Magnetic properties of thin films of Co and of (CoPt) superstructures on Pt(100) and Pt(111)

The magnetic properties of Coₙ/Pt(100), Coₙ/Pt(111), (Co0.5Pt0.5)ₙ/Pt(100), (CoPt)ₙ/Pt(100), and (CoPt)ₙ/Pt(111), $n<~15$, are investigated using the relativistic spin-polarized screened Korringa-Kohn-Rostoker method. It is found that only the artificial superstructures (CoPt)ₙ/Pt(100) and (CoPt)ₙ/Pt(111) show a perpendicular magnetic anisotropy beyond $n=10$. For the free surfaces of Co on Pt in the case of the (100) orientation, a multiple reorientation transition below $n=7$ is found, while along (111) such a transition is predicted at about four layers of Co. For the homogeneous, statistically disordered alloy Co0.5Pt0.5 on Pt(100) the orientation of the magnetization remains in-plane for all n investigated. A comparison to experiment yields interesting insight into aspects of order and disorder, surface segregation, and phase separation frequently encountered in experimental studies of perpendicular magnetism in the Co/Pt system.