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(up arrow) states transform to the unfilled Co 3d(down arrow) 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.