Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X30: Focus Session: Cobaltites |
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Sponsoring Units: DMP GMAG Chair: Igor Zaliznyak, Brookhaven National Laboratory Room: 334 |
Thursday, March 19, 2009 2:30PM - 3:06PM |
X30.00001: Doping fluctuation-driven magneto-electronic phase separation in La$_{1-x}$Sr$_{x}$CoO$_{3}$ single crystals Invited Speaker: The doped perovskite cobaltites, in particular La$_{1-x}$Sr$_{x}$CoO$_{3}$ (LSCO), have emerged as productive systems for the study of the magneto-electronic phase separation phenomenon widely observed in complex oxide materials. It is now well established that this system phase separates into hole-rich ferromagnetic clusters embedded in a hole-poor non-ferromagnetic insulating matrix at low doping. These clusters percolate at a critical doping value near $x$ = 0.17 leading to a crossover from short- to long-range ferromagnetic order and a simultaneous insulator-metal transition. In this work we have used multiple complementary experimental probes (e.g. small-angle neutron scattering (SANS), heat capacity, and magnetotransport) to establish that high quality single crystals actually exhibit magnetic phase separation only over a well-defined doping range, 0.04 $< \quad x \quad <$ 0.22. We further show that these limits can be perfectly reproduced by simple statistical simulations where the existence of local ferromagnetism is driven solely by the inevitable local compositional fluctuations that are present at such small length scales. These length scales are defined by the mean cluster size which is determined directly by SANS. The same simulations also reproduce the doping dependence of both the observed magnetic phase fractions and SANS intensity. A remarkable consequence of this analysis is that it suggests that the magnetic phase diagram measured on macroscopic specimens is applicable even at length scales as small as 1 nm. Most importantly, it is clear from this work that models based on true electronic phase separation are not required to explain the physical properties of these cobaltites. Co-authors: C. He, S. El-Khatib, J. Wu, (UMN), J.W. Lynn (NIST), H. Zheng, J.F. Mitchell (Argonne National Lab). Work supported by DoE and NSF. [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X30.00002: Heat capacity study of magneto-electronic phase separation in La1-xSrxCoO3 single crystals. C. He, S. Eisenberg, C. Jan, H. Zheng, J.F. Mitchell, J.W. Lynn, C. Leighton We present an investigation of the specific heat (0.35 K $< \quad T \quad <$ 270 K) and ordinary Hall effect (300 K) in La$_{1-x}$Sr$_{x}$CoO$_{3}$ single crystals (0.00 $< \quad x \quad <$ 0.30). The data reveal new information on the nature of the percolation transition, the crystal and electronic structure, and the magneto-electronic phase separation. The observations include a discontinuity in Debye temperature accompanying the percolation-type insulator-metal transition and a large electron mass enhancement, likely due to strong electron correlation effects. The various contributions to the heat capacity provide a detailed picture of the evolution of the phase-separated state with doping. Most importantly, this provides strong evidence for the unanticipated result that the phase separation is restricted to a well-defined doping range, 0.04 $< \quad x \quad <$ 0.22, in close agreement with recent small-angle neutron scattering. [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X30.00003: Magnetic Phase Separation and its Relation to the Tolerance Factor in Cobaltites Juan Yu, Despina Louca, Daniel Phelan, Keisuke Tomiyasu, Kazumasa Horigane , Kazuyoshi Yamada The doping of holes into the non-magnetic Mott insulator, LaCoO$_{3}$, induces a magnetic inhomogeneous state resulting from competing magnetic phases. Elastic neutron scattering measurements on Ca, Sr, and Ba doped single crystals showed that phase competition is strongly dependent on the tolerance factor, $t$. When $t$ is small as in La$_{1-x}$Ca$_{x}$CoO$_{3}$, only a ferromagnetic (FM) phase is present. As $t$ gets large as in La$_{1-x}$Ba$_{x}$CoO$_{3}$, an incommensurate (IC) phase coexists with the FM phase. The IC phase becomes commensurate and as strong as the FM phase by x = 0.18, with a very long correlation length. This is in stark contrast from La$_{1-x}$Sr$_{x}$CoO$_{3}$ where the IC phase remains short-range and with its intensity reduced when the system orders ferromagnetically. Our observation shows that increasing $t$ enhances the presence of two phases and favors the growth of nanoscale spin-ordered superstructures. The subtle lattice changes brought about by changing $t$ have a direct effect on the Co-O hybridization that in turn affects the magnetic interactions. Double exchange interactions between Co$^{3+}$ and Co$^{4+}$ result in FM correlations while the superexchange between Co$^{3+}$ ions result in antiferromangetic correlations giving rise to the second phase. This may be mediated by a Jahn-Teller mechanism that sets in at high temperatures. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X30.00004: Incommensurate charge and spin ordering in doped layered Co perovskite oxides: small-polaron charge glass I. Zaliznyak, N. Sakiyama, S.-H. Lee, Y. Mitsui, H. Yoshizawa Using neutron diffraction, we have investigated two families of cobalt-based layered perovskite oxides, Pr$_{2-x}$Ca$_{x}$CoO$_{4}$ (0.39 $<$ x $<$ 0.73) and La$_{2-x}$Sr$_{x}$CoO$_{4}$ (x=0.5, 0.61), which are relatives of high-Tc cuprate superconductors. In the range of heavy doping, 0.5 $<$ x $<$ 0.75, we have discovered the doping-dependent incommensurate short-range ordering of charges and magnetic moments, whose scattering signatures look somewhat similar to those previously found in cuprates and nickelates. The average incommensurability of charge order (CO) propagation vector, \textbf{Q}c = ($\varepsilon _{c}$,0,l), scales roughly linearly with doping and is proportional to the concentration of Co$^{2+}$ ions, $\varepsilon _{c} \sim $ (1-x). CO exists already at room temperature and shows no change on cooling. In cobaltites, this CO can be understood as a glassy state formed by nano-scale patches of commensurate small-polaron superlattices, whose average period is determined by the doping, x, but the long-range coherence is frustrated by the charge neutrality requirement. Static magnetic spin order in cobaltites only develops at low T $<$ 40 K. Its period is roughly twice that of CO, indicating dominant antiferromagnetic correlation between the nearest Co$^{2+}$ spins. [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X30.00005: Core level line shape analysis of LaCoO$_3$ E. M. Paisley, J. Stanley, J. Hinton, N. Sundaram, B. S. Mun, A. Bostwick, E. Rotenberg, J. F. Mitchell, D. P. Belanger, G.-H. Gweon The spin state of LaCoO$_3$ is a topic of high interest lately. Here we investigate the electronic structure of LaCoO$_3$ using core level and valence band photoemission spectroscopy. We compare the competing spin models in the literature by using our data obtained as a function of incident photon energy and temperature. Using line shape simulation of the Co 3s core level spectroscopy data and the Co 2p core level spectroscopy data, we address the issue of extracting the spin state information of the ground state from the photoemission data. [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X30.00006: Low Temperature Interactions of High SpinMagnetic Excitons in LaCoO$_3$ Sean Giblin, Ian Terry, Andrew Boothroyd, Chris Leighton The low temperature magnetic behavior of LaCoO$_3$, containing oxygen vacancies, is reported. Magnetic susceptibility measurements made in the temperature range 0.5~K to 35~K on a single crystal and a polycrystalline sample provide strong evidence for the existence of magnetic excitons as fundamental entities within in the bulk of the material system. Specifically, two distinct types of excitons form, isolated and interacting excitons, both of which are associated with oxygen vacancies. Isolated magnetic excitons act as high spin paramagnetic particles, whilst the interacting excitons appear to be coupled antiferromagnetically. It is proposed that the interaction arises from the overlap of magnetic excitons as a consequence of the statistical clustering of oxygen vacancies. The striking similarity of these results with those of the lightly doped La$_{0.97}$Sr$_{0.03}$CoO$_3$ suggests that the observed excitons are a precursor to magneto-electronic phase separation and supports the idea that phase separation is initiated by disorder in the material system. [Preview Abstract] |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X30.00007: Spin Glass Behavior in the new cobaltite series (BaSr)$_{4-x}$La$_{x}$Co$_{4}$O$_{15}$. Ovidiu Garlea, Rongying Jin, Radu Custelcean, Hao Sha, Jiandi Zhang We report on the structural and magnetic properties of a new class of cobaltites with the chemical formula (BaSr)$_{4-x}$La$_{x}$Co$_{4}$O$_{15}$. These compounds crystallize in a hexagonal structure, where Co ions occupy two different sites with octahedral and tetrahedral oxygen environments. Four Co ions of the unit cell define the vertices of a tetrahedron and their mutual antiferromagnetic superexchange interactions are topologically frustrated. Partial substitution of Sr and Ba atoms for La allows one to adjust the degree of Co valence mixing and finely tune their magnetic interactions. A strong irreversibility between FC and ZFC magnetizations and the absence of magnetic reflections in the neutron diffraction patterns suggest a spin glass-like ground state for all the compositions. [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:30PM |
X30.00008: Competition between Jahn-Teller instability and uniaxial magnetism in Ca$_{3}$CoMO$_{6}$ (M = Mn, Co, Rh) Yuemei Zhang, Hongjun Xiang, Erjun Kan, A. Villesuzanne, M.-H. Whangbo Ca$_{3}$CoMO$_{6}$ (M = Mn, Co, Rh) exhibits a uniaxial magnetism, because the Co$^{n+}$ ions of their CoO$_{6}$ trigonal prisms (n = 2 or 3) possess an electron configuration with unevenly filled degenerate d-states, so the Co$^{n+}$ ions have a nonzero magnetic moment only along the axis of the rotational symmetry causing the degeneracy (i.e., the 3-fold rotational axis along the CoMO$_{6}$ chain). Such ions lead to Jahn-Teller (JT) instability, and the associated distortion removes the rotational symmetry responsible for the uniaxial magnetism. We investigated how these opposing factors compete in Ca$_{3}$CoMO$_{6}$ (M = Mn, Co, Rh) on the basis of first principles DFT calculations. [Preview Abstract] |
Thursday, March 19, 2009 4:30PM - 4:42PM |
X30.00009: Various magnetic ground states linked to sodium ordering pattern via controlled cooling in Na$_{x}$CoO$_{2}$ (x $\approx $ 0.75-0.85) J. Kanter, V. Wittwer, T. Schulze, P. Haefliger, S. Petitjean, D. Sheptyakov, Ch. Niedermayer, K. Mattenberger, B. Batlogg Detailed characterization is presented of a recently found link between the low temperature magnetic properties of Na$_{x}$CoO$_{2}$ and a sodium rearrangement process at the onset of sodium mobility around 200 K. Switching between different sodium ordering patterns is possible by adjusting the cooling speed, due to the long time constant of the sodium rearrangement. The various magnetic states (with T$_{c}$ of 8, 15 and 22 K) are characterized by transport, magnetization, specific heat and thermal expansion measurements and linked to the sodium ordering process. The magnetic field dependence of the transition temperatures and the magnetic anisotropy were studied in detail. Muon Spin Rotation experiments confirmed the true bulk character of the magnetic transitions and locally probed the different phases. Single crystal diffraction data links the different magnetic ground states to structural changes. [Preview Abstract] |
Thursday, March 19, 2009 4:42PM - 4:54PM |
X30.00010: A quantum Monte Carlo study of magnetism in the frustrated Hubbard model Matthew Enjalran Motivated by the observation of complex phases in materials with quasi-two-dimensional triangular lattice structures, Na$_x$CoO$_2$ $\cdot$ yH$_2$O and $\kappa$-(ET)$_2$X, where nearest neighbor interactions are frustrated, we investigate the magnetic correlations in the 2D Hubbard model using constrained path quantum Monte Carlo. In order to develop our understanding of the effect of geometric frustration on the magnetic correlations in an itinerant electron model, we report results for the square and triangular lattice geometries at half-filling. [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X30.00011: Does disorder destroy e$_{g'}$ pockets in Na$_{0.3}$CoO$_{2}$? A new ab initio method for disorder Tom Berlijn, Dimitri Volja, Wei Ku Hydrated Na$_{0.3}$CoO$_{2}$ shows interesting superconductivity[1], with evidence of a nodal order parameter[2]. One possible origin of the nodal structure is {\it f}-wave pairing[3] due to the six e$_{g'}$ pockets predicted by the local density approximation[4]. However, ARPES experiments[5] showed no sign of these hole pockets. In this talk, we will investigate a recent proposal[6] of destruction of the e$_{g'}$ pockets due to disorder. An affordable {\it ab initio} Wannier function based method will be presented that takes into account spatial distributions of disorder, beyond existing mean-field approximations (e.g. VCA, CPA). We also use our Wannier functions to analyse the crystal field splitting, the sign of which critically determines the role of correlation in DMFT. \\[3pt] [1] K. Takada et al, Nature {\bf 422}, 53 (2003)\\[0pt] [2] Zheng G. et al, JPCM {\bf 18}, L63 (2006)\\[0pt] [3] Kuroki K. et al, PRL {\bf 93}, 077001-1 (2004)\\[0pt] [4] D. Singh, PRB {\bf 61}, 13397 (2000)\\[0pt] [5] Hasan M.Z. et al, PRL {\bf 92}, 246402 (2004)\\[0pt] [6] D. Singh et al PRL {\bf 97}, 016404 (2006) [Preview Abstract] |
Thursday, March 19, 2009 5:06PM - 5:18PM |
X30.00012: Novel electronic states in the sodium rich phases of cobaltates Na$_x$CoO$_2$ Meng Gao, Ziqiang Wang The cobaltates display many unusual properties in the sodium rich regime. We study the effects of strong local and finite range correlation and the sodium dopant order within the framework of an extended Hubbard model on the triangular lattice. We find that despite the proximity to the band insulating state at $x=1$, the interplay of strong electronic correlation and sodium order leads to the formation of various unconventional inhomogeneous electronic states. We compare these findings to recent experimental observations around $x=0.84$ [Preview Abstract] |
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