Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B23: Focus Session: Perovskite Cobaltites |
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Sponsoring Units: DMP GMAG Chair: Stephan Rosenkranz, Argonne National Laboratory Room: Morial Convention Center 215 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B23.00001: Direct Electron Spectroscopy Investigation of the Spin State of LaCoO$_3$ J. Stanley, J. Hinton, N. Sundaram, B. S. Mun, A. Bostwick, E. Rotenberg, D. P. Belanger, G.-H. Gweon The spin state of LaCoO$_3$ and related properties have been a topic of high interest lately. Here we investigate the electronic structure of LaCoO$_3$ using core level and valence band photo-electron spectroscopy. We compare the competing spin models in the literature by using our data obtained as a function of incident photon energy and temperature. We are able to make an interesting contribution to the discussion of the spin state of this material. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B23.00002: Field Effect on Spin Correlations in La$_{1-x}$Sr$_x$CoO$_3$ Daniel Phelan, Despina Louca, Seunghun Lee, Stine Ancona, Stephan Rosenkranz, John Mitchell La$_{1-x}$Sr$_x$CoO$_3$ evolves from a non-magnetic, insulating ground state in the parent compound (x=0) into a metallic, ferromagnetic cluster-glass (x$>$0.18). For intermediate concentrations, the ground state is characterized by a competition between ferromagnetic and incommensurate spin correlations, which leads to short-range clustering. The field effect on these competing spin correlations has been investigated by neutron diffraction from a single crystal of La$_{.85}$Sr$_{.15}$CoO$_3$ under an externally applied vertical magnetic field ranging from 0 to 7 T. The intensity of the incommensurate reflections is signficantly reduced under 7 T. On the other hand, the ferromagnetic signal is enhanced by more than an order of magnitude under 7 T, while simultaneously narrowing in reciprocal space, implying an increase in the correlation length, which cannot be understood simply by a reorientation of ferromagnetic clusters along the direction of the applied field. Instead, the data is interpreted in a phase-separated scenario in which the applied field polarizes regions which are initially not ferromagnetic in zero field. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B23.00003: Ionic Size Effects and Magnetic Incommensurability in Cobaltite Juan Yu, Despina Louca, Daniel Phelan, K. Yamada LaCoO$_{3}$ in which Co$^{3+}$ has nearly degenerate spin states shows unusual magnetic behavior attributed to the fact that the different states can co-exist. The cobalt ions interact with each other via complex magnetic coupling, that is further complicated by the addition of Co4+ ions with hole doping. In the case of hole doped La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO), we found that the system exhibits a rich phase diagram, dominated by two competing magnetic phases, one that is ferromagnetic and metallic and another that is incommensurate with the lattice and insulating. Our recent elastic neutron scattering measurements on single crystals of La$_{1-x}$Ba$_{x}$CoO$_{3}$ (LBCO, with x = 0.03, 0.06, 0.10 and 0.15) also identified the coexistence of these two phases. Compared to LSCO, the LBCO series shows different incommensurability, but has a similar trend in the progression of intensity with increasing x. However, the LBCO series shows a much longer correlation length particularly in the direction perpendicular to (111), and a significantly stronger incommensurate peak than in LSCO. The incommensurate and ferromagnetic-like intensities exhibit identical temperature dependence in LBCO unlike in LSCO, in which ferromagnetic scattering had a higher onset temperature than the incommensurate phase at identical compositions. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B23.00004: Electronic structure of La$_{1-x}$Sr$_{x}$CoO$_{3}$ investigated from doping evolution of its soft x-ray spectra Yinwan Li, David Ederer, Thomas CallCott, J. W. Freeland Perovskite cobalt oxide LaCoO$_{3}$ attracted a lot of attention because of its spin state transition around 90K, the interpretation of which is still under debate. We performed soft X-ray absorption and emission experiments on La$_{1-x}$Sr$_{x}$CoO$_{3}$ at O-K edge and Co-L edge with various doping from x = 0 to x = 30. The doping evolution of the spectra can be understood as a result of introduction of holes at the top of valence band accompanied with a lowering of the Fermi level. The significant change of oxygen K edge spectra with doping indicates that the top of the valence band is largely of oxygen character, indicating strong Co 3d -- O 2p hybridization. The bottom of unoccupied band has a double peak feature indicating a splitting e$_{g}$ band. This split is the largest in the undoped sample and becomes smaller with doping. A gap of less than 1eV is observed from the excitation feature in the emission spectra and its change with doping again indicates a lower Fermi level in doped samples. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B23.00005: Thickness dependent magnetotransport properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_{3}$(001) films M. Sharma, M. Torija, C. Leighton, M. Varela Thin films of the doped perovskite cobaltite La$_{1-x}$Sr$_{x}$CoO$_{3}$ offer an ideal system to study the effect of dimensional confinement on spin state transitions and magnetoelectronic phase separation, and have application possibilities as electrodes in ferroelectric memory and solid oxide fuel cells. In this work we present the magnetotransport properties of epitaxial La$_{0.5}$Sr$_{0.5}$CoO$_{3}$ (001) thin films deposited on SrTiO$_{3}$ (001) by high pressure reactive sputtering. The films were structurally characterized by high-resolution x-ray diffraction, scanning probe microscopy, and STEM. Films with thickness $>$ 100 {\AA} exhibit bulk-like ferromagnetic metallic characteristics with the conventional negative MR in the vicinity of T$_{C}$ and a large AMR at low T. In stark contrast, films with thickness below 60 {\AA} exhibit reduced magnetization and a crossover to an insulating-like temperature dependence of the resistivity. This crossover is accompanied by a large negative MR at low T which bears a striking resemblance to that seen in bulk at x $<$ 0.17, which is known to be due to an intercluster ``GMR'' effect. In essence, x = 0.5 films on STO, when sufficiently thin, behave much like x $<$ 0.17 bulk samples, i.e. phase separation is evidenced at the LSCO/STO interface. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B23.00006: Heat capacity investigation of phase separation and spin-state transitions in La$_{1-x}$Sr$_{x}$CoO$_{3}$ Chunyong He, Heng Zheng, John Mitchell, C. Leighton We present a heat capacity study (to 0.3 K) on La$_{1-x}$Sr$_{x}$CoO$_{3}$ single crystals (0.00 $<$ x $<$ 0.30). In doped samples we observe three contributions at low $T$; a lattice term ($\propto \quad T^{3})$, an electronic term ($\propto \quad T)$, and a third term proportional to $T^{2}$. Remarkably, the x dependence of the electronic and $T^{2}$ contributions reflects very clearly the known magnetic phase separation, indicating that the $T^{2}$ term is a signature of the non-F matrix. Possible origins related to AF fluctuations will be discussed. At the lowest $T$ the nuclear hyperfine contribution provides a further probe of magnetic order. The electronic contribution also gives the density of states at the Fermi level which, in combination with the hole density from Hall effect, suggests a large effective mass indicative of strong correlations. Finally, the end-member LaCoO$_{3}$ shows a striking Schottky anomaly providing new information on the controversial spin-state transition. In particular, we find further evidence of the around 0.5 meV excitation recently observed by inelastic neutron scattering. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B23.00007: Phase separation and inhomogeneity in La$_{1-x}$Sr$_{x}$CoO$_{3}$ single crystals W.G. Moulton, Robert Smith, Michael Hoch, Phillip Kuhns, Arneil Reyes, Gregory Bobinger, John Mitchell, Chris Leighton Nanoscale inhomogeneity in La$_{1-x}$ Sr$_{x}$CoO$_{3}$ has been studied in single crystal samples for 0.05$\le $ x $\le $0.3 using $^{139}$La NMR as a probe of the internal magnetic field distribution. The results show that phase separation occurs for x near the metal-insulator critical concentration x$_{C }$=0.17. The phase separation is confined to a much smaller range of x than that previously found in polycrystalline samples even though the bulk magnetic properties are similar. In single crystals with increasing x large, spin polarized (FM) regions merge into networks while smaller regions, identified as spin polarons, coexist with the larger regions. Insight into phase separation has been obtained by simulating the spectra with statistical model A phase diagram summarizing the evolution of the inhomogeneous internal field distribution with x and T will be presented. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B23.00008: Strongly Inhomogeneous Conduction in Cobaltite Films. Vladimir Orlyanchik, Michael Weissman, Maria Torija, Manish Sharma, Chris Leighton The prototypical doped perovskite cobaltite, La0.5Sr0.5CoO3, has received considerable attention in bulk form. It was shown to be a ferromagnetic material with metallic conductivity. Here we show that the films, like bulk samples of the same composition, show resistance increasing with temperature when measured by Van der Pauw technique. However, when the resistance is measured in strip-shaped samples it reveals negative temperature dependence. Noise measurements performed in relatively large (e.g. 10-10 cm3) samples, revealed strongly non-Gaussian conductance fluctuations in the form of random telegraph noise. Field and temperature dependencies of Boltzmann factors of individual switchers show them to represent magnetic rotations rather than fluctuations between different thermodynamic phases (metallic and insulating). We argue that the inhomogeneous magnetic orientation dependent conductivity arises from strain-driven domain texture. The highly inhomogeneous conductivity may account for the unusual geometry dependent conductance. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B23.00009: The spin state issue in the $R$BaCo$_2$O$_{5.5}$ cobaltates Hua Wu, Z. Hu, T. Burnus, D. I. Khomskii, L. H. Tjeng The double perovskites $R$BaCo$_2$O$_{5+\delta}$ ($R$=rare earth, 0$\le\delta\le$1) display intriguing phenomena such as charge and orbital ordering, as well as antiferromagnetic to ferromagnetic transition, depending on the oxygen concentration. In particular, the $\delta$=0.5 system shows a giant magnetoresistance effect, and its metal-insulator transition has been often interpreted in terms of a spin-state transition [1,2], which, however, is fiercely debated [3,4]. To address the spin-state issue, we performed density-functional theory calculations which include a mean-field correction for the correlation effects caused by the Co $3d$ electrons. We have investigated various scenarios with different combinations of the low-, intermediate- and high-spin (LS, IS, and HS) states. Our results show that the pyramidally coordinated Co$^{3+}$ ions are exclusively in the HS state since [3], in disagreement with [1,2]. The octahedrally coordinated Co$^{3+}$ can be stabilized into a LS-HS ordered state if we take into account the superstructure recently reported [4]. Our results put limits as to how much spin-state transition could accompany the metal-insulator transition. [1] C. Frontera $et$ $al$., Phys. Rev. B {\bf 65}, 180405(R) (2002). [2] A. A. Taskin $et$ $al$., Phys. Rev. Lett. {\bf 90}, 227201 (2003). [3] Z. Hu $et$ $al$., Phys. Rev. Lett. {\bf 92}, 207402 (2004). [4] D. D. Khalyavin $et$ $al$., Phys. Rev. B {\bf 75}, 134407 (2007) [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B23.00010: Magnetic anisotropy and switching in Pr$_{0.5}$Sr$_{0.5}$CoO$_{3}$ using RF transverse susceptibility N.A. Frey, H. Srikanth, D.D. Stauffer, C. Leighton Pr$_{0.5}$Sr$_{0.5}$CoO$_{3}$ has been a system of current interest that appears very different from manganites or even other cobaltites. It has a ferromagnetic transition at 230 K and an anomalous magnetic transition at 120 K which is manifested in steps in temperature dependent magnetization and coercivity. The origin of this transition is still under investigation and recent experiments have suggested a change in magnetic anisotropy associated with this anomalous transition. In this work, we report systematic investigations of the magnetic anisotropy and switching fields across the 120 K transition in polycrystalline Pr$_{0.5}$Sr$_{0.5}$CoO$_{3}$ using a very sensitive RF transverse susceptibility method based on a tunnel diode oscillator resonant at 12 MHz. Our experiments reveal evidence for a sharp change in the anisotropy at 120 K with the anisotropy field (H$_{k})$ dropping from around 1.8 kOe to 750 Oe on cooling. By tracing the evolution of the anisotropy and switching peaks, we are able to discern behavior associated with separate ferromagnetic phases with 120 K signaling the transition between the two. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B23.00011: Influence of cation and oxygen vacancy ordering on magnetic properties of RE$_{1-x}$Sr$_{x}$CoO$_{3-\delta }$ (RE=Ho, La) S. Kolesnik, B. Dabrowski, O. Chmaissem, J. Mais, A. Baszczuk The attractive physical properties of RE$_{1-x}$Sr$_{x}$CoO$_{y}$ (RE - rare earth and Y) are strongly dependent upon overall oxygen content y and the ordering of oxygen and RE/Sr, as well as charge and spin ordering on the Co$^{3+}$/Co$^{4+}$- sites. Structural, magnetic and thermoelectric properties of Ho$_{1/3}$Sr$_{2/3}$CoO$_{y}$ have been studied within the oxygen content range 2.67$<$y$<$2.80. The studied samples demonstrate antiferromagnetic order (G-type) and semiconductive behavior. No ferromagnetic ordering has been observed and compared with A. Maignan et al. on a closely related Sr$_{2/3}$Y$_{1/3}$CoO$_{y}$ with similar y. We have observed that a the same high degree of oxygen ordering and antiferromagnetic structure exists for both cation disordered (La) and ordered (Y,Ho) materials. We conclude that the high degree of oxygen vacancy ordering determines the observed properties via strong localization of Co$^{3+ }$electrons. Work at NIU was supported by the NSF-DMR-0302617 and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B23.00012: Orbital Ordering in Room Temperature Ferromagnet Sr$_3$YCo$_4$O$_{10.5}$ Studied by a Resonant X-ray Scattering Hironori Nakao, Tetsuya Murata, Daisuke Bizen, Youich Murakami, Shintaro Ishiwata, Wataru Kobayashi, Ichiro Terasaki Sr$_{1-x} R_x$Co$_4$O$_{10.5}$ ($R=$Y and lanthanide,
$0.2 |
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