Films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excluded that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5K at 60kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect was observed in ferromagnetic ZnO as well as in non-ferromagnetic Cu-doped ZnO films, indicating that the anomalous Hall effect did not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO was attributed to intrinsic defects.

Room Temperature Ferromagnetism in ZnO Films Due to Defects. Q.Xu, H.Schmidt, S.Zhou, K.Potzger, M.Helm, H.Hochmuth, M.Lorenz, A.Setzer, P.Esquinazi, C.Meinecke, M.Grundmann: Applied Physics Letters, 2008, 92[8], 082508