We report results of ab initio calculations on (GaN)n nanoclusters with n = 12, 16, 22,
and 24 using generalized gradient approximation (GGA) for the exchange-correlation
energy. We find cage structures of GaN clusters to be lower in energy as compared to
bulk fragments which have been fully optimized. Selected cages have been doped with
Gd and Nd atoms by substituting Gd and Nd on Ga sites, respectively. The atomic
structures of such doped nanoclusters have been optimized using spin-polarized GGA
and by including all f electrons on the rare earth atoms. We find the doped nanoclusters
to be magnetic with 7 μB (3 μB) magnetic moments when 1 Gd (Nd) atom is doped.
When 2 Gd (Nd) atoms are substituted on different Ga sites, there is often a
ferromagnetic coupling between the rare earth atoms, but we also find zero net
magnetic moment in the nanocluster depending upon the separation between the doped
atoms suggesting that both ferromagnetic and antiferromagnetic couplings may occur
between the rare earth dopants.