Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P20: Focus Session: Cobaltites, Nickelates and Vanadates |
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Sponsoring Units: DMP GMAG Chair: Owen Vajk, National Institute of Standards and Technology Room: Baltimore Convention Center 317 |
Wednesday, March 15, 2006 11:15AM - 11:27AM |
P20.00001: Intrinsic nanoscale electronic phase separation and simple percolation in La$_{1-x}$Sr$_{x}$CoO$_{3}$ J. Wu, J. Parker, C. Perrey, B. Carter, J. Lynn, H. Zheng, J. Mitchell, C. Leighton The doped pervoskite cobaltite La$_{1-x}$Sr$_{x}$CoO$_{3}$ has been advanced as a model system for studying magnetoelectronic phase separation. We present here a combination of chemically sensitive high-resolution TEM, SANS, and transport data that reveal interesting new features of this phase separation. The TEM data show that the material is chemically homogenous down to nm length scales, proving that the phase separation is truly intrinsic electronic phase separation. The SANS data, which were performed at several compositions below x = 0.18 (where long-range ferromagnetism (FM) sets in), reveal that the FM clusters have a maximum size of about 2-3 nm, \textit{independent} \textit{of doping}. This demonstrates that the percolation transition that occurs at x = 0.18 is due to an increasing density of clusters with increasing x, \textit{not} an expansion of cluster size. These observations naturally explain the simple percolation observed in single crystal transport, i.e. conductivity exponents close to predicted values and a critical composition (x = 0.18) close to the expected value for the 3-D percolation limit. [Preview Abstract] |
Wednesday, March 15, 2006 11:27AM - 11:39AM |
P20.00002: Short-range magentic correlations and dynamic orbital ordering in the thermally activated spin state of LaCoO$_3$ S. Rosenkranz, D. Phelan, D. Louca, S.H. Lee, P.J. Chupas, R. Osborn, H. Zheng, J.F. Mitchell The cobalt perovskites La$_{1-x}$Sr$_x$CoO$_3$ show intriguing spin, lattice, and orbital properties similar to the ones observed in colossal magnetoresistive manganites. The x=0 parent compound is a non-magnetic insulator at low temperatures, but shows evidence of a spin-state transition of the cobalt ions above 50K from a low-spin to an intermediate or high-spin configuration. Using high resolution, inelastic neutron scattering, we observe a distinct low energy excitation at 0.6meV coincident with the thermally induced spin state transition observed in susceptibility measurements. The thermal activation of this excited spin state also leads to short-range, dynamic ferro- and antiferromagnetic correlations. These observations are consistent with the activation of a zero-field split intermediate spin state as well as the presence of dynamic orbital ordering of these excited states. \\ \\ Work supported by US DOE BES-DMS W-31-109-ENG-38 and NSF DMR-0454672 [Preview Abstract] |
Wednesday, March 15, 2006 11:39AM - 11:51AM |
P20.00003: Emergence of Magnetism in La$_{1-x}$Sr$_x$CoO$_3$ D. Phelan, Despina Louca, S.-H. Lee, S. Rosenkranz, J.F. Mitchell Orbital, spin, and charge degrees of freedom play a central role in the physics of CMR-type transition metal perovskite oxides. La$_{1-x}$Sr$_x$CoO$_3$ is a system in which a ferromagnetic, metallic state emerges when holes are doped into the parent compound, a non-magnetic, Mott insulator in the ground state. We have studied this system using elastic and inelastic neutron scattering techniques on single crystals with 0$\leq$x$\leq$0.2. With hole doping the ferromagnetic correlations between Co spins become static and isotropically distributed due to the formation of ferromagnetic droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism. Diffuse spin dynamics appear as the correlation length increases. The dynamics are broad in energy indicative of a spin wave continuum. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P20.00004: Local Matrix-Cluster Interactions In La$_{1-x}$ Sr$_{x}$CoO$_{3}$. Sean Giblin, Ian Terry, Andrew Boothroyd, Dharmalingiam Prabhakaran , Jing Wu, Chris Leighton Magneto-electronic phase separation plays an integral part in many recent advances in the understanding of correlated electron systems. We have studied the magnetically phase separated material La$_{1-x}$ Sr$_{x}$CoO$_{3}$ and the parent compound LaCoO$_{3}$, using muon spectroscopy and magnetic susceptibility measurements. The muon as a local magnetic probe is sensitive to the magnetic field distribution in LaCoO$_{3}$ in the LS state, which is a direct consequence of magnetic excitons. We believe that these excitons are interacting with the Co ions undergoing the known thermally induced spin transition. By directly comparing the results of the parent compound with La$_{1-x}$ Sr$_{x}$CoO$_{3 }$we can observe the hole-rich ferromagnetic clusters interacting with the neighboring hole poor matrix for low x. This mechanism, detected here for the first time, may play an important role in the rich electrical and magnetic properties of La$_{1-x}$ Sr$_{x}$CoO$_{3}$. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P20.00005: Anisotropy-driven magnetic anomalies in Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ Chris Leighton, Doug Stauffer, Jing Wu, Qing Huang, Jeff Lynn, Brian Toby, John Mitchell Interest in the perovskite cobaltites has been growing steadily due to the intriguing phenomena they exhibit. It is well known that the availability of various Co ion spin states in the cobaltites provides an additional degree of freedom in comparison to the manganites. In this work we demonstrate that the cobaltites also possess another factor of considerable importance not present in the manganites -- large magnetocrystalline anisotropy. As previously reported [Mahendiran et al PRB 68 024427 (2003)] at x $>$ 0.30 Pr$_{1-x}$Sr$_{x}$CoO$_{3}$ displays an additional anomaly below the Curie temperature, where the magnetization can dramatically increase or decrease depending on applied field. We demonstrate here, using magnetometry, transport, heat capacity, and neutron diffraction, that this results from a structural phase transition from a low symmetry to higher symmetry (tetragonal) phase on reducing T. Although the Co moment is unaffected, a sharp change in the magnetocrystalline anisotropy takes place and is reflected in the hysteresis loop shape, coercivity, and remnance. The complex and puzzling behavior of the field dependence of the magnetization vs. T curves is then simply explained by the T dependent variations in hysteresis loop shape. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P20.00006: Magnetic properties and phase separation in Pr$_{1-x}$Sr$_{x}$CoO$_{3}$, using $^{59}$Co NMR R. Smith, P.L. Kuhns, M.J.R. Hoch, W.G. Moulton, D. Stauffer, J. Wu, C. Leighton Doped transition metal oxides including manganites and cobaltites have revealed a rich variety of properties that may be technologically important. The mixed valence cobaltite Pr$_{1-x}$Sr$_{x}$CoO$_{3 }$(PSCO) has a phase diagram reminiscent of La$_{1-x}$Sr$_{x}$CoO$_{3 }$(LSCO) but with a number of significant differences. For x=0.5 the system is ferromagnetic (FM) below $T_{C}$= 240 K but anomalous magnetization behavior is found close to 120 K with an associated crystal structure change from low symmetry to tetragonal with decreasing $T$. For x $<$ 0.3 no change in magnetic properties or crystal structure is found below $T_{C}$. Zero-field $^{59}$Co NMR spectra show that differences in FM character between x=0.5 and x=0.3 samples are negligibly small at temperatures in the range 3-30 K No FM line was observed for x=0.2; a narrow paramagnetic- like signal only slightly shifted from the diamagnetic $^{59}$Co spectrum is observed in high- field NMR for all three x values, providing evidence of some form of phase separation where a paramagnetic phase coexists with the FM phase. The results will be compared with the very different phase separation data previously obtained for LSCO. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 12:39PM |
P20.00007: Structural and Magnetic Properties of the Kagom\'{e} Antiferromagnet YbBaCo$_{4}$O$_{7}$ John Mitchell, Ashfia Huq, Laurent Chapon, Paolo Radaelli, Kevin Knight, Peter Stephens, Hong Zheng The mixed-valent compound YbBaCo$_{4}$O$_{7}$ is built up of Kagom\'{e} sheets of CoO$_{4}$ tetrahedra, linked in the third dimension by a triangular layer of CoO$_{4}$ tetrahedra in an analogous fashion to that found in the known geometrically frustrated magnets such as pyrochlores and SrCr$_{9x}$Ga$_{12-9x}$O$_{19}$ (SCGO). We have undertaken a study of the structural and magnetic properties of this compound using combined high resolution powder neutron and synchrotron X-ray diffraction. YbBaCo$_{4}$O$_{7}$ undergoes a first order trigonal to orthorhombic phase transition at 175 K that breaks the trigonal symmetry of the structure. We show that this transition occurs as a response to a markedly underbonded Ba$^{2+}$ site in the high-temperature phase and does not appear to involve charge-ordering of Co$^{2+}$/Co$^{3+}$ ions in the tetrahedra. The symmetry-lowering relieves the geometric frustration of the structure, and a long-range ordered 3-D antiferromagnetic state develops below 80 K. [Preview Abstract] |
Wednesday, March 15, 2006 12:39PM - 12:51PM |
P20.00008: Crystal Structure and Magnetic Properties of an oxygen deficient n = 2 Ruddlesden-Popper phase Sr$_{3}$Co$_{2}$O$_{5.67}$ Julienne M. Hill, John F. Mitchell, Bogdan Dabrowski Interest in charge, orbital, and spin state phenomena in perovskite and related cobalt oxides is a growing area of transition metal oxide physics. Recently, J. Matsuno \textit{et al.}\footnote{ J. Matsuno \textit{et al}., PRL \textbf{93}, 167202 (2004).} have found that epitaxial films of the n = 1 Ruddlesden-Popper (R-P) phase Sr$_{2}$CoO$_{4}$ are metallic ferromagnets with relatively high T$_{C} \quad \sim $ 250 K. This is particularly interesting in light of the formal oxidation state of Co, Co$^{4+}$, offering no clear source of carriers. To extend the materials chemistry and physics of the R-P series of cobaltites, we have synthesized the n = 2 R-P phase Sr$_{3}$Co$_{2}$O$_{7-\delta }$ in bulk form. The crystal structure [from neutron powder diffraction (NPD) data] of our most oxygen-deficient sample, Sr$_{3}$Co$_{2}$O$_{5.67}$ is orthorhombic \textit{Immm} with a = 3.94025(9) {\AA}, b = 3.67479(9) {\AA} and c = 20.6642(5) {\AA}. The magnetization versus temperature data show two antiferromagnetic transitions at approximately 170 K and 220 K. To further elucidate the magnetic properties of this material, we have conducted a temperature-dependent NPD study. The low temperature magnetic structure is surprisingly complex and suggestive of an incommensurate ordering wave vector. Full details and results of the NPD study will be given. [Preview Abstract] |
Wednesday, March 15, 2006 12:51PM - 1:03PM |
P20.00009: Complex magnetic structure of YBaCo4O7 Laurent C. Chapon, John F. Mitchell, Paolo G. Radaelli The new series of mixed-valent oxides RBaCo4O7 (R=Yb,Tb,Y) show complex structural and magnetic behavior. We have recently revealed, for the Yb analog, that a stuctural phase transition occurs in response to an extremely underbounded Ba2+ site. The symmetry lowering from orthorhombic to tetragonal, releases the frustration and allows the system to order magnetically below ~80K. Here we present our neutron diffraction study of the analog compound YBaCo4O7, that shows the same structural phase transition at high temperature and a magnetic transition at around 110K, where the system orders antiferromagnetically with propagation vector k=0. The magnetic structure, solved by global optimization algorithms, shows a non colinear Co-spins arrangement that results from the unique topology of the Co interactions. The magnetic structure is found to be strongly temperature dependent between 1.6K and 110K, which provide crucial information about the relative strengths of competing interactions. [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P20.00010: The effect of transition metal ions distribution on magnetic properties of Li$_{x}$(Ni$_{y}$Mn$_{y}$Co$_{1-2y})$O$_{2}$. Natasha A. Chernova, Miaomiao Ma, Jie Xiao, M. Stanley Whittingham, Peter Y. Zavalij Li$_{x}$(Ni$_{y}$Mn$_{y}$Co$_{1-2y})$O$_{2}$ compounds have layered O(3) structure with an occupancy disorder as Ni ions migrate to the lithium layer. Ni ions provide strong antiferromagnetic (AF) exchange between the transition metal (TM) layers; therefore the degree of disorder has a pronounced effect on the magnetic properties. Ni migration is reduced when the amount of Co or Li is increased. In this work we study temperature and magnetic field dependences of magnetization and the ac susceptibility of Li$_{x}$(Ni$_{y}$Mn$_{y}$Co$_{1-2y})$O$_{2}$ with various Li and Co contents. We have shown that in LiNi$_{0.5}$Mn$_{0.5}$O$_{2}$ compound large amount of Ni on Li sites facilitates AF order within the TM layer, while interlayer Ni ions contribute to the net magnetic moment. This is consistent with the ``flower'' order of the TMs proposed from the Monte-Carlo simulations. With increasing Co content, the ``flower'' structure is destroyed and a spin glass state is observed in Co-containing compounds. This work is financially supported by the US Department of Energy, Office of FreedomCAR and Vehicle Technologies, through the BATT program at LBNL. [Preview Abstract] |
Wednesday, March 15, 2006 1:15PM - 1:27PM |
P20.00011: Observation of Magnetic Memory Effect and Photo-induced Magnetism in Y$_{0.33}$Sr$_{0.67}$CoO$_{3-\delta}$ M. Izumi, Y.F. Zhang, S. Sasaki, O. Yanagisawa We prepared the Y$_{0.33}$Sr$_{0.67}$CoO$_{3-\delta }$by the conventional solid state method which sintered under the O$_{2}$ flow. The sample was finally annealed under the oxygen and nitrogen atmosphere. A DC magnetization jump was found about 200 K with a large thermal hysteresis at 0.01 T indicating a kind of magnetic memory effect. The magnetization jump comes from the inter-spin state transition on Co$^{3+}$ ion from low to intermediate spin state. The magnetic memory effect gradually disappears with the magnetic field increase and the jump temperature ($T_{J})$ shifts to low temperature. Annealed samples indicate high $T_{J}$, $T_{C}$ and the magnetization coming from the oxygen content difference. Under the irradiation of a pulsed near-infrared laser ($\lambda $ = 1050 nm), the $T_{J}$ shifts to low temperature and the magnetization below T$_{J}$ decreases. Photo-induced effect is weakened with the magnetic field. Laser irradiation may suppress spin-state transition of the part Co$^{3+}$ ions. [Preview Abstract] |
Wednesday, March 15, 2006 1:27PM - 1:39PM |
P20.00012: Local Electronic and Spin Structure of GdBaCo2O5.5 from X-ray Absorption Spectroscopy Kyle Shen, David Hawthorn, Darren Peets, Ilya Elfimov, George Sawatzky, Alexey Taskin, Yoichi Ando The family of RBaCo$_{2}$O$_{5+\delta}$ cobaltates is known to exhibit a rich variety of magnetic behavior as a function of oxygen content and temperature. We present x-ray absorption measurements on detwinned single crystals of GdBaCo$_{2}$O$_{5.5}$, where the structure is comprised of alternating rows of CoO$_{6}$ octahedra and CoO$_{5}$ pyramids. GdBaCo$_{2}$O$_{5.5}$ exhibits successive paramagnetic, ferromagnetic, and antiferromagnetic phases, and also exhibits a ``spin blockade'' effect upon doping. These unusual behaviors are believed to stem from the nearly degenerate spin states of the Co$^{3+}$ ions which can potentially vary from low (S=0), intermediate (S=1), to high (S=2) spin states. Our recent x-ray absorption measurements provide the first measurements of the local electronic and spin states. Measurements of the temperature and polarization dependence of the x-ray absorption at the oxygen K edge clearly indicate an abrupt change in the orbital populations at the metal-insulator transition at T $\sim$ 360 K. We combine our spectroscopic measurements with atomic multiplet and LSDA+U calculations to provide a first insight into the true nature of the spin state transitions which govern the unusually rich magnetic properties of the RBaCo$_{2}$O$_{5+\delta}$ cobaltates. [Preview Abstract] |
Wednesday, March 15, 2006 1:39PM - 1:51PM |
P20.00013: High Pressure and High Resolution Magnetization of GdBaCo$_{2}$O$_{5.5}$ R. P. Guertin, E. S. Choi, P. Schlottmann, Z. X. Zhou We present the results of two rather diverse experiments designed to reveal new features of the complex magnetic properties of GdBaCo$_{2}$O$_{5.5}$ and, in particular, lanthanide/transition metal coupling in oxide materials. First, high resolution magnetization, M (H,T), measurements on an untwinned single crystal show a small but non-zero coupling between the 3d-shell Co-based magnetic order and the 4f shell Gd paramagnetism. Second, high pressure magnetization measurements on a polycrystalline sample suggest a weakening of the ferromagnetic interplane coupling at the expense of a strengthening of the antiferromagnetic interplane coupling. In the measured pressure range, however, no unambiguous pressure-induced spin transition was observed. Finally, low temperature isothermal magnetization measurements under pressure indicate a slight weakening of the effective Co-generated molecular field, affecting the Gd paramagnetism. The totality of data contained in this work suggests that there is a small but definitive molecular field effect at the Gd sites, which is a function of the strength of transition metal magnetism ordering at higher temperatures. [Preview Abstract] |
Wednesday, March 15, 2006 1:51PM - 2:03PM |
P20.00014: Electron-phonon coupling and low-temperature structure of NaV$_2$O$_5$ Juergen Spitaler, E. Ya. Sherman, H.G. Evertz, Claudia Ambrosch-Draxl NaV$_2$O$_5$ is an extraordinary example of a structure where charge, spin and lattice degrees of freedom strongly interact. This low-dimensional compound is characterized by V atoms arranged in the form of ladders. At ambient conditions it is found to be quarter-filled with one electron distributed over one rung of the ladder. Going below T$_c$ = 34K, NaV$_2$O$_5$ undergoes a phase transition involving a reordering of the V charges, a lattice deformation and a spin pairing. In order to investigate the mechanisms driving the phase transition, parameters of electron-phonon and spin-phonon for the $\Gamma$ point phonons are determined from ab initio calculations within density functional theory. They are compared to the corresponding parameters of the isostructural CaV$_2$O$_5$, where no phase transition occurs. Moreover, ab initio results of several candidates for the low-temperature supercell of NaV$_2$O$_5$ are presented and analyzed in terms of total energies and electric field gradients. [Preview Abstract] |
Wednesday, March 15, 2006 2:03PM - 2:15PM |
P20.00015: Quasi-one-dimensional electronic structure of $\beta^{\prime}$-Cu$_x$V$_2$O$_5$ ($x$=0.33$\sim$0.65) studied by photoemission S.-K. Mo, F. Wang, J. W. Allen, J. He, R. Jin, D. Mandrus, H. H\"ochst $\beta^{\prime}$-Cu$_x$V$_2$O$_5$ is a quasi-one-dimensional (quasi-1D) oxide that undergoes a metal to insulator transition (MIT) when $x$ decreases from 0.65 to below 0.60. It becomes a superconductor below 6K under pressure around 3GPa. In the metallic phase, the electrical resistivity along the chain direction is 30 times larger than that across the chain direction, making this material suitable for studies of quasi-1D electronic structures. We present the first photoemission spectra of $\beta^{\prime}$-Cu$_x$V$_2$O$_5$ in both metallic (x=0.60, 0.65) and insulating (x=0.33, 0.55) phases. Angle-integrated spectra show a clear indication of the MIT. Nonetheless the intensity near the Fermi energy ($E_{\rm F}$) is heavily suppressed in the metallic phase, just as in the spectra of Li$_{0.9}$Mo$_6$O$_{17}$ and certain other low dimensional oxides [1]. We observe a single band crossing $E_{\rm F}$ along the chain direction, around the $\Gamma$-point of the Brillouin zone, only in the metallic phase angle-resolved spectrum. Fermi surface intensity maps have clear 1D character and the Fermi wavevector changes according to the concentration of the dopant. [1] G.-H. Gweon, J.W. Allen, and J.D. Denlinger, Phys. Rev. B {\bf 68}, 195117 (2003). [Preview Abstract] |
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