Bound magnetic polarons and p-d exchange interaction in ferromagnetic insulating Cu-doped ZnO

A systematic study on the magnetic and electrical transport properties of single-phase wurtzite Zn1−xCuxO is performed. Efros variable range hopping dominates the conduction, which is accompanied by a ferromagnetic order up to 700 K for x>1%. Both the first-principles calculations and Cu/Al co-doping experiments suggest that the spontaneous spin polarization originates from the p-d exchange interaction between O 2p and Cu 3d orbitals. Furthermore, our results are consistent with the scenario that the intrinsic ferromagnetism is established through indirect interactions between bound magnetic polarons mediated by magnetic impurities.