Ph. Sainctavit

Fluorescence yield detection: Why it does not measure the X-ray absorption cross section

Abstract It is shown that fluorescence yield (FY) detection does not measure a pure X-ray absorption spectrum if multiplet effects are important. The reason is that while the Auger decay is approximately constant, the fluorescence decay shows a strong dependence on the final state. With a coherent description of the X-ray absorption and X-ray emission processes, the observed differences between TEY and FY detected nickel 2 p edges can be reproduced accurately. In the, approximate, incoherent description the FY spectral shape identifies with the X-ray absorption spectrum multiplied with its state dependent fluorescence decay. Self-absorption effects are calculated and the implications for X-ray Magnetic Circular Dichroism (X-MCD) and generalisations to other systems are discussed.

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 Optimisée de Lumière à Energie Intermédiaire 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.

Magnetism of (Zn,Co)O thin films probed by x-ray absorption spectroscopies

We report on the electronic and magnetic properties of Co-doped ZnO thin films investigated by x-ray absorption spectroscopies and element selective magnetometry. For a low Co concentration (around 5%), we evidence a paramagnetic phase clearly correlated to Co2+ ions substituted to Zn in the ZnO matrix. For higher Co concentrations (around 25%), we demonstrate the coexistence of both paramagnetic and ferromagnetic phases. The use of advanced element and orbital selective techniques allows us through the distinct spectral signature of Co in ionic or metallic states to assign the ferromagnetic phase to the presence of Co in a metallic state as a consequence of Co metal clustering in our films.

Restoration of bulk magnetic properties by strain engineering in epitaxial CoFe2O4 (001) ultrathin films

We report on the significantly enhanced in-plane magnetic properties of CoFe2O4 (001) ultrathin layers (5 nm) grown on MgAl2O4 (001) in comparison to films deposited on MgO (001). The predicted inverse spinel structure is confirmed by x-ray magnetic circular dichroism measurements and transmission electronic microscopy studies reveal a significant in-plane compressive strain, responsible for the strong film anisotropy. These results show that strain engineering can be used to tailor the magnetic properties of oxide ultrathin films. A large compressive strain restores bulk magnetic properties for CoFe2O4 films at tunnel barrier thicknesses.

Use of an asymmetric wiggler to study the magnetic circular x‐ray dichroism of Ni, Ho, and Tb

The asymmetric wiggler, inserted since the end of 1990 on the Super‐Aco storage ring in Orsay, has been built to produce a high rate and high flux of circularly polarized soft x rays in the 100–3000 eV range. This insertion device is particularly well suited to measure x‐ray absorption spectra corresponding to dipole transitions towards electronic orbitals that are directly responsible for magnetism (3d states of first row transition metals, 4f states of rare earths). In this article we report the first magnetic circular x‐ray dichroism spectra obtained with this insertion device. They exemplify the wide prospects of this experimental setup.

Understanding the Photomagnetic Behavior in Copper Octacyanomolybdates

The mechanism of photomagnetism in copper octacyanomolybdate molecules is currently under debate. Contrary to the general belief that the photomagnetic transition occurs only due to a photoinduced electron transfer from the molybdenum to the copper atom, recent X-ray magnetic dichroic (XMCD) data clearly indicate that this phenomenon is associated at low temperature to a local low-spin-high-spin transition on the molybdenum atom. In this article we provide theoretical justification for these experimental facts. We show the first simulation of X-ray absorption (XAS) and magnetic circular dichroism (XMCD) spectra at the L(2,3) edges of molybdenum from the joint perspective of density functional theory (DFT) calculations and ligand field multiplet (LFM) theory. The description of electronic interactions seems mandatory for reproducing the photomagnetic state.

Investigation of uranium edges in by x-ray magnetic circular dichroism

We report a high statistic x-ray magnetic circular dichroism study of carried out at the uranium edges. Using the sum rules, the detailed analysis of our data yields orbital and spin moments of the 5f uranium electrons in reasonable agreement with values extracted from a localized moment approach.

Distortions of X-ray absorption spectra measured with fluorescence yield

The 2p X-ray absorption spectra of 3d transition metal compounds measured with fluorescence yield (FY) appear distorted. It is shown that this distortion is intrinsic to FY detection and not due to self-absorption. It results from the final-state dependent variation of the fluorescence decay, which is more than 400%. The Auger decay is also final-state dependent but the variation is only 25%. In the approximation that the final states do not interfere with each other (which is shown to be approximately correct for Ni 2+ ), the FY spectral shape identifies with the X-ray absorption spectrum multiplied with its symmetry-dependent fluorescence decay. Results are given for the nickel 2p edge of Cs[NiCr(CN) 6 ].2H 2 O measured synchronously with FY and electron yield

Structure, magnetic ordering, and spin filtering efficiency of NiFe2O4(111) ultrathin films

NiFe2O4(111) ultrathin films (3–5 nm) have been grown by oxygen-assisted molecular beam epitaxy and integrated as effective spin-filter barriers. Structural and magnetic characterizations have been performed in order to investigate the presence of defects that could limit the spin filtering efficiency. These analyses have revealed the full strain relaxation of the layers with a cationic order in agreement with the inverse spinel structure but also the presence of antiphase boundaries. A spin-polarization up to +25% has been directly measured by the Meservey-Tedrow technique in Pt(111)/NiFe2O4(111)/γ-Al2O3(111)/Al tunnel junctions. The unexpected positive sign and relatively small value of the spin-polarization are discussed, in comparison with predictions and previous indirect tunnelling magnetoresistance measurements.

Analysis of the circular polarization rate delivered by an asymmetric wiggler and a two beryl crystal monochromator—Calculation and experiment

Synchrotrons are powerful sources of an x‐ray beam whose polarization state is well defined. The recent apparition of insertion devices able to produce high flux of circularly polarized light has impulsed new fields of solid‐state physics in spectroscopy or diffraction techniques. In this paper, the calculation of circular polarization rates taking into account the characteristics of the synchrotron source and the depolarization effect introduced by the beam monochromatization on crystals is presented here. From measurements of the circular polarization rates by x‐ray magnetic circular dichroism at nickel L2,3 edges, the experimental results are in excellent agreement with the calculations.