  Even though it has been known for decades that the magnetocrystalline
anisotropy is linked to the spin-orbit coupling (SOC), the mechanism how it
arises for specific systems is still subject of debate. We focused on finding
markers of SOC in the density of states (DOS) and on employing them for
understanding the source of magnetocrystalline anisotropy for the case of
adatoms and monolayers. Fully relativistic ab-initio KKR-Green function
calculations were performed for Fe, Co, and Ni adatoms and monolayers on
Au(111) to investigate changes in the orbital-resolved DOS due to a rotation of
magnetization. In this way one can see that a significant contribution to the
magnetocrystalline anisotropy for adatoms comes from pushing of the SOC-split
states above or below the Fermi level. As a result of this, the
magnetocrystalline anisotropy energy crucially depends on the position of the
energy bands of the adatom with respect to the Fermi level of the substrate.
This view is supported by model crystal field Hamiltonian calculations.
