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

Abstract

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 $\text{Ga}\text{ }4{p}^{\ensuremath{\uparrow}}$ states transform to the unfilled $\text{Co}\text{ }3{d}^{\ensuremath{\downarrow}}$ states. The strong hybridization between the charge carriers in the $\text{Co}\text{ }3d$ and $\text{Ga}\text{ }4p$ states and electronic polarization for surrounding O ions at Co ions are detected. Finally, the $\text{Ga-}4p$ 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.