Origin of ferromagnetism in ZnO codoped with Ga and Co: Experiment and theory

In view of the recent controversies on the above room-temperature ferromagnetism in pure ZnO and the transition-metal doped ZnO, the present paper aims to shed some light on the origin of ferromagnetism by investigating the detailed atomic structure of (Co, Ga)-codoped ZnO experimentally as well as theoretically. Above room-temperature ferromagnetism in nonmagnetic Co-doped ZnO, nanoparticle powders prepared by sol-gel technique were obtained by codoping with Ga. It is found that Co ions substitute Zn sites while Ga ions were located at octahedral interstitial sites together with one O vacancy. Electrons from the Ga 4p^↑ states transform to the unfilled Co 3d^↓ states. The strong hybridization between the charge carriers in the Co 3d and Ga 4p states and electronic polarization for surrounding O ions at Co ions are detected. Finally, the Ga-4p electrons merged with conduction band and polarized O ions act as medium for an indirect exchange between the Co ions, which could be the origin of ferromagnetism in the (Co, Ga)-codoped ZnO.