Seminars Archive

Surface EXAFS characterization of magnetic and resonant photoemission in Ce compound

Abstract
The knowledge of the crystallography of metastable magnetic films prepared by deposition on suitable substrates is the key for the understanding of their unusual magnetic properties. Surface EXAFS is a very attractive technique to measure it since it allows to characterize with the same precision films of any thickness, coated films and multilayers. This can be illustrated by the examples of Fe-Cu (fcc iron) and Co-Cu metastable structures. The analysis up to the fourth shell of neighbours can be done using multiple scattering simulations (like FEFF code). A very precise characterization of the structural anisotropy of the films can be deduced, and in some cases, a detailed representation of the interface (adaptation of adsorbate to the substrate network) can be obtained. In most surface EXAFS experiments, the linear polarisation of the synchrotron radiation is used to infer direct information about the crystallographic anisotropy. For K absorption edge, the polarisation dependence of the EXAFS signal is simple (cos2a dependence). For L2,3 absorption edges, the polarisation dependence is more complicated since the initial p state can be excited to a final state of s or d symmetry. The angular dependence of these two terms is strongly different. With a study on a bulk anisotropic system, a cadmium single crystal, we have evaluated the relative importance of the p to s transitions for L3 edge absorption spectrum. Finally, some results of high energy resonant photoemission will be presented : strong resonant effects are observed at the Ce L3 absorption edge in Ce-Rh compounds, both for the Ce 3d photoemission peaks and the L3M4,5N4,5 Auger lines. These resonances show unambiguously that interference effects exist in core level spectroscopies. In mixed valent compounds, they occur at different photon energies making it possible to unravel the X-ray absorption profiles. Moreover resonant L3M4,5N4,5 Raman Auger lines are observed up to 10 eV above the L3 absorption edge jump, showing the strong localization of the core excited state.