Structure of epitaxial ferromagnetic layers by multishell analysis of polarization dependent XAFS data

X-ray absorption fine structure (XAFS) has been often used to determine the structure of metastable magnetic films and multilayers deposited on single crystal substrates. These metastable phases provide important extensions of the family of simple elemental magnetic crystals and give new elements for the comparison with the predictions of electronic structure calculations. Further the well-known strengths of XAFS, a quantitative simulation of the spectra including distant shells contributions allows to identify unambiguously stacking sequences and the symmetry of the adsorption sites at the interface. This is now possible by using multiple scattering calculations which take the polarization of the light into account. It is illustrated by the Co-Cu system. The stable structure of cobalt is hexagonal closed packed (hcp), but cobalt can be stabilized in a distorted face centered cubic structure (fcc) by epitaxy on Cu(001). The deviation from the isotropic fcc structure is here determined precisely by polarization dependent XAFS. On the other hand, for a cobalt film grown on a (111) oriented copper single crystal, a complete simulation of the X-ray absorption spectra, using a multiple scattering formalism, clearly shows that the thick films have a local hcp stacking. The analysis of the thinner films reveals details of the growth of cobalt on Cu(111): the first two deposited layers follow the fcc (111) copper lattice, whereas the third one already starts an hcp (0001) stacking