Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P36: Focus Session: Bulk Properties of Complex Oxides -- Cobaltites |
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Sponsoring Units: DMP GMAG Chair: Christopher Leighton, University of Minnesota Room: E146 |
Wednesday, March 17, 2010 8:00AM - 8:36AM |
P36.00001: Spin state transitions in cobaltites: spectroscopic perspective Invited Speaker: The class of cobalt-oxide based materials has attracted increasing interest in the last decade. A key aspect of the cobaltites that distinguishes them clearly from the Cu, Ni, and Mn oxides is the spin state degree of freedom of the Co3+ and Co4+ ions: the ions can be low spin, high spin, and perhaps even intermediate spin. This aspect comes on top of the orbital and charge degrees of freedom that already make the Cu, Ni, Mn systems so exciting. It is, however, also precisely this aspect that causes considerable debate in the literature. In this presentation we would like to show how synchrotron based soft-x-ray spectroscopies can successfully resolve the local electronic structure of the Co ions and thus contribute to a better understanding of the physical properties of the cobaltites. In particular, we will address the issue of spin state transitions, metal insulator transitions and the newly proposed spin-blockade phenomenon in several layered cobalt materials. --- Work done in collaboration with Z. Hu, M.W. Haverkort, C.F. Chang, H. Wu, T. Burnus, Y.Y. Chin, N. Hollmann, C. Schussler- Langeheine, M. Benomar, T. Lorenz, D.I. Khomskii (Univ. Cologne), A. Tanaka (Univ. Hiroshima), S.N. Barilo (NAS, Minsk), J. Cezar, N.B. Brookes (ESRF-Grenoble), H.H. Hsieh, H.J. Lin, C.T. Chen (NSRRC-Hsinchu). Supported by the DFG through SFB 608. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 8:48AM |
P36.00002: Dynamical behavior of spins in La$_{1-x}$Sr$_{x}$CoO$_{3}$ Robert Smith, Michael Hoch, William Moulton, Philip Kuhns, Arneil Reyes, Greg Boebinger, John Mitchell The thermally induced spin state transition of Co$^{3+}$ ions in the cobaltite LaCoO$_{3}$ , found at temperatures in the range 30 to 120 K, has been the subject of extensive experimental and theoretical investigation. Much less is known about the spin state transition in hole-doped La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO). Phase diagrams for the doped material, that are based on available experimental evidence, show low temperature (4 K) conversion of the majority of Co$^{3+}$ ions from low spin to intermediate spin states. It has been suggested that a low temperature transition is induced in LSCO for all x $>$ 0.02, by lattice relaxation effects. However, the present $^{139}$La NMR experiments have provided information on the spin state transition in hole-poor regions of the nanoscale phase separated material which shows that the thermally induced transition remains important in these regions for x approaching the metal-insulator critical concentration x$_{C}$ = 0.17. The finding should be taken into account in investigation of the magnetoelectronic properties of LSCO. [Preview Abstract] |
Wednesday, March 17, 2010 8:48AM - 9:00AM |
P36.00003: Self-assembly of an exchange-spring composite via magnetic phase separation in Pr$_{1-x}$Ca$_{x}$CoO$_{3}$ S. El-Khatib, S. Bose, C. He, J. Kuplic, Q. Huang, J.W. Lynn, J. Borchers, J.F. Mitchell, C. Leighton We report structural and magnetic properties in bulk polycrystalline Pr$_{1-x}$Ca$_{x}$CoO$_{3}$ (0.00$\le \quad x \quad \le $0.30) from neutron diffraction, thermogravimetic analysis, magnetometry, and small-angle neutron scattering (SANS). Upon cooling, the Pr$_{0.70}$Ca$_{0.30}$CoO$_{3}$ composition (deep in the FM phase) first undergoes a transition around 250 K where short-range FM clusters emerge with a size of order 1-2 unit cells. The magnetization and SANS intensity slowly increase on cooling to 70 K, where the system undergoes a transition to a long-range ordered FM state, but with low magnetization, indicative of a small FM volume fraction. Magnetometry and SANS data indicate coexistence of the short-range clusters within a network of long-range FM. The coercivities H$_{C}$ of the short-range and long-range FM regions are very different, and a non-monotonic $T$ dependence of the H$_{C}$ reveals clear evidence of FM exchange coupling between the phase-separated regions. In essence the phase separation leads to natural formation of a hard/soft composite, which displays classic exchange spring behavior. Work at UMN supported by DoE. [Preview Abstract] |
Wednesday, March 17, 2010 9:00AM - 9:12AM |
P36.00004: Coupling of the magnetic structure to crystal phase transition in La$_{1-x}$Ba$_{x}$CoO$_{3}$ Peng Tong, Juan Yu, Despina Louca, Qingzhen Huang The addition of charge carriers into the perovskite LaCoO$_{3}$ leads to a ferromagnetic (FM) state that becomes metallic via the double-exchange interaction mechanism. Neutron scattering showed that in La$_{1-x}$Ba$_{x}$CoO$_{3}$, the FM order coexists with an antiferromagnetic (AFM) order that$_{ }$is initially incommensurate and becomes commensurate when $x >$ 0.15. We investigated the magnetic structures of $x $= 0.17 - 0.22 by performing powder neutron diffraction at 10 K. For $x $= 0.17, only the AFM phase is present and can be indexed with a propagation vector of (0, -0.5, 0.5). The Co moments lie in the \textit{ab} plane of the rhombohedral ($R\bar {3}c)$ lattice in the hexagonal setting. With increasing $x$, the FM phase appears with the Co moments pointing along (001)$_{rhomb}$ and becomes dominant. At $x $= 0.22, the AFM order vanishes while the crystal structure transforms from rhombohedral to orthorhombic (Pnma) with the moments pointing along the (001)$_{ortho}$ direction. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P36.00005: Low temperature Schottky anomalies in the specific heat of LaCoO$_{3}$: Defect-stabilized finite spin-states C. He, H. Zheng, J.F. Mitchell, M.L. Foo, R.J. Cava, C. Leighton The nature of the thermally-induced spin-state transition in LaCoO$_{3}$ continues to be a matter of vigorous debate. Recent inelastic neutron spectroscopy studies revealed a thermally excited 0.6 meV excitation associated with this transition, close to earlier electron spin resonance work. We show here that measurement of the low temperature specific heat of LaCoO$_{3}$ single crystals reveals a previously unobserved Schottky anomaly with an energy level splitting, 0.5 meV, associated with the first excited spin-state of the Co$^{3+}$ ion. These states persist well below 2 K and have a $g$-factor around 3.5, consistent with the high-spin spin-orbit triplet, implying the existence of a low density (approximately 0.1 {\%} of the sites) of finite-spin Co ions \textit{even in the T = 0 limit}. We propose that these states are trapped at defects and are consistent with the magnetic excitons observed in earlier work. [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P36.00006: Low energy magnetic excitation in lightly electron doped LaCo1-yNiyO$_{3}$ Juan Yu, Daniel Phelan, Despina Louca We investigated the magnetic excitations in lightly doped LaCo$_{1-y}$Ni$_{y}$O$_{3}$ (y=0.005, 0.01) via inelastic neutron scattering measurements (INS). The substitution of Ni$^{3+}$ (3d$^{7})$ for Co$^{3+}$ (3d$^{6})$ nominally introduces an electron in the system. At 4 K, a low energy magnetic excitation is present with a characteristic energy of 1.2 meV. On warming above 30 K, the excitation smears out due to the appearance of the more intense 0.6 meV excitation previously observed in pure LaCoO$_{3}$, the latter associated with single ionic transitions within the excited state manifold. The new excitation is associated with dynamic ferromagnetic correlations as measured from INS measurements on single crystals. The effective magnetic moment is estimated to be about 12 $\mu _{B}$/Ni for the y = 0.005 sample at 10 K. The magnetic field dependence of the 1.2 meV excitation yielded an estimate for the value of g to be about 10 for the y = 0.01 sample. The large magnetic moment and g value at low temperatures may be an indication of the existence of a magnetic ground state of the Co$^{3+}$ ions where the Ni$^{3+}$ and Co$^{3+}$ ions engage in ferromagnetic coupling. [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P36.00007: Magnetic phase separation in SrCoO$_{2.5+x}$ Changkun Xie, Yuefeng Nie, Barrett Wells, Joseph Budnick, William Hines, Bogdan Dabrowski We study phase separation and inhomogeneities induced by oxygen non-stoichiometry in SrCoO$_{2.5+x}$. In previous work [A. Nemudry, et. al. Chem. Mater. 8, 2232(1996)], it has been shown that as oxygen is driven into the SrCoO$_{2.5 }$electrochemically, the material structurally separates into two different phases: one is antiferromagnetic SrCoO$_{2.5}$, and the other is ferromagnetic SrCoO$_{2.75}$. We show that two distinct ferromagnetic phases appear for SrCoO$_{2.88}$ and SrCoO$_{3}$, with T$_{c}$ = 220 K and 280 K, respectively. The phase diagram of SrCoO$_{2.5+x}$ suggests the four magnetic line phases are the only stable ground states in the system. While antiferromagnetic SrCoO$_{2.5}$ is orthorhombic, the three ferromagnetic phases are pseudo-cubic and, unlike the magnetic properties, do not show the coexistence of different structures. The existence of distinct T$_{c}$=220 K and T$_{c}$=260 K magnetic phases with no structural differentiation indicates the presence of magnetic phase separation. [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P36.00008: Early stage spin-state transition in LaCoO$_{3}$ investigated by first principles Han Hsu, Renata Wentzcovitch, Matteo Cococcioni, Chris Leighton The existence of intermediate-spin (IS) state cobalt and its role in the thermally-induced spin-state transition in LaCoO$_{3}$ has been controversial. Using the local density approximation + Hubbard $U$ (LDA+$U)$ method, we have successfully stabilized isolated single high-spin (HS) and IS excited cobalt ions~in an array of low-spin (LS) cobalt in LaCoO$_{3}$. The Hubbard $U$ is self-consistently determined from first principles. We show that at low temperatures, for isolated single ions, HS cobalt is the first excited state. Low concentrations of isolated IS cobalt produce a metallic state. This result supports the conclusions of quantum chemistry calculations on clusters, and contrasts with results from periodic calculations with 100{\%} IS and HS cobalt population.~It is also consistent with recent experiments at very low densities of excited states at low temperatures. [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P36.00009: Enhancement of the Curie temperature in NdBaCo$_{2}$O$_{5.5}$ by Ca substitution S. Kolesnik, B. Dabrowski, O. Chmaissem, K. Swierczek RBaCo$_{2}$O$_{5.5}$ (R=rare earth or Y) undergoes a sequence of magnetic and electronic transitions between antiferromagnetic/ferrimagnetic/paramagnetic and insulating/paramagnetic metallic states with respective transition temperatures T$_{N}$(230-260 K)$<$T$_{C}$(250-290 K)$<$T$_{MI}(\sim $360 K). We have synthesized a Nd$_{1-x}$Ca$_{x}$BaCo$_{2}$O$_{5.5}$ series (0$\le $x$\le $0.2) of cation- [(Nd,Ca)/Ba] and oxygen vacancy ordered materials and investigated them by neutron diffraction, magnetization, electronic and thermal transport. We observe that upon Ca doping T$_{N}$ is decreasing to 0 for x=0.1 and T$_{C}$ is increasing and coincides with T$_{MI}$ for x$>$0.12, which weakly changes with Ca substitution from $\sim $360 to $\sim $340 K. This is the largest enhancement of T$_{C}$ ever observed for these cobaltites. Unlike the hole doping by adding oxygen, the Ca doping does not disrupt the cation and oxygen vacancy orderings up to x=0.2. [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P36.00010: Atomic-scale structure-property relationship of ferroelastic LaCoO$^{3}$ Tiantian Yuan, Nina Orlovskaya, Mihaela Tanase, Stefan Kell, Robert Klie The ferroelastic oxide LaCoO$^{3}$ has attracted increasing attention by exhibiting room-temperature creep, which is usually only observed at temperatures close to a material's melting point. To advance our understanding of these unusual properties, a combination of TEM techniques, including electron diffraction, atomic-resolution Z-contrast imaging and electron energy-loss spectroscopy have been used to study the LaCoO$^{3}$ microstructures as a function of applied strain. In polycrystalline samples compressed at room temperature above the coercive strain, we observed the formation of superlattice domains with lattice constant 3a$^{0}$, which have been attributed to monoclinic distortions within the rhombohedral lattice.$_{1}$ While in untreated LaCoO$^{3}$ and samples compressed below the coercive strain we only found twin boundaries within the grain. We will further show how these superstructure domains evolve as a function of time, and correlate the transformation of the monoclinic superlattice into highly twinned rhombohedral bulk to the room-temperature strain recovery observed in bulk LaCoO$^{3}$ after unloading.$_{2}$ $_{1--}$ J.C. Walmsley et al., J. Mat Sci, 35, 4251-60 (2000) $_{2}$ Funded by: NSF CAREER Award DMR-0846748 [Preview Abstract] |
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