75 research outputs found
Measurement of Magnetic Moment at the Atomic Scale in a High TC Molecular Based Magnet
The molecular-based magnet Cs^(I) [Ni^(II) Cr^(III) (CN)6]-2H2O is a ferromagnetic with a Curie temperature TC ) 90
K. Its structure consists of face-centered cubic lattice of Ni^(II) ions connected by Cr(CN)6 entities. We have
recorded X-ray magnetic circular dichroism (XMCD) at nickel L2,3 edges. It clearly evidences that nickel(II)
is in a high-spin configuration and ferromagnetically coupled to the surrounding Cr^(III) . Through ligand field
multiplet calculations, we have determined the total magnetic moment carried by Ni^(II) . Special attention has
been paid to the magnetic anisotropy that complicates the calculation of the cross section for a powder. By
using sum rules derived for XMCD, it has been possible to extract the orbital and spin contributions to the
total magnetic moment. A somewhat too small magnetic moment is found on nickel. A complete calculation
taking into account the multiplet coupling effect and the covalent hybridization allowed to determine the
precise ground state of nickel and showed that hybridization cannot be responsible for the experimental low
nickel magnetic moment. The origin of this effect is discussed
Experimental evidence of thermal fluctuations on the X-ray absorption near-edge structure at the aluminum K-edge
After a review of temperature-dependent experimental x-ray absorption
near-edge structure (XANES) and related theoretical developments, we present
the Al K-edge XANES spectra of corundum and beryl for temperature ranging from
300K to 930K. These experimental results provide a first evidence of the role
of thermal fluctuation in XANES at the Al K-edge especially in the pre-edge
region. The study is carried out by polarized XANES measurements of single
crystals. For any orientation of the sample with respect to the x-ray beam, the
pre-edge peak grows and shifts to lower energy with temperature. In addition
temperature induces modifications in the position and intensities of the main
XANES features. First-principles DFT calculations are performed for both
compounds. They show that the pre-edge peak originates from forbidden 1s to 3s
transitions induced by vibrations. Three existing theoretical models are used
to take vibrations into account in the absorption cross section calculations:
i) an average of the XANES spectra over the thermal displacements of the
absorbing atom around its equilibrium position, ii) a method based on the crude
Born-Oppenheimer approximation where only the initial state is averaged over
thermal displacements, iii) a convolution of the spectra obtained for the atoms
at the equilibrium positions with an approximate phonon spectral function. The
theoretical spectra so obtained permit to qualitatively understand the origin
of the spectral modifications induced by temperature. However the correct
treatment of thermal fluctuation in XANES spectroscopy requires more
sophisticated theoretical tools
Determination of the cation site distribution of the spinel in multiferroic CoFe2O4 / BaTiO3 layers by X-ray photoelectron spectroscopy
International audienceThe properties of CoFe2O4/BaTiO3 artificial multiferroic multilayers strongly depend on the crystalline structure, the stoichiometry and the cation distribution between octahedral (Oh) and tetrahedral (Td) sites (inversion factor). In the present study, we have investigated epitaxial CoFe2O4 layers grown on BaTiO3, with different Co/Fe ratios. We determined the cation distribution in our samples by X-ray magnetic circular dichroism (XMCD), a well accepted method to do so, and by X-ray photoelectron spectroscopy (XPS), using a fitting method based on physical considerations. We observed that our XPS approach converged on results consistent with XMCD measurements made on the same samples. Thus, within a careful decomposition based on individual chemical environments it is shown that XPS is fully able to determine the actual inversion factor
Direct Evidence of the Role of Hybridization in the X-Ray Magnetic Circular Dichroism of a-Ce
We present an x-ray magnetic circular dichroism (XMCD) study of a [Ce(10 Å)/Fe(30 Å)] multilayer
performed at the Ce-M4,5 absorption edges. In this system the Ce-4f electrons are strongly hybridized
with the valence band. XMCD experiments show that they carry an ordered magnetic moment. The
differences of the shape of the XMCD signals of a typical g-like compound (CeCuSi) and of the Ce/
Fe multilayer highlight the role of hybridization in determining the ground state of cerium atoms in the
multilayer, which results in a mixing of J = 5/2 and J = 7/2 coupled states
Ultralow-temperature device dedicated to soft X-ray magnetic circular dichroism experiments
A new ultralow-temperature setup dedicated to soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD) experiments is described. Two experiments, performed on the DEIMOS beamline (SOLEIL synchrotron), demonstrate the outstanding performance of this new platform in terms of the lowest achievable temperature under X-ray irradiation (T = 220 mK), the precision in controlling the temperature during measurements as well as the speed of the cooling-down and warming-up procedures. Moreover, owing to the new design of the setup, the eddy-current power is strongly reduced, allowing fast scanning of the magnetic field in XMCD experiments; these performances lead to a powerful device for X-ray spectroscopies on synchrotron-radiation beamlines facilities
DEIMOS: A beamline dedicated to dichroism measurements in the 350-2500 eV energy range:
The DEIMOS (Dichroism Experimental Installation for Magneto-Optical Spectroscopy) beamline was part of the second phase of the beamline development at French Synchrotron SOLEIL (Source Optimisee de Lumiere a Energie Intermediaire du LURE) and opened to users in March 2011. It delivers polarized soft x-rays to perform x-ray absorption spectroscopy, x-ray magnetic circular dichroism, and x-ray linear dichroism in the energy range 350-2500 eV. The beamline has been optimized for stability and reproducibility in terms of photon flux and photon energy. The main end-station consists in a cryo-magnet with 2 split coils providing a 7 T magnetic field along the beam or 2 T perpendicular to the beam with a controllable temperature on the sample from 370 K down to 1.5 K. (C) 2014 AIP Publishing LLC
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