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

Determination of Mg Site Geometry in Pyroxenes by Full Multiple Scattering Calculations

We present a comparison between experiment and Full Multiple Scattering calculations at the magnesium K-edge for two pyroxenes: diopside where magnesium atoms are in M1 site and enstatite where magnesium atoms are in two distinct sites M1 and M2. The purpose of this work is to determine how the number of octahedral sites on the one hand and the site distortion on the other hand affect the magnesium K-edge XANES spectra

A1K-XANES Formation in Some Tetrahedral and Octahedral Compounds

Experimental and theoretical study of Al K-XANES in AlPO4 (berlinite) and in KAl(S04)212H2O(K-alum) with different types of Al environment is performed. Calculations proved that the experimental XANES are well reproduced within EXAFS-like approach to electron scattering description i.e. considering the single-, double- and triple-scattering processes on two and approximately linear three-atoms chains originated at Al-absorbing atom and consisting mainly of O-atoms chosen under the same for both compounds empirical "selection rule"