Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X27: Focus Session: Chemical Control of the Properties of Complex Oxides I |
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Sponsoring Units: DCP Chair: Patrick Woodward, Ohio State University Room: D137 |
Thursday, March 18, 2010 2:30PM - 3:06PM |
X27.00001: Chemical control of geometrically frustrated magnets Invited Speaker: The chemistry of geometrically frustrated magnets - materials which are composed of networks of spins which typically cannot find conventional ordered ground states - has in the past been largely restricted to a few archetypes, including the pyrochlore lattice, the spinel lattice, and variants of the perovskite lattice. Even though these compounds have exhibited a rich variety of low-temperature ground states (such as spin glassiness in the absence of chemical disorder, spin ice states, and spin liquid states), new materials are now being discovered which test current models of magnetism within solids. This talk will outline new directions in geometrically frustrated magnetism research, and in particular will concentrate on how chemical control within these materials can be used to bring about unusual physical phenomena. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:18PM |
X27.00002: Bismuth Perovskites: Bi$_{2}$Mn$_{4/3}$Ni$_{2/3}$O$_{6}$ John Claridge, Helen Hughes, Matthew Suchomel, Mathieu Allix, Matthew Rosseinsky Materials in which dielectric and magnetic properties are coupled are of interest for multiple state memory and information storage applications, and fundamentally in terms of the mechanisms for coupling these properties. In studies of Bi$^{3+}$ A site perovskites, we have isolated a number of interesting and structurally complex phases. Bi$_{2}$Mn$_{4/3}$Ni$_{2/3}$O$_{6}$ is a very unusual material as it adopts the perovskite structure with Bi$^{3+}$ on the A site -- there has only been one previous example of perovskite stable to ambient pressure synthesis, BiFeO$_{3}$. Structurally the material is very interesting -- it is very slightly incommensurate, it can be described in terms of the 3+2 dimmensional space group Ibmm(0-p0,q00)mm.ss ($\surd $2a$_{p}$ x 2a$_{p}$ x $\surd $ 2a$_{p}$ ; p,q $\sim \quad \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$. This modulation is responsible for suppressing the polarisation apparent in single crystal diffraction studies with conventional q-resolution. The modulation can be thought of as arising due to the need to accommodate three distinct cations on the octahedral site and the asymmetric low coordination number environment preferred by Bi$^{3+}$. [Preview Abstract] |
Thursday, March 18, 2010 3:18PM - 3:30PM |
X27.00003: Studies of the electronic structures of tantalum oxynitride perovskites, $A$TaO$_{2}$N (A = Ca, Sr, Ba) and PrTaON$_{2}$ Spencer Porter, Young-Il Kim, Patrick Woodward Transition metal oxynitrides are of interest for use as photocatalysts, pigments, battery electrodes, high-permittivity dielectrics, etc. The tantalum oxynitride perovskites are of particular interest because they have band gaps that fall in the visible range of the spectrum and show excellent chemical stability. For applications such as photocatalysis the absolute positions of the valence and conduction bands play an important role. The valence and conduction band positions of $A$TaO$_{2}$N (A = Ca, Sr, Ba) and RTaON$_{2}$ (R = La, Pr) are characterized using a combination of diffuse reflectance UV-Visible spectroscopy, density functional theory calculations, X-ray photoelectron spectroscopy and photoelectrochemical measurements. Changes in the position of the conduction band edge, resulting from changes in the width of the conduction band, are found to play the major role in changing the band gap energy. Studies of the band positions are augmented with photocatalytic measurements demonstrating the link between band positions and photocatalytic activity. [Preview Abstract] |
Thursday, March 18, 2010 3:30PM - 4:06PM |
X27.00004: Chemical control of the properties of perovskite oxides Invited Speaker: Perovskite oxides show a variety of interesting properties that can be tuned by chemical control. In this talk, I will present three examples of how such approach can be used to study the nature of functional properties in perovskites: (1) $R$MnO$_{3}$ ($R$=rare earth) show a variety of unusual states, including the spiral spin ordering and ferroelectricity in $R$=Tb and Dy. In [1], $R$=Ho-Lu have been obtained under high pressure, and their magnetic and structural properties have been studied. Combined with the data on larger $R$, the results show the importance of competing magnetic interactions on the complex phase diagram of $R$MnO$_{3}$. (2) $R$CoO$_{3}$ show a spin-state transition and an insulator-metal transition as a function of temperature. The nature of the excited states has been studied since the 1950's, but remains elusive. Here [2], I provide the complete electronic phase diagram of $R$CoO3 that has been obtained from high-pressure synthesis and heat capacity measurements. The results support a picture involving a high-spin state above the spin-state transition and an intermediate-spin state above the insulator-metal transition. (3) Pb(Mg$_{1/3}$Nb$_{2/3})$O$_{3}-x$PbTiO$_{3}$ (PMN-$x$PT) is a relaxor ferroelectric system with extraordinary dielectric and piezoelectric properties. The average structure of the system changes from cubic to rhombohedral, monoclinic, and tetragonal with $x$. However, this system is also characterized by nanoscale phase inhomogeneities, and the role of polar nanoregions on the enhanced properties is not clear. Here [3], I will show that thermal conductivity and heat capacity of PMN-$x$PT show a systematic evolution from glasslike to crystalline behavior as a function of $x$. The results provide interesting perspectives on how polar nanoregions are transformed into macroscopic polarizations with increasing $x$. \\[4pt] [1] M. Tachibana et al., Phys. Rev. B 75, 144425 (2007). \\[0pt] [2] M. Tachibana et al., Phys. Rev. B 77, 094402 (2008). \\[0pt] [3] M. Tachibana et al., Phys. Rev. B 79, 100104(R) (2009). [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X27.00005: Chemical, magnetic and orbital order in the substituted double perovskite Sr$_{(1{\-}x)}$Ca$_{(x)}$Mn$_{0.5}$Ru$_{0.5}$O$_{3}$ Rebecca Ricciardo, Patrick Woodward, Heather Cuthbert, Qingdi Zhou, Brendan Kennedy, Zhaoming Zhang, Maxim Avdeev, Ling-Yun Jang The structural and magnetic properties of Sr$_{(1{\-}x)}$Ca$_{(x)}$Mn$_{0.5}$Ru$_{0.5}$O$_{3}$ have been investigated. SrMn$_{0.5}$Ru$_{0.5}$O$_{3}$ exhibits antiferromagnetic ordering, T$_{N}$~$\approx $~200~K. Neutron powder diffraction of this perovskite indicates the presence of orbital ordering of the occupied Mn$^{+3}$ d$_{z}$2 orbitals, stabilizing the spin ordering corresponding to the AFM C-type structure. The substitution of smaller Ca$^{+2}$ for Sr$^{+2}$ on the A-site induces a change in the octahedral tilt system, (a$^{0}$a$^{0}$c$^{-})$ to (a$^{-}$b$^{+}$a$^{-})$ forcing a loss of this type of orbital ordering for x~$\ge $~0.2. This is accompanied by a crossover to a ferromagnetic ground state with 200K $\le $ T$_{C }\le $ 300K even in the absence of long range chemical order. Magnetic data and neutron powder diffraction of CaMn$_{0.5}$Ru$_{0.5}$O$_{3}$ and Sr$_{0.5}$Ca$_{0.5}$Mn$_{0.5}$Ru$_{0.5}$O$_{3}$ reveal reduced magnetic moments and imply a complex magnetic behavior as well as a complete loss of orbital ordering for CaMn$_{0.5}$Ru$_{0.5}$O$_{3}$. X-ray absorption studies show increased electron transfer from Mn to Ru. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X27.00006: First principles study of magnetism and defect energetics in non-stoichiometric Sr$_{2}$FeMoO$_{6}$ R. Mishra, O.D. Restrepo, W. Windl, P.M. Woodward The influence of disorder and stoichiometry-breaking point defects on the structural and magnetic properties of Sr$_{2}$FeMoO$_{6}$ have been investigated with the help of electronic structure calculations within the GGA+U approach. Defining the chemical potentials of the constituent elements from constitutional defects, we calculate the energetics of the possible point defects in non-stoichiometric Sr$_{2}$FeMoO$_{6}$ and find transition metal-ion antisites and oxygen vacancies to be the dominant point defects. In non-stoichiometric Sr$_{2}$Fe$_{1+x}$Mo$_{1-x}$O$_{6}$, both Fe$_{Mo}$ antisites and Mo$_{Fe}$ antisites lead to a systematic decrease in saturation magnetization ($M_{s})$. Only Mo$_{Fe}$ antisites destroy the half metallic character of the electronic structure. Oxygen vacancies also reduce the $M_{s}$, but the half-metallicity is retained. The optimized unit cell lattice parameters stay within a relatively narrow range despite large changes in composition. In stoichiometric Sr$_{2}$FeMoO$_{6}$, the $M_{s}$ decreases linearly with increasing Fe/Mo antisite disorder with loss in half-metallicity. The calculated results are in excellent quantitative agreement with experimental values. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 5:06PM |
X27.00007: Realizing Half Metallicity in Sr$_{2}$FeMoO$_{6}$ Epitaxial Films: Roadblocks and Successes Invited Speaker: Half-metallic Sr$_{2}$FeMoO$_{6}$ has attracted much attention due to its high Tc of 420 K for magnetoelectronic applications. However, the potential of its half metallicity is far from being realized due to the unusual challenges in the fabrication of Sr$_{2}$FeMoO$_{6}$ films. In this talk, I will discuss a number of hurdles that troubled the deposition of Sr$_{2}$FeMoO$_{6}$ films, some of which have been largely ignored to date. We have succeeded in overcoming some of the dominant problems in the epitaxial film growth and obtained phase-pure, fully epitaxial, and stoichiometric Sr$_{2}$FeMoO$_{6}$ films on SrTiO$_{3}$ with high structural and magnetic ordering using off-axis ultrahigh vacuum sputtering. First, by precisely controlling the growth environment, we achieved pure double perovskite phase and complete epitaxy in Sr$_{2}$FeMoO$_{6}$ films without any detectable secondary phases (such as SrMoO$_{4})$ as confirmed by Bragg-Brentano and triple-axis X-ray diffraction (XRD) and aberration-corrected TEM. Secondly, we discovered using Rutherford backscattering (RBS) that the films have much more Mo than Fe under typical growth conditions (high sputter pressure) for complex oxides. The optimal pressure for obtaining stoichiometric films is around 10 mTorr at certain off-axis geometry. Next, we focused on improving the Fe/Mo ordering by tuning growth rate, substrate temperature and sputtering pressure. To date, the highest Fe/Mo ordering parameter we have is around 90{\%} obtained by Rietveld refinements on epitaxial Sr$_{2}$FeMoO$_{6}$ (111) films with ordered double perovskite XRD peaks. More importantly, the Sr$_{2}$FeMoO$_{6}$ films exhibit strong magnetic shape anisotropy, i.e. the in-plane hysteresis loops are fairly square and the out-of-plane loops are perfectly slanted lines with a saturation filed of $\sim $3800 Oe. The clear shape anisotropy, which has never been seen before in Sr$_{2}$FeMoO$_{6}$ films, indicates strong magnetic coupling across the films. We are pursuing further improvement of the Sr$_{2}$FeMoO$_{6}$ film quality and incorporating it into magnetic heterostructures for magnetotransport studies. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X27.00008: Effective Spin Hamiltonian, Magnetic Ground States and Low-Energy Excitations of Double Perovskites Onur Erten, Anamitra Mukherjee, Nandini Trivedi, Mohit Randeria, Patrick Woodward We investigate the T=0 properties of the generalized double exchange model for double perovskites (DP) A$_{2}$BB$^{\prime}$O$_{6}$. We present exact analytical results for the effective spin Hamiltonian of a system consisting of two unit cells. This is a generalization of the classic Anderson-Hasegawa analysis for manganites to DPs. Using a variational approach for the core spins, together with an exact diagonalization of the conduction electrons, we determine the magnetic phase boundaries as a function of electron concentration. We find that the wide region of stability for the FM phase depends on the magnitude and the sign of direct $B^\prime$-$B^\prime$ hopping and a metallic AFM phase at larger doping. Coulomb correlations on the $B^\prime$ site in a self-consistent Hartree-Fock theory, shows that these are unable to stabilize the FM state beyond a critical filling. We compare our results with experiments on La$_{x}$Sr$_{(2-x)}$FeMoO$_{6}$. We compute the spin wave spectrum, and determine the doping dependence of the effective ferromagnetic exchange and compare with exact results on small clusters. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X27.00009: Systematic study of the effect of growth conditions on the composition, structure and properties of Sr$_{2}$FeMoO$_{6}$ films grown via pulsed laser deposition Tricia Meyer, Rebecca Ricciardo, Patrick Woodward, Thomas Lemberger Perfect Sr$_{2}$FeMoO$_{6}$ is thought to be a 100{\%} spin polarized half metallic ferrimagnet (HMF). This feature coupled with its Curie temperature (420 K) well above room temperature, make it an attractive material for spintronic applications. Thin films of Sr$_{2}$FeMoO$_{6}$ have been grown by pulsed laser deposition (PLD) and their properties characterized using X-ray diffraction, Rutherford Backscattering, transmission electron microscopy, atomic force microscopy and SQUID magnetometry. Films have been grown at substrate temperatures ranging from 800-850 \r{ }C and pressures ranging from 30-150~mTorr in both inert (Ar) and reducing (Ar+H$_{2})$ atmospheres. The role of substrate lattice match has also been explored. As will be shown in this presentation the phase purity, stoichiometry, Fe/Mo order and magnetic properties are sensitive to all of these variables. [Preview Abstract] |
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