Electronic structure and magnetism of Mn-doped GaN

Mn-doped semiconductors are extremely interesting systems due to their novel magnetic properties suitable for the spintronics applications. It has been shown recently by both theory and experiment that Mn-doped GaN systems have a very high Curie temperature compared to that of Mn-doped GaAs systems. To understand the electronic and magnetic properties, we have studied Mn-doped GaN system in detail by a first-principles plane-wave method. We show here the effect of varying Mn concentration on the electronic and magnetic properties. In agreement with previous studies, d states of Mn form an impurity band completely separated from the valence-band states of the host GaN for dilute Mn concentration. This is in contrast to the Mn-doped GaAs system where Mn d states in the gap lie very close to the valence-band edge and hybridize strongly with the delocalized valence-band states. To study the effects of electron correlation, $\mathrm{LSDA}+U$ calculations have been performed. Calculated exchange interaction in (Mn,Ga)N is short ranged contrary to that in (Mn,Ga)As where the strength of the ferromagnetic coupling between Mn spins is not decreased substantially for large Mn-Mn separation. Also, the exchange interactions are anisotropic in different crystallographic directions due to the presence or absence of connectivity between Mn atoms through As bonds.