CRYSTALLOGRAPHIC STRUCTURE OF COBALT FILSM ON CU(001) - ELASTIC-DEFORMATION TO A TETRAGONAL STRUCTURE

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 determined by surface-extended X-ray absorption fine structure (EXAFS). A polarization dependent first shell analysis of the EXAFS spectra shows that the Co/Cu(001) films have a face-centered tetragonal structure (fct): the mean nearest-neighbour distance parallel to the surface is 2.55 angstrom (same value as in bulk copper) and the interlayer bond length is 2.50 angstrom: the films are in perfect epitaxy on copper (001) with a contraction of the lattice parameter perpendicular to the surface of 4%, in agreement with the continuum elasticity theory. A constant tetragonalization is observed for films of 2 to 15 monolayers. A simulation of the EXAFS spectra is also done using the FEFF code. In order to simulate the polarization dependence of the experimental spectra, we have introduced in this theoretical calculation a polarization dependence of the paths (single scattering and multiple scattering ones) based on a plane wave approximation. This simple model gives a good agreement between calculated and experimental polarization dependent EXAFS spectra both on bulk hcp cobalt and on the face-centered tetragonal films Co/Cu(001); this agreement confirms the structure determined from the first shell analysis.