The mechanism for the room temperature magnetic coupling and electric conduction in oxide diluted magnetic semiconductors (DMSs) has been simultaneously studied on the Co:ZnO thin film by utilization of the electric field effect. We find that the carriers are bound on a defect in a radius much larger than the bounded magnetic polaron (BMP) radius and can move by variable-range hopping (VRH) over a relatively small distance. Here, we propose a concentric bounded model consisting of a concentric localization configuration with a limited carrier VRH capability. In this model, the carriers localized around defects strongly couple with the doped magnetic ions forming a BMP in the inner sphere and can only itinerate with no spin coherence in the outer shell. Carriers can hop either by spin-polarized or by spin-independent VRH directly between or not directly between adjacent inner spheres, respectively. This model can explain both the electric and magnetic properties of the oxide DMS and depicts an evolution of electric and magnetic properties associated with defect concentration. The spin-current ratio depends on the ratio of the inner and outer radii.

• Received 19 January 2008

DOI:https://doi.org/10.1103/PhysRevB.77.245210