In spite of considerable interest in ferromagnetism of the dilute magnetic semiconductor GaN:Mn, the nature of the ferromagnetism was still quite controversial. Experimental values for the Curie temperature TC vary widely depending upon the details of the growth conditions which might affect the impurity concentrations and possibly the magnetic properties. In order to gain insight into the effects of the impurities, ab initio density functional studies of the magnetic interactions in GaN in the presence of vacancies were performed. Both N and Ga vacancies were considered. The N vacancy released electrons into the system which changed the Mn d4-state to a half-filled Mn d5 state, so that the antiferromagnetic super-exchange became dominant. Previous studies had found the N vacancy had the lowest formation energy, so the presence of these vacancies was predicted to lower TC. The naive picture of Ga vacancies was the release of holes into the system which should increase ferromagnetism due to the increased hole concentration compared to the vacancy-free material. However, an antiferromagnetic interaction for the Ga vacancy was found as well, in agreement with Mahadevan's work. This could be attributed to the localized nature of the hole states which did not participate in the transport. The effects of localization of the holes from the Ga vacancy was demonstrated using the virtual crystal approximation. Thus both the N and Ga vacancy were found to impede ferromagnetism.

Effect of Vacancies on Ferromagnetism in GaN:Mn Dilute Magnetic Semiconductors from First-Principles. P.Larson, S.Satpathy: Physical Review B, 2007, 76[24], 245205 (8pp)