${\text{Zn}}_x{\left(\text{ZnO}\right)}_{1-x}$ granular films with nominal atomic concentration of $x=0\sim 1$ were prepared by magnetron cosputtering method. Ferromagnetism is observed in films with $0.04\le x<0.60$. The room-temperature saturated magnetization increases with increasing $x$ and reaches its maximum value of about 3.34 emu/cc at $x=0.31$. The temperature-dependent magnetization curve could be fitted within the framework of Stoner model in a large temperature range from 50 to 800 K. The obtained Curie temperature is higher than $500°\text{C}$. It is found that the main point defects in ZnO are Zn interstatial and oxygen vacancy. Room-temperature photoluminescence analysis and high-temperature x-ray diffraction measurement show conclusive evidence that the native point defect of Zn interstitial plays a crucial role in the observed magnetic behaviors. By implicating the shallow donor related carriers and/or extending the charge-transfer mechanism to metal/semiconductor heterostructure, the result could be qualitatively explained based on the Stoner theory of band magnetism. These findings may help to get further insight into the ferromagnetic origin in nonmagnetic ion doped ZnO systems.

• Received 17 May 2009

DOI:https://doi.org/10.1103/PhysRevB.80.174427