  A first-principles DFT-GGA+U study of the doping of GaN with Cr, Mn, or Fe
confirms a strong tendency for the formation of embedded clusters occupying Ga
sites of the wurtzite and zinc blende phases of GaN. Within the employed
computation model, the tendency for aggregation is larger for Cr and Mn than
for Fe. In contrast to previous DFT-GGA calculations, we predict a
ferromagnetic ordering of the Cr and Mn clusters having more than two atoms
while the Fe clusters are all antiferromagnetic. We have also investigated the
magnetic ordering of nearest-neighboring ionic pairs that substitute gallium
atoms at the (0001) wz-GaN surface. We find that Fe dopants tend to aggregate,
whereas there is a repulsive interaction in the case of Cr and Mn. Nearest
neighbor Mn and Fe pairs are coupled antiferromagnetically whereas the Cr pair
is coupled ferromagnetically. The relevance of our finding to recent
experimental findings is discussed.
