Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W17: Focus Session: CMR Manganites |
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Sponsoring Units: DMP GMAG Chair: Jeffrey Lynn, NIST Center for Neutron Research Room: 319 |
Thursday, March 21, 2013 2:30PM - 2:42PM |
W17.00001: Paramagnetic Spin Fluctuations in Optimally Doped CMR Manganites La$_{0.7}A_{0.3}$MnO$_{3}$ ($A$ = Ca, Sr, Ba) Joel Helton, Jeffrey Lynn, Yiming Qiu, Yang Zhao, Dmitry Shulyatev, Yakov Mukovskii, Georgii Bychkov, Sergei Barilo Hole doped perovskites of the form La$_{0.7}A_{0.3}$MnO$_{3}$ (where $A$ = Ca, Sr, or Ba) display colossal magnetoresistance at a combined ferromagnetic and metal-insulator transition. The spin fluctuation spectrum of these materials develops a quasielastic spin diffusive central component that dominates the spectrum near $T_{C}$. We report inelastic neutron scattering measurements that reveal an additional and unexpected component to the spin fluctuation spectrum, in the form of anisotropic ridges of surprisingly strong quasielastic scattering running along ($H$~0~0) and equivalent directions. Temperature and field dependent measurements show that this scattering is most pronounced at temperatures in the paramagnetic phase and is suppressed by applied magnetic fields exceeding 10 Tesla. [Preview Abstract] |
Thursday, March 21, 2013 2:42PM - 2:54PM |
W17.00002: Role of covalency in ``Charge Ordering'' perovskite ferrates Antonio Cammarata, James Rondinelli Transition-metal oxides (TMO) with the perovskite crystal structure exhibit strong electron--electron correlation effects and complex structural distortions. The balance of those factors determines the stability of charge ordered states in chemistries susceptible to valence instabilities. We use first-principles density functional calculations to investigate the role of symmetry-unique structural distortions on covalent bonding in the ``charge-ordered'' insulator CaFeO$_3$. We evaluate the electronic density distribution along the Fe--O bonds to assess the ground state stability by tracing the evolution in the oxygen environment, which appears as octahedral expansion/contractions and rotations. We show that nearly zero charge transfer occurs; the insulating phase results from a complex interplay of symmetry-lowering structural distortions and enhanced covalent interactions. Finally, we discuss possible routes to control the metal--insulator transition by fine-tuning the covalency. [Preview Abstract] |
Thursday, March 21, 2013 2:54PM - 3:06PM |
W17.00003: Magnetic, structural and magneto-resistance studies of doped LaMnO$_3$ bulk samples prepared by citrate combustion process Himani Khanduri, Mukesh Chandra Dimri, Arvo Mere, Valdek Mikli, J\"{u}ri Krustok, Raivo Stern We present the structural and magnetic properties of polycrystalline samples of La$_{0.95}$A$_{0.05}$MnO$_{3}$ (where A = Na, Sr, Er, Dy and Ce) prepared by chemical citrate combustion method. Er substituted samples (La$_{1-x}$Er$_{x}$MnO$_{3}$ with x = 5, 10, 20 and 30\%) were also investigated, because their studies lack in the literature. The pervoskite structure was confirmed from X-ray diffraction and Raman spectra in these doped samples, excluding higher Er substituted samples (x $>$ 0.1). The grain sizes were around 2-3 micrometres for all of the sintered samples (at 1300$^{\circ}$C), whereas it was below 100 nm for the powders calcined at 600$^{\circ}$C, determined from the SEM images. Curie transition temperatures in those doped LMO bulk samples were found to be around 250K, which is higher than the ideal value ($\sim$140 K) for undoped samples. The increase in the Curie temperature can be related to non-stoichiometry and cation vacancies created due to higher/lower valence substitutions for trivalent La$^{3+}$ ions. The temperature dependence of resistivity also confirms the MIT transition in some of these samples. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W17.00004: Theory of K-edge resonant inelastic x-ray scattering and its application for La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ T.F. Seman, X. Liu, J.P. Hill, M. van Veenendaal, K.H. Ahn We present a formula based on tight-binding approach for the calculation of K-edge resonant inelastic x-ray scattering spectrum for transition metal oxides, by extending the previous result [K. H. Ahn, A. J. Fedro, and M. van Veenendaal, Phys. Rev. B 79, 045103 (2009).] to include explicit momentum dependence and a basis with multiple core hole sites. We apply this formula to layered charge, orbital, and spin ordered manganites, La$_{0.5}$Sr$_{1.5}$MnO$_{4}$. The K-edge RIXS spectrum is found not periodic with respect to the actual reciprocal lattice, but approximately periodic with respect to the reciprocal lattice for the hypothetical unit cell with one core hole site. With experimental strcuture and reasonable tight-binding parameters, we obtain good agreement with experimental data, in particular, with regards to the large variation of the intensity with momentum. We find that the screening in La$_{0.5}$Sr$_{1.5}$MnO$_{4}$ is highly localized around the core hole site and demonstrate the potential of K-edge RIXS as a probe for the screening dynamics in materials. [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W17.00005: LSCO: Consistent agreement for electronic structure and experimental X-ray spectra Y. Lee, B.N. Harmon, S. Medling, F. Bridges, H. Zheng, J.F. Mitchell, J.W. Freeland We have investigated magnetic properties of La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CoO$_{\mathrm{3}}$ (LSCO) as a function of Sr doping with X-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism(XMCD) at the O K edge and a first principles method. Experiment shows the peak of the oxygen XAS at beginning of the edge is increased with increasing Sr doping. The calculations, using supercells, are in good agreement with detailed XAS of the O K-edge as a function of doping. XMCD calculations reproduce the full experimental spectrum well, and show an increase of the magnetic moment on the oxygen with the number of Sr nearest neighbors. The calculations show that the hybridization involving Co d- and O-p electrons is the key factor for obtaining agreement with the changing XAS spectra as a function of doping. In this talk, we will discuss the XAS, XMCD results and the large external magnetic field effects on the ground state of LSCO(x$=$0). [Preview Abstract] |
Thursday, March 21, 2013 3:30PM - 3:42PM |
W17.00006: Crossover from Polaronic to Magnetically Phase-Separated Behavior in La$_{1-x}$Sr$_x$CoO$_3$ D. Phelan, S. El Khatib, S. Wang, J. Barker, J. Zhao, H. Zheng, J.F. Mitchell, C. Leighton Dilute hole-doping in La$_{1-x}$Sr$_x$CoO$_3$ leads to the formation of ``spin-state polarons'' where a non-zero spin-state is stabilized on the nearest Co3+ ions surrounding a hole [1]. Here, we discuss the development of electronic/magnetic properties of this system from non-magnetic x=0, through the regime of spin-state polarons, and into the region where longer-range spin correlations and phase separation develop. We present magnetometry, transport, heat capacity, and small-angle neutron scattering (SANS) on single crystals. Magnetometry indicates a crossover with x from Langevin-like behavior (polaronic) to a state with a freezing temperature and finite coercivity. Fascinating correlations with this behavior are seen in transport measurements, the evolution from polaronic to clustered states being accompanied by a crossover from Mott variable range hopping to intercluster hopping. SANS data shows Lorentzian scattering from short-range ferromagnetic clusters first emerging around x = 0.03 with correlation lengths of order two unit cells. We argue that this system provides a unique opportunity to understand in detail the crossover from polaronic to truly phase-separated states.\\[4pt] [1] A. Podlesnyak et al., Phys. Rev. Lett. 101, 247603. [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 3:54PM |
W17.00007: Energy-loss magnetic circular dichroism measurements of ferromagnetic ordering in LaSrCoO$_3$ Ahmet Gulec, Robert F. Klie, John F. Mitchell Experimental results show that tuning the ferromagnetism of LaSrCoO$_3$ can be achieved at various temperatures by doping bulk sample with smaller atoms or straining thin film sample. In this work, we will use atomic-resolution Z-contrast imaging, annular bright field (ABF) imaging and electron energy-loss spectroscopy in the aberration-corrected JEOL JEM-ARM200CF in combination with in-situ heating and cooling experiments to examine the magnetic and spin-state transitions in La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CoO$_3$ (x$=$0-0.3) between 80 K and 600 K. Using energy-loss magnetic circular dichroism method, we confirm the magnetic ordering transition at room temperature with increasing doping concentrations. Differences in the O K- and Co-L-edges will be utilized to determine the Co valence of the samples. A magnetic transition is observed in 5{\%} doped sample during in-situ cooling experiment to 95 K. Additionally, with increasing the doping concentration, a change in crystal structure is measured using ABF imaging, more specifically a distortion of the CoO$_6$ octahedra. [Preview Abstract] |
Thursday, March 21, 2013 3:54PM - 4:06PM |
W17.00008: Metastable low-spin character of Co$^{2+}$ and the control of spin state transition Bongjae Kim, B. I. Min We have studied different spin states of the octahedrally coordinated Co$^{2+}$ systems. For every tested systems, we found metastable character of low-spin phase and, interestingly, the energy differences between the high-spin and low-spin phases are similar regardless of the anion (X) type, Co$^{+2}$-X bond lengths and CoX$_{6}$ octahedron distortion. For CoCl$_{2}$ as a model system, we studied pressure-induced high-spin to low-spin state transition, which is governed by $J$/$\Delta_{CF}$ value ($J$: exchange parameter, $\Delta_{CF}$: crystal-field parameter). CoCl$_{2}$ shows sudden collapse of volume and spin moment at the point of spin state transition together with the insulator-to-metal transition. Unlike the other transition-metal oxides, which shows pressure-driven Mott-type transition, physics of CoCl$_{2}$ is determined mainly by $J$ and $\Delta_{CF}$, not by $U$ and $W$. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W17.00009: Pressure Effect on the Structural Transition and Suppression of the High-Spin State in the Triple-Layered T'-La4Ni3O8 J.-G. Cheng, J.-S. Zhou, J.B. Goodenough, H.D. Zhou, K. Matsubayashi, Y. Uwatoko, P.P. Kong, C.Q. Jin, W.G. Yang, G.Y. Shen We have carried out a comprehensive high-pressure study on the triple-layer T'-La4Ni3O8 with a suite of experimental probes, including structure determination, magnetic, and transport properties up to 50 GPa. Consistent with a recent ab inito calculation [1], application of hydrostatic pressure suppresses an insulator-metal spin-state transition at Pc $\sim$ 6 GPa. However, a low-spin metallic phase does not emerge after the high-spin state is suppressed to the lowest temperature. For P $>$ 20 GPa, the ambient T' structure transforms gradually to a T'-type structure, which involves a structural reconstruction from fluorite La-O2-La blocks under low pressures to rock-salt LaO-LaO blocks under high pressures. Absence of the metallic phase under pressure has been discussed in terms of local displacements of O2- ions in the fluorite block under pressure before a global T* phase is established [2]. Ref. [1] V. Pardo and W. E. Pickett, Phys. Rev. B 85, 045111 (2012). [2] J.-G. Cheng, et al. Phys. Rev. Lett. 108, 236403(2012). [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W17.00010: Lattice and transport properties of the misfit-layered oxide thermoelectric Ca$_3$Co$_4$O$_9$ from first principles Alejandro Rebola, Robert Klie, Peter Zapol, Serdar Ogut The misfit-layered oxide Ca$_3$Co$_4$O$_9$ (CCO) has recently been the subject of many experimental and some theoretical investigations due to its remarkable thermoelectric properties. CCO is composed of two incommensurate subsystems, a distorted rocksalt-type Ca$_2$CoO$_3$ layer sandwiched between hexagonal CoO$_2$ layers. Taking into account that the composition ratio between these subsystems is very close to the golden mean, which is the limit of the sequence of the ratios of consecutive Fibonacci numbers $F(n)$, we model CCO from first principles\footnote{A. Rebola, R. F. Klie, P. Zapol, and S. Ogut, Phys. Rev. B {\bf 85}, 155132 (2012)} by using rational approximants of composition [Ca$_2$CoO$_3$]$_{2F(n)}$[CoO$_2$]$_{2F(n+1)}$. In the present study, we use 3/2 and 5/3 rational approximants and PBE+U computations to calculate the {\em ab initio} phonon dispersion curves, related thermal properties, as well as {\em ab initio} electronic transport properties such as DC conductivity and thermopower within the relaxation time approximation by applying the Boltzmann transport theory. Results are compared with available experimental data and potential routes for increasing the thermopower of CCO are discussed. [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W17.00011: Magnetostructural transitions and metamagnetism induced by Ising spins in spinel-rock salt intergrowth Co$_{10}$Ge$_3$O$_{16}$ Phillip Barton, Ram Seshadri Co$_{10}$Ge$_3$O$_{16}$ crystallizes in an intergrowth structure featuring alternating layers of spinel and rock salt, making it related to GeCo$_2$O$_4$. Variable-temperature synchrotron X-ray powder diffraction, magnetometry, and heat capacity experiments reveal a magnetostructural transition at antiferromagnetic $T_N$ = 205 K. This rhombohedral-to-monoclinic transition involves a slight elongation of the CoO$_6$ octahedra. Curie-Weiss analysis suggests that the Co$^{2+}$, with $S$ = 3/2 and $L$ = 3, acts as a Kramer's doublet due to spin-orbit coupling. Below $T_N$, the Ising-like Co$^{2+}$ causes spin reorientation at high applied magnetic field that is first seen as an upward kink in $M$-$H$ near $H_C$ = 3.9 T. A ``butterfly'' loop emerges when $T <$ 150 K, with the transition causing hysteresis at high fields while linear and reversible behavior persists at low fields. $H_C$ decreases as temperature is lowered and the loops at positive and negative fields merge beneath $T$ = 20 K. The low-temperature behavior is complicated by a field-induced first-order transition that is observed in temperature-dependent measurements for $H >$ 1000 Oe. We discuss the $H$-$T$ phase diagram with reference to other measurements including neutron powder diffraction and high-field magnetometry. [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W17.00012: Anisotropy and Magnetostriction in Cobalt-Modified Magnetite: A Crystal Field Approach Cajetan Nlebedim, David Jiles The anisotropy and magnetostrictive properties of magnetite are altered by the introduction of cobalt ions into the spinel crystal lattice. 4{\%} of Co$^{2+}$ substituted for Fe$^{2+}$ changes both the sign and magnitude of magnetocrystalline anisotropy coefficient. Such strong dependence can be useful for tailoring the properties of cobalt-iron oxides for applications. This is especially important, considering that cobalt ferrite materials prepared for magnetostrictive, multiferroic and other related applications often deviate from targeted or stoichiometric compositions. In this study, magnetite has been systematically modified by substitution of cobalt. The changes in anisotropy and magnetostriction have been studied and can be explained using the single ion model. The agreement between the trend observed in this experimental investigation and previous theoretical studies is noteworthy. The variation in anisotropy and magnetostriction will be presented on the basis of two competing factors; the unquenched orbital angular momentum of Co$^{2+}$ and changes in the crystal field due to Co$^{2+}$ substitution. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W17.00013: Thermal expansion study of anisotropic magnetolattice coupling and antiferromagnetic transition in CuO John J. Neumeier, Alwyn Rebello, Samuel Viall, Zachary C.M. Winter Transition metal oxides have been the subject of intense research over the past few decades since they form the basic building block of many materials showing exotic properties such as high temperature superconductivity, spin and charge ordering, magnetoresistance, multiferroicity etc. Recently, Kimura et al. demonstrated an intriguing coupling between electric and magnetic dipole ordering in CuO, which opened a new route for finding materials exhibiting induced multiferroic behavior.\footnote{T. Kimura et al., Nature Mater. \textbf{7}, 291 (2008).} Here we present results on anisotropic thermal expansion in single crystalline CuO in the temperature range $5 < T < 350$ K. Our results demonstrate anisotropic magnetolattice coupling in CuO around the two known antiferromagnetic phase transitions at $T_{N1}$= 230 K and $T_{N2}$= 213 K. We also discuss the pressure dependence of $T_{N1}$ and critical behavior in CuO using the scaling of heat capacity and thermal expansion data. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W17.00014: Ab initio study on magnetic coupling in A-site-ordered perovskite CaCu3B4O12 (B=Ti, Ge, Zr, and Sn) Masayuki Toyoda, Kunihiko Yamauchi, Tamio Oguchi Magnetism of A-site-ordered perovskites, CaCu$_3$Ti$_4$O$_{12}$, CaCu$_3$Ge$_4$O$_{12}$, CaCu$_3$Sn$_4$O$_{12}$, and CaCu$_3$Zr$_4$O$_{12}$, is comprehensively studied by ab initio electronic structure calculations. The magnetic exchange constants between Cu spins, $J_1$, $J_2$ and $J_3$, are estimated via an effective Heisenberg model, which reveals relative importance of $J_3$ despite its long interaction length. The ground-state magnetic order is reasonably explained by combination of relatively weak ferromagnetic super-exchange interaction ($J_{1}$ and $J_{2}$) and dominant super-exchange interaction ($J_{3}$) which can be tuned by replacement of the B-site element. We will also discuss the effect of A-site-cation replacement by comparing with the results of other A-site-ordered perovskite materials. [Preview Abstract] |
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