  The magnetic coupling between doped Mn atoms in clusters as well as crystals
of GaN has been studied from first principles using molecular orbital theory
for clusters and linearized muffin tin orbitals-tight binding formulation
(LMTO-TB) for crystals. The calculations, based on density functional theory
and generalized gradient approximation for exchange and correlation, reveal the
coupling to be ferromagnetic with a magnetic moment ranging from 2.0 to 4.0
Bohr magnetons per Mn atom depending on its environment. Mn atoms also tend to
cluster and bind more strongly to N atoms than to Ga atoms. The significant
binding of Mn to GaN clusters further indicates that it may be possible to
increase the Mn concentration in GaN by using a porous substrate that offers
substantial interior surface sites.
