It was noted that experimental values for the Curie temperature TC varied widely; depending upon the details of the growth conditions; which could affect the impurity concentrations and possibly the magnetic properties. In order to obtain some insight into the effects of the impurities, ab initio density functional studies of 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 superexchange became dominant. Previous studies had found that the N vacancy had the lowest formation energy, so the presence of these vacancies was predicted to decrease TC. The naive picture of Ga vacancies was that the release of holes into the system should increase ferromagnetism due to the increased hole concentration compared to the vacancy-free material. However, an antiferromagnetic interaction was found for the Ga vacancy as well. 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 by using the virtual crystal approximation. Thus both the N and the 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)