Ferromagnetism in GaN:Gd: A density functional theory study*

First principle calculations of the electronic structure and magnetic interaction of GaN:Gd have been performed within the Generalized Gradient Approximation (GGA) of the density functional theory (DFT) with the on-site Coulomb energy U taken into account (also referred to as GGA+U). The ferromagnetic $p-d$ coupling is found to be over two orders of magnitude larger than the$ s-d$ exchange coupling. The experimental colossal magnetic moments and room temperature ferromagnetism in GaN:Gd reported recently are explained by the interaction of Gd 4$f$ spins via $p-d$ coupling involving holes introduced by intrinsic defects such as Ga vacancies.