}$, Na ordering is also observed. We have tried to solve the Na ordering pattern from our NMR spectra. The results will be discussed and be compared with the existing structural models. [Preview Abstract] |
Monday, March 21, 2011 3:54PM - 4:06PM |
D17.00006: Synthesis and anisotropic magnetic and transport properties of cubic SrCoO$_{3}$ single crystal Youwen Long, Yoshio Kaneko, Shintaro Ishiwata, Yasujiro Taguchi, Yoshinori Tokura Solid state oxides containing transition metals with unusually high valence states exhibit interesting physical properties. However, due to the unstableness of these high valence states, high pressure is often needed to stabilize such high valence states. We were successful in growing a large-size SrCoO$_{3}$ single crystal by using high-pressure technique. This material shows good metallic behavior with high ferromagnetic Curie temperature about 305 K, and the easy magnetization axis is $<$111$>$ direction. The spin moment of Co$^{4+}$ ion measured at 2 K and 7 T is about 2.50 $\mu _{B}$, suggesting an intermediate spin configuration as predicted by theoretical calculations. Although SrCoO$_{3}$ has a highly symmetric cubic crystal structure (Pm-3m), it exhibits significant anisotropic magnetoresistance at low temperatures. [Preview Abstract] |
Monday, March 21, 2011 4:06PM - 4:18PM |
D17.00007: The origin of the temperature dependence of the magnetic susceptibility and the large thermoelectric power in metallic layered cobaltites Iv\'an Gonz\'alez, Camilo X. Quintela, Manuel Ba\~nobre-L\'opez, Francisco Rivadulla We perform detailed measurements of the thermoelectric power and the static magnetic susceptibility on metallic Na$_{x}$CoO${_2}$ and Ca$_{3}$Co$_{4}$O$_{9}$, as representatives of layered Co oxides with a triangular Co-lattice. We propose that the observed large thermoelectric power and the Curie-Weiss temperature dependence of the susceptibility have a common origin related to metallic character of these compounds. Thermoelectric power measurements are compared to Boltzmann transport theory calculations. The Curie-Weiss behaviour of the susceptibility is explained within the framework of the self-consistent renormalization theory for spin fluctuations proposed by Moriya for itinerant magnets. Our results clarify the apparent duality in the localised/itinerant behaviour of the electron spin in these systems and provide a unifying view on the physics of metallic layered cobaltites. [Preview Abstract] |
Monday, March 21, 2011 4:18PM - 4:30PM |
D17.00008: TEM imaging and in-situ EELS study of multiple ferroic transitions in LaCoO3 Tiantian Yuan, Robert Klie, Nina Orlovskaya The perovskite oxide LaCoO3 has attracted increasing attention due to its reported room-temperature ferroelastic behavior, and a ferromagnetic transition observed at around 90K in epitaxially strained thin films. To advance our understanding of these nanoscale properties of LaCoO3, a combination of analytical TEM techniques, including the atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy in combination with in-situ cooling experiments have been used to study the relationship of the multiple ferroic transitions in bulk LaCoO3. In particular, we find that the bulk LaCoO3 samples compressed above the coercive stress exhibit ferromagnetic transitions, similar to the ferromagnetic behavior of the epitaxially strained LaCoO3 thin film. While the bulk LaCoO3 samples compressed below the coercive stress do not exhibit any ferromagnetic transitions down to 5K. We will further correlate this ferromagnetic property to the ferroelastic property of LaCoO3, and show how the strain of the LaCoO3 affects the ferromagnetic property of the sample. [Preview Abstract] |
Monday, March 21, 2011 4:30PM - 4:42PM |
D17.00009: Magnetocaloric effect across the coupled structural/magnetocrystalline anisotropy transition in Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ (x=0.3-0.5) N.S. Bingham, M.H. Phan, H. Srikanth, M.A. Torija, C. Leighton Large magnetocaloric effects (MCE) are often observed in materials exhibiting a first order magnetic transition coupled with a crystal structure change. Since the magnetic and structural changes are coupled, it is difficult to decouple the structural entropy contribution from the magnetic entropy contribution to the total MCE. Therefore a clear understanding of the structural entropy change and its field dependence in such materials is lacking. A recent study revealed that Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ (x$>$0.35) undergo a coupled structural/magnetocrystalline anisotropy transition at T$_{A}$, in addition to the paramagnetic-ferromagnetic transition at T$_{C}$. Since the structural change at T$_{A}$ in PSCO is not associated with any magnetic transition, it is an excellent system for studying the structural entropy change and its contribution to the MCE. We report systematic studies of the MCE in Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ (x=0.3, 0.35, 0.4, 0.5) compounds. The results show significant entropy change at T$_{A}$, whose magnitude can be tuned by controlling the magnetocrystalline anisotropy. [Preview Abstract] |
Monday, March 21, 2011 4:42PM - 4:54PM |
D17.00010: Magneto-electronic Phase Separation in Pr$_{1-x}$Ca$_{x}$CoO$_{3-\delta }$: Intrinsic Exchange Spring Magnetism S. El-Khatib, S. Bose, C. He, J. Kuplic, M. Laver, J.A. Borchers , Q. Huang, J.W. Lynn, J.F. Mitchell, C. Leighton We present a neutron diffraction, small-angle scattering, and magnetometry study of the narrow bandwidth perovskite cobaltite Pr$_{1-x}$Ca$_{x}$CoO$_{3}$, demonstrating an unusual form of magneto-electronic phase separation where long-range ordered ferromagnetism coexists spatially with short-range ferromagnetism. The two phases have very different coercivities and, remarkably, are strongly exchange coupled. The electronic phase separation thus leads to spontaneous formation of a hard-soft nanocomposite, exhibiting prototypical exchange-spring behavior in the absence of chemical interfaces. [Preview Abstract] |
Monday, March 21, 2011 4:54PM - 5:06PM |
D17.00011: The nature of magneto-elastic coupling with the isovalent substitution at the B-site in LaCo$_{1-y}$B$_{y}$O$_{3}$ Juan Yu, Despina Louca The influence of magnetic ion doping on the interplay of the lattice~with magnetism in LaCo$_{1-y}$B$_{y}$O$_{3}$ (B = Ni or Fe, y= 0.1, 0.4) has been investigated via neutron scattering techniques. The substitution of~either Ni$^{3+}$ (3d$^{7})$ or Fe$^{3+}$ (3d$^{5})$ does not alter the crystal symmetry~which remains rhombohedral (R-3c) at all temperatures. With doping, the~degree of cooperative octahedral rotations about the (111) axis~increases, but it is only with Ni that such a rotation is accompanied by~a compression along the trigonal axis. The observed crystal distortion~is invoked to break the degeneracy of the magnetic Co$^{3+}$ ions, while maintaining the Co-O bonds at a constant length. The absence of two~distinct types of Co-O bond lengths in the local structure with the~substitution of Fe$^{3+}$ or Ni$^{3+}$ for Co$^{3+}$ (3d$^{6})$ is indicative that, unlike~in the hole doped cobaltites with Ba$^{2+}$ or Sr$^{2+}$ previously studied, the~intermediate spin state of Co is either absent or suppressed. This~leaves us to question the origin of the magnetic interactions, which~most likely arises from a high-spin state of the Co ion. [Preview Abstract] |
Monday, March 21, 2011 5:06PM - 5:18PM |
D17.00012: First Principles Study of Misfit-Layered Calcium Cobaltite Using Fibonacci Approximants Alejandro Rebola, Robert Klie, Serdar Ogut Cobalt oxides have been the focus of many recent studies due to the wide variety of electrical, magnetic, structural and thermoelectrical properties they exhibit. In this talk we present a first-principles study on the misfit-layered Ca$_{3}$Co$_{4}$O$_{9}$. This material can be more accurately described as [Ca$_{2}$CoO$_{3}$][CoO$_{2}$]$_{1.61}$ and consists of two substructures that are incommensurate to each other. Taking into account that the composition ratio is very close to the golden mean (1.6180...), and that this number can be obtained as the limit of the sequence of the ratios of consecutive Fibonacci numbers: 3/2, 5/3, 13/8, {\ldots}, F(n+1)/F(n)..., we model the structure by using supercells of composition [Ca$_{2}$CoO$_{3}$]$_{F(n)}$[CoO$_{2}$]$_{F(n+1) }$. In this way, structural, electronic, transport and lattice properties can be calculated as a function of cell size. We compute the atomic and electronic structures, defect energetics of a series of rational approximants to Ca$_{3}$Co$_{4}$O$_{9 }$within the framework of DFT+U, and examine the convergence of such properties with respect to size, thus allowing us to identify the most realistic and smallest structural model for this misfit-layered compound. [Preview Abstract] |
Monday, March 21, 2011 5:18PM - 5:30PM |
D17.00013: Complex ferrimagnetic state induced by zigzag oxygen-vacancy stripes in Sr$_3$YCo$_4$O$_{10.72}$ D.D. Khalyavin, L.C. Chapon, E. Suard, J.E. Parker, S.P. Thompson, A.A. Yaremchenko, V.V. Kharton The nature of high temperature ferromagnetic behaviour in Sr$_3 $YCo$_4$O$_{10+\delta}$ perovskite has been studied by neutron powder diffraction supplemented with synchrotron X-ray diffraction measurements. The present analysis of the magnetic structure takes into account the complex superstructure formed by oxygen vacancy ordering. These vacancies create zigzag strips in the oxygen-deficient CoO$_{4+\delta}$ layers providing three distinct coordinations for Co ions. The values of the ordered moments were found to be essentially different for the distinct coordinated units and clearly correlate with the coordination number. The symmetry of the superstructure in conjunction with strong antiferromagnetic interactions between neighbour spins results in a net moment whose origin has been the subject of considerable debates. [Preview Abstract] |
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