Magnetic anisotropy of Fe and Co adatoms and monolayers: Need for a proper treatment of the substrate

The magnetocrystalline contribution to the magnetic anisotropy energy (MAE) of Fe and Co adatoms, monolayers, and surface superstructures on Pt(111) is investigated. It is shown that the thickness of the slab representing the substrate and the interaction between the atoms in neighboring surface supercells affect the calculated MAE much more profoundly than they affect magnetic moments. Reliable theoretical values of MAE cannot be obtained if the substrate is represented by a slab of less than about ten atomic layers. If a surface superstructure is meant to represent an adatom by means of supercell approach, then decoupling has to be ensured by using very large supercells.