Showing papers by "Markus Janousch published in 2006"

LUCIA, a microfocus soft XAS beamline

Abstract: The beamline “LUCIA” (line for ultimate characterization by imaging and absorption) is a “tender” (0.8–8 keV) X-ray microprobe with capabilities for chemical speciation by micro-X-ray absorption spectroscopy (μ-XAS) and for elemental mapping by X-ray micro-fluorescence (μ-XRF). It allows the possibility to study heterogeneous samples at a micrometer scale and to combine these two element-specific and non-destructive techniques. A monochromatic beam of a few micrometer in size is incident on a sample which is mounted on a scanning x–y–z stage. μ-XRF shows the location of the elements, their relative abundances, and their association with other elements. One can take advantage of the monochromatic beam which allows separating out different elements by their absorption edges. After mapping the fluorescence, spots of interest can be analysed by XAS to determine the speciation (local chemistry, quantitative determination of the local geometric structure around the absorbing atom) of the elements and how they depend on the different components. Installed at first at the SLS of the Paul Scherrer Institute (Switzerland), the LUCIA beamline will be transferred to SOLEIL by the beginning of 2008. The energy range offered by the beamline corresponds to the best performances of SLS and SOLEIL in terms of brightness. It allows XAS experiments at the K edge of elements ranging from Na to Fe, L edges from Ni to Gd, and M edges of rare earths and actinides.

Role of magnetic and orbital ordering at the metal-insulator transition in NdNiO 3

Abstract: Soft x-ray resonant scattering at the Ni ${L}_{2,3}$ edges is used to test models of magnetic- and orbital-ordering below the metal-insulator transition in ${\mathrm{NdNiO}}_{3}$. The large branching ratio of the ${L}_{3}$ to ${L}_{2}$ intensities of the $(1∕2\phantom{\rule{0.3em}{0ex}}0\phantom{\rule{0.3em}{0ex}}1∕2)$ reflection and the observed azimuthal angle and polarization dependence originates from a noncollinear magnetic structure. The absence of an orbital signal and the noncollinear magnetic structure show that the nickelates are materials for which orbital ordering is absent at the metal-insulator transition.

Charge/orbital ordering vs. Jahn-Teller distortion in La0.5Sr1.5MnO4

Abstract: Temperature dependences of a magnetic and an orbital Bragg reflection at the Mn L2,3 edges of La0.5Sr1.5MnO4 have been collected. The temperature evolution of the orbital reflection depends strongly on the X-ray energy and reflects the different order parameters, which are assigned to orbitals and Jahn-Teller distortions. The Jahn-Teller order parameter saturates at 150 K, whereas the orbital order parameter gradually increases. These results are discussed in context with the possible driving forces, the orbital-orbital super exchange and the Jahn-Teller strain field interaction.