Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U43: Focus Session: Phase Complexity and Enhanced Functionality in Magnetic Oxides V |
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Sponsoring Units: DMP GMAG Chair: Andrei Sushkov, University of Maryland Room: LACC 150C |
Thursday, March 24, 2005 8:00AM - 8:36AM |
U43.00001: X-ray Absorption spectroscopic investigation of Novel Magnetic oxides Invited Speaker: Magnetic oxides have attracted so much attention as candidates of magnetic materials for next generation magnetic devices such as spintronics device, tera-bit magnetic storages, high efficient magnetic sensors, etc. The magnetic oxides display variety of magnetic behaviors at a function of doping, pressure, external fields, etc., and those behaviors can be even controllable. X-ray absorption spectroscopy including the magnetic circular dichroism is a powerful microscopic probe to characterize and elucidate the origin and the mechanism for the magnetic behaviors. Here I will discuss recent findings in the spectroscopic studies on various magnetic oxides, including magnetic oxide nano-particles, a multiferroic oxide, and spin- orbit-lattice coupled magnetic oxide films. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U43.00002: Anomalous electronic state in CaCrO$_3$ and SrCrO$_3$ J.-S. Zhou, J.B. Goodenough, Y.W Long, C.-Q. Jin Measurements of thermal conductivity, thermoelectric power, electrical conductivity, magnetization and the equation of state have been carried out on ceramic samples of CaCrO$_{3}$ and SrCrO$_{3}$ that were synthesized under high pressure. Contrary to earlier reports, both compounds have been found to be a spin-glass insulator. While the magnetic susceptibility $\chi $(T) of SrCrO$_{3}$ becomes completely incompatible with the Curie-Weiss law, the $\mu _{eff}$=3.4 $\mu _{B}$ obtained in CaCrO$_{3}$ is close to the spin-only moment of a localized electronic state. Suppression of the thermal conductivity in both compounds indicates that orbital fluctuations are present, which confirms further the ``localized'' electronic state. Factors such as a higher $\kappa $(T) and weaker temperature dependence of $\chi $(T) for SrCrO$_{3}$ than CaCrO$_{3}$ suggest that SrCrO$_{3}$ is close to the crossover from the localized to the itinerant electronic state. More importantly, the Cr-O bond length in SrCrO$_{3}$ is much smaller than that calculated from the ionic radii. An anomalous small bulk modulus found for SrCrO$_{3}$ at P $>$ 40 kbar confirms unambiguously that the electronic state transition is induced under high pressure. The bulk modulus of SrCrO$_{3}$ below 40 kbar and CaCrO$_{3}$ falls in line with other perovskite oxides. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U43.00003: Phase separation and Jahn-Teller effect in spinels Sunmog Yeo, Y. Horibe, S. Mori, C.H. Chen, S. Guha, S-W. Cheong Inter-relationship between phase separation and the Jahn-Teller effect has been investigated in various spinel compounds containing Cu2+ and Mn3+ ions. We have employed comprehensive experiments of resistivity and magnetic susceptibility measurements, x-ray diffraction, and TEM. Particular attention was given to study the evolution of physical properties with the substitution of non Jahn-Teller ions to the Jahn-Teller-active Cu or Mn sites. The results of x-ray and TEM clearly show nanometer-scale chemical/structural phase separation and our phase diagram demonstrates a close relationship between phase separation and the Jahn-Teller effect. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U43.00004: Tuning of magnetic and electronic states by control of oxygen content in lanthanum strontium cobaltates S. Kolesnik, B. Dabrowski, J. Mais, M. Majjiga, A. Baszczuk We report on the magnetic, resistive, and structural studies of perovskite La$_{1/3}$Sr$_{2/3}$CoO$_{3-\delta}$. By using the relation between the temperature, partial oxygen pressure and the oxygen content from the thermogravimetric analysis, we have to synthesized a series of samples with precisely controlled $\delta=0.00-0.49$. The samples show significant coupling among the structural, magnetic and transport properties as a function of $\delta$. The stoichiometric material with $\delta=0.00$ is a cubic ferromagnetic metal with the Curie temperature $T_C=274$ K. The increase of $\delta$ to 0.15 is followed by a linear decrease of $T_C$ to $\approx$ 160 K and a metal- insulator transition within the cubic structure range. Further increase of $\delta$ results in formation of orthorhombic $a_p\times a_p \times 2a_p$ phase for $\delta\approx 0.25$ and brownmillerite phase for $\delta\approx 0.5$. Those phases are weak ferromagnetic insulators with $T_C=230$ K and 120 K, respectively. The present data show that the control of oxygen stoichiometry in lanthanum strontium cobaltates allows to modify the crystal structure and physical properties of these materials. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:48AM |
U43.00005: Electronic Structure at Mott Insulator/Band Insulator Interfaces Invited Speaker: A conceptual dilemma arises in the study of oxide thin film heterostructures. The question is: how should one consider the correlated equivalent of band bending and quantum confinement? These semiconductor concepts are based on rigid single particle band diagrams, which are known to be an inadequate description for strongly correlated electrons. In addition to presenting an interesting scientific challenge, this underlies attempts to develop new applications for doped Mott insulators in device geometries. They further offer a unique approach to creating new two-dimensional states in artificial structures. Modern thin film growth techniques can be used to fabricate highly idealized oxide heterostructures on the atomic scale, allowing a direct examination of these issues. Here we present our recent study of three examples of a Mott insulator/band insulator interface. By measuring the electronic properties and using spatially-resolved electron energy-loss spectroscopy, we can deduce the distribution and response of the correlated electrons. In LaTiO$_3$/SrTiO$_3$, we find the charge distribution length is dominated by the strong polarizability of the lattice, arising from a nearby ferroelectric instability. In LaTiO$_3$/LaAlO$_3$, we demonstrate a dramatic narrowing of the charge distribution by quantum confinement. Finally, in LaMnO$_3$/SrTiO$_3$ we find the charge distribution can be modified by a magnetic field, reflecting the strong charge-spin coupling arising from the double-exchange mechanism. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U43.00006: Field-Induced Orbital and Magnetic Phases in the Layered Ruthenates J. F. Karpus, R. Gupta, H. Barath, S. L. Cooper, G. Cao Magnetic-field- and Temperature-dependent Raman scattering studies have been performed on Ca$_{3}$Ru$_{2}$O$_{7}$, which undergoes an antiferromagnetic transition at 56K and a metal-insulator transition at 48K. Although no appreciable changes in the magnon or phonon spectra are observed for fields oriented along the c-axis, a field applied in the a-b plane reveals dramatic magnetic-induced changes to both the magnon and phonon spectra. For fields aligned along the magnetic easy-axis (a-axis), a splitting in the magnon mode occurs with increasing field, as well as evidence for a metamagnetic transition above 5 T, based upon which we can deduce several magnetic parameters for this material. Furthermore, by monitoring the field-induced changes in the Ru-O phonon frequency at various temperatures, we are able to map-out various field-induced orbital and magnetic phases for fields applied along both the hard and easy axis directions. *Work supported by NSF DMR02-44502, DOE DEFG02-91ER45439, and the Sony Scholar Fund. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U43.00007: Structural and Magnetic Behavior of a New Low Dimensional Cobalt Oxide John Mitchell, Hong Zheng, Laurent Chapon, Paolo Radaelli The study of transition metal oxide physics has been dominated by octahedral coordination of the transition metal, such as in perovksite manganites and cobaltites. A less common coordination geometry is the tetrahedron, whose weaker crystal field [10 \textit{Dq} (tetrahedron) = 4/9 10 \textit{Dq} (octahedron)] favors high-spin complexes across the periodic table. Here we discuss the crystal and magnetic structure of a recently-identified class of tetrahedrally coordinated mixed-valent cobalt oxides, RBaCo$_{4}$O$_{7}$ (R=Y, Tm, Yb, Lu). The structure of these compounds consists of planes of corner-sharing CoO$_{4}$ tetrahedra that form a Kagome net when considering only the Co ions. These planes are connected in the third dimension by yet another CoO$_{4}$ tetrahedral layer with a density 1/3 that of the Kagome plane. A full temperature-dependent neutron diffraction study on the Yb compound reveals a structural phase transition from trigonal ($P31c)$ to monoclinic (\textit{Cc}) on cooling through T=180 K. This first order transition is accompanied by an anomaly in the magnetization and a pronounced increase in resistivity. Below 75 K, broad superlattice lines appear. We discuss these findings in terms of Co spin states, the possibility of charge order of the Co$^{2+} $ and Co$^{3+}$ ions (formally a 3:1 ratio Co$^{2+}$/Co$^{3+})$, and low-dimensional magnetism engendered by the crystal structure. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U43.00008: Magnetic Field Induced Phases of the Strontium Ruthenates Rajeev Gupta, Minjung Kim, Harini Barath, S. Lance Cooper, Gang Cao Magnetic-field- and temperature-dependent Raman scattering has been used to investigate the magnetic-field-induced structural and magnetic phases of the triple-layer ruthenate system Sr$_{4}$Ru$_{3}$O$_{10}$ (Sr4310), which is a low temperature ferromagnet with T$_{C }$= 105 K. Magnetic-field-induced changes in the phonon spectra reveal dramatic spin-reorientation transitions and strong magnetoelastic coupling in this material. Further, the highly anisotropic field-induced effects observed for magnetic fields along the c-axis (magnetic easy axis) and ab-plane provide insight into the complex magnetic and structural (H,T) phase diagram of this material. We compare our magnetic-field dependent Raman results in Sr4310 with those of the quantum-critical bilayer material Sr$_{3}$Ru$_{2}$O$_{7}$, as well as with magnetic and transport measurements of Sr4310. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U43.00009: Coexistence of superconductivity and antiferromagnetism in multilayered high-$T_{\rm c}$ cuprates Michiyasu Mori, Sadamichi Maekawa We propose a mechanism for high critical temperature ($T_c$) in the coexistent phase of superconducting- (SC) and antiferromagnetic (AF) CuO$_2$ planes in multilayered cuprates. The Josephson coupling between the SC planes separated by an AF insulator (Mott insulator) is calculated perturbatively up to the fourth order in terms of the hopping integral between adjacent CuO$_2$ planes. The perturbative processes comprises two parts: The first provides a positive value of Josephson coupling called {\it 0-Josephson coupling}, while the second makes a negative contribution called {\it $\pi$-Josephson coupling}. We find that the AF exchange interaction suppresses the latter process, and allows the Cooper pair to tunnel through the Mott insulator. The fluctuations of the SC phase are suppressed by this long-ranged Josephson coupling, and it is this which enables the coexistence and a rather high value of $T_c$. [Preview Abstract] |
Thursday, March 24, 2005 10:36AM - 10:48AM |
U43.00010: Superconducting Clusters and Colossal Effects in Underdoped Cuprates Gonzalo Alvarez, Matthias Mayr, Adriana Moreo, Elbio Dagotto Phenomenological models for the antiferromagnetic vs. $d$-wave superconductivity competition in cuprates are studied[1] using conventional Monte Carlo techniques. The analysis suggests that cuprates may show a variety of different behaviors in the very underdoped regime: local coexistence, stripes, or, if disorder is present, states with nanoscale superconducting clusters. The transition from an antiferromagnetic to a superconducting state does not seem universal. In particular, inhomogeneous states lead to the possibility of colossal effects in some cuprates, analogous of those in manganites. Under suitable conditions, non-superconducting Cu-oxides could rapidly[2] become superconducting by the influence of weak perturbations that align the randomly oriented phases of the superconducting clusters in the mixed state. Consequences of these ideas for angle resolved photoemission and scanning tunneling microscopy experiments[3] will also discussed. {[}1{]} Alvarez et al., cond-mat/0401474, to appear in PRB. {[}2{]} I. Bozovic et al., PRL 93, 157002, (2004) {[}3{]} A. Ino et al., PRB 62, 4127 (2000); K. Lang et al, Nature 415, 412 (2002). Research performed in part at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-00OR22725. [Preview Abstract] |
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U43.00011: Magnetic coupling near the metal-insulator transition in RNiO3 J.-S. Zhou, J.B. Goodenough, B. Dabrowski In the temperature versus geometric tolerance factor t phase diagram of the RNiO$_{3}$ perovskite family, the N\'{e}el temperature T$_{N}$ increases with t, $i.e.$ the (180\r{ }- $\phi )$ Ni-O-Ni bond angle, until it is intercepted by an insulator-metal transition occurring at T$_{IM}$ that decreases with increasing t. Recent XAS data reveal that as T$_{N}$ approaches T$_{IM}$ in the insulator phase, large and small NiO$_{6/2}$ octahedra emerge locally although neutron and x-ray diffraction are fit well by an orthorhombic rather than monoclinic space group. The bonding has been shown to be vibronic where T$_{N}$ approaches T$_{IM}$. In order to probe how the interatomic exchange interactions evolves where the bonding is vibronic with T$_{N} \quad <$ T$_{IM}$, we have carried out a systematic measurement of the pressure dependence of T$_{N}$. This dependence was determined by tracking under pressure an anomaly of the resistivity $\rho $(T) that occurs at T$_{N}$. The coefficient dlnT$_{N}$/dP of GdNiO$_{3}$ falls into the range of values for magnetic insulators well-described by superexchange theory. However, this coefficient increases dramatically as t increases, reaching a maximum at lower pressure in SmNiO$_{3}$ before falling to zero; in Nd$_{0.5}$Sm$_{0.5}$NiO$_ {3}$ it is zero in the pressure interval where T$_{N} \quad <$ T$_{IM}$. A schematic magnetic phase diagram of T$_{N}$ versus the Ni-O-Ni electron transfer integral is presented. [Preview Abstract] |
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