Unconventional magnetism in semiconductors: Role of localized acceptor states

Magnetism induced by the localized defect states in the otherwise nonmagnetic sp semiconductors-GaN and ZnO-is investigated using ab initio methods. The defects studied include the cation vacancy in ZnO, a potassium substitutional in GaN, and an acceptor like defect complex (gallium vacancy along with oxygen as an anion substitutional). In all three cases, spontaneous spin-polarized ground states are obtained within density-functional theory (DFT). Magnetic coupling between defect-induced local moments is also studied by mapping the DFT total energy to a nearest-neighbor Heisenberg model. The coupling between magnetic moments is found to be ferromagnetic for the case of cation vacancies in ZnO. This is also found to be the case for the potassium substitutional in GaN. However, the magnetic coupling is antiferromagnetic for the acceptorlike defect complex. A kinetic exchange model is used to explain this diverse magnetic behavior shown by the different systems.