Crystallographic structure and morphology of cobalt films on Cu(1 1 11)

We present a detailed study of the crystallographic structure and morphology of Co/Cu(1 1 11) films by surface extended X-ray absorption fine structure (SEXAFS) and scanning tunneling microscopy (STM). At the very beginning of the growth, the step array is profoundly altered, and large terraces about two times larger than on the clean substrate are formed. For higher thicknesses, islands are observed on the surface. They are first isotropic, and then begin to elongate along the initial step edge direction around 1 ML, and relative straight steps are retrieved around 5 ML. The SEXAFS measurements show that the lattice parameter of cobalt is expanded parallel to the surface but Co is not in a pseudomorphic epitaxy at the beginning of growth because of the relaxation processes at islands free edges. An in-plane structural anisotropy is found only for a Co coverage around 0.7 ML. Upon increasing the thickness, Co adopts a face centred tetragonal structure like on a flat Cu(0 0 1). These results show that the step-induced magnetic anisotropy does not originate from an in-plane crystallographic anisotropy but it is more likely due to the shape anisotropy (i.e. morphology of the films).