Structure and local order in Co magnetic thin films on Au(111): A surface EXAFS study

The structure of cobalt thin films on an Au(111) surface has been studied using x-ray absorption spectroscopy at the cobalt K edge. The polarization dependence of x-ray absorption near-edge spectra and extended x-ray absorption fine structure (EXAFS) spectra evidences a hexagonal stacking for films thicker than 4 ML. For all the thicknesses, the analysis of the first-nearest-neighbor shell shows that this hexagonal structure is very close to that of the Co hcp bulk one: the cobalt does not grow in a coherent epitaxy on the Au(111) surface. This incoherent epitaxy leads to a wide radial distribution of the Co-Au bonds at the interface: this effect and the contribution of Au atoms to the EXAFS signal are discussed. The weak strains inside the Co magnetic thin films allow us to neglect the contribution of the magnetoelastic anisotropy to the perpendicular-magnetic anisotropy existing in this system.