  The orbital magnetic moment is still surprisingly not well understood, in
contrast to the spin part. Its description in finite systems, such as isolated
atoms and molecules, is not problematic, but it was only recently that a
rigorous picture was provided for extended systems. Here we focus on an
intermediate class of systems: magnetic adatoms placed on a non-magnetic
surface. We show that the essential quantity is the ground-state charge current
density, in the presence of spin-orbit coupling, and set out its
first-principles description. This is illustrated by studying the magnetism of
the surface Pt electrons, induced by the presence of Cr, Mn, Fe, Co and Ni
adatoms. A physically appealing partition of the charge current is introduced.
This reveals that there is an important nonlocal contribution to the orbital
moments of the Pt atoms, extending three times as far from each magnetic adatom
as the induced spin and local orbital moments. We find that it is as sizable as
the latter, and attribute its origin to a spin-orbital susceptibility of the Pt
surface, different from the one responsible for the formation of the local
orbital moments.
