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Wang X.D.,Zhejiang University | Cao Q.P.,Zhejiang University | Jiang J.Z.,Zhejiang University | Franz H.,Hamburger Synchrotronstrahlungslabor Am Deutschen Elektronen Synchrotron | And 8 more authors.
Scripta Materialia

Structural evolution of an Au-based bulk metallic glass (BMG) after severe plastic deformation (SPD) was investigated. The newly formed glass contains high-density shear bands, a reduced ordering and a concomitant excess free volume. Moreover, it exhibits a less-changed local structure even in the supercooled liquid region, but a reduced thermal stability reflected in an accelerated crystal nucleation and growth process. These results suggest that SPD modifies the atomic structure of BMGs by localized shear band formation, thus producing so-called nanoglasses. © 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Wunderlich F.,TU Dresden | Leisegang T.,TU Dresden | Weissbach T.,TU Dresden | Zschornak M.,TU Dresden | And 9 more authors.
Physical Review B - Condensed Matter and Materials Physics

In YMn2 O5, the Mn atoms occupy two nonequivalent Wyckoff sites within the unit cell exhibiting different oxygen coordinations, i.e., the system can be characterized as a mixed-valence compound. For the formation of the orthorhombic crystal structure, Jahn-Teller distortions are assumed to play an important role. In this study, we aimed at the investigation of the crystal structure changes upon the substitution of Mn by the non-Jahn-Teller cation Fe3+. Therefore, we synthesized a series of YMn2-x Fex O5 powder samples with x=0, 0.5, and 1 by a citrate technique. We utilized extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) analysis as well as density-functional theory (DFT) to investigate the two nonequivalent Wyckoff sites within the orthorhombic crystal structure (confirmed for all compositions) occupied by transition-metal atoms. For quantitative determination of structural short-range order, all plausible options of substitution of Fe for Mn are discussed. On the basis of these evaluations, the EXAFS and XANES behavior is analyzed and appropriate crystallographic weights are assigned to the subset of structural models in accordance with the experimental data. From EXAFS analysis, using multiple-scattering theory, we conclude only the 4h Wyckoff site to be occupied by Fe [occupancy refined is (100±3) % in case of x=1]. Furthermore, taking the XANES spectra into account, we are able to verify the EXAFS results and additionally explain the differences in the MnK XANES spectra in dependence on x to be caused by changes in the dipole transitions to 4p final states. From quantitative pre-edge analysis an oxidation number of +4 for the Mn atom for x=1 is determined whereas the Fe valence is shown to be unchanged. Since the substitution process only involves one Wyckoff site, the experimentally observed limit to a maximum amount of x=1 is explained. Additionally, a possible disorder, discussed in the literature, is not proven for our samples. With DFT calculations, the experimental findings are verified on the basis of the total energy of the different possible electronic configurations. Crystal-field effects are identified to be responsible for the site-selective substitution of Fe for Mn. © 2010 The American Physical Society. Source

Mikhailova D.,Max Planck Institute for Chemical Physics of Solids | Mikhailova D.,Leibniz Institute for Solid State and Materials Research | Schwarz B.,Leibniz Institute for Solid State and Materials Research | Senyshyn A.,TU Munich | And 14 more authors.
Physical Review B - Condensed Matter and Materials Physics

We have studied the magnetic properties and crystal structure of Sr 3CoIrO 6 and Sr 3NiIrO 6 as a function of temperature. Two characteristic temperatures, T 1=90 K and T 2=25 K for Sr 3CoIrO 6, and T 1=85 K and T 2=15 K for Sr 3NiIrO 6, were observed. Below T 1 a significant increase of magnetization and below T 2 a weak temperature dependence of magnetization in the field-cooled and practically zero magnetization values in the zero-field-cooled mode were detected for both compounds. The existence of Ir4 + in Sr 3CoIrO 6 was confirmed by an Ir-L III x-ray absorption measurement. Magnetoelastic effects have been observed in the temperature dependence of the lattice parameters of Sr 3CoIrO 6 and Sr 3NiIrO 6. The magnetic structure of Sr 3CoIrO 6 in zero fields can be described as a commensurate modulated antiferromagnet with a propagation vector k=(0,0,1). Neutron powder diffraction with polarized neutrons gave evidence of short-range magnetic order, above and below the magnetic ordering temperature. © 2012 American Physical Society. Source

Kuschel T.,University of Osnabruck | Becker T.,University of Osnabruck | Bruns D.,University of Osnabruck | Suendorf M.,University of Osnabruck | And 3 more authors.
Journal of Applied Physics

Thin Co films of different thickness deposited on glass are investigated by magnetooptic Kerr effect to study the uniaxial magnetic anisotropy of these films. The direction of the uniaxial magnetic anisotropy is determined from the azimuthal dependence of the magnetic remanence and differs with increasing thickness of the Co film investigated by x-ray reflectivity. Our experiments reveal that preparation conditions like temperature, deposition rate, or obliqueness of deposition cannot be the reason for this rotation effect of the uniaxial magnetic anisotropy. Also, strain in the substrate and possible textures in the film structure can be excluded as the origin of the magnetic behavior as studied by grazing incidence wide angle x-ray scattering. Thus, probably only the substrate shape in connection with the amorphous or polycrystalline film structure can explain the rotation of the uniaxial magnetic anisotropy. © 2011 American Institute of Physics. Source

Bertram F.,Hamburger Synchrotronstrahlungslabor Am Deutschen Elektronen Synchrotron | Deiter C.,Hamburger Synchrotronstrahlungslabor Am Deutschen Elektronen Synchrotron | Hoefert O.,Friedrich - Alexander - University, Erlangen - Nuremberg | Schemme T.,University of Osnabruck | And 4 more authors.
Journal of Physics D: Applied Physics

Epitaxial ultrathin iron oxide films of different thicknesses were grown by reactive molecular beam epitaxy in 10 -6mbar oxygen atmosphere on MgO(001) single crystal substrates at room temperature. Afterwards, the films were studied by x-ray diffraction, x-ray reflectivity and x-ray photoelectron spectroscopy to provide information regarding film structure as well as chemical composition of the films. Except for a very thin interface layer of subnanometre thickness, the iron oxide films have magnetite stoichiometry and structure and Mg does not diffuse from the substrate into the iron oxide film. The interface layer has a wuestite structure as determined by kinematic diffraction analysis. The magnetite films exhibit very homogeneous thickness while the vertical lattice constant decreases gradually towards its bulk value. © 2012 IOP Publishing Ltd. Source

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