Noble metal capping effects on the spin-reorientation transitions of Co/Ru(0001)

Thin films of Co/Ru(0001) are known to exhibit an unusual spin reorientation transition (SRT) coupled to the completion of Co atomic layers for Co thicknesses under four layers. By means of spin-polarized low-energy electron microscopy, we follow in real space the magnetization orientation during the growth of atomically thick capping layers on Co/Ru(0001). Capping with noble metal (Cu, Ag and Au) elements modifies the SRT depending on the Co and overlayer thickness and on the overlayer material, resulting in an expanded range of structures with high perpendicular magnetic anisotropy. The origin of the SRT can be explained in terms of ab initio calculations of the layer-resolved contributions to the magnetic anisotropy energy. Besides the changes in the SRT introduced by the capping, a quantitative enhancement of the magnetic anisotropy is identified. A detailed analysis of the interplay between strain and purely electronic effects allows us to identify the conditions that lead to a high perpendicular magnetic anisotropy in thin hcp Co films.