Study of the anomalous Hall effect, magnetoresistance, and magnetic anisotropy in ZnO:Co and ZnO:Mn thin films

ZnO-based diluted magnetic semiconductor (DMS) materials have attracted much attention in these years due to the theoretical prediction of above room-temperature ferromagnetism. So far, most of the experiments were focused to the study of the magnetization of the ZnO DMS materials. However, the magnetization study, by only providing a global information on the moment of the material, cannot distinguish the intrinsic magnetic properties from the extrinsic contributions. The ZnO thin films were grown on sapphire substrates by molecular-beam epitaxy. The Co- and Mn-implantations were performed on the as-grown ZnO samples with different free carrier concentrations. Anomalous Hall effect (AHE) and magnetoresistance measurements were performed on the ZnO:Co and ZnO:Mn thin films. AHE hysteresis loops were observed in both ZnO:Co and ZnO:Mn thin films, which confirm the intrinsic ferromagnetism in both films. However, the AHE hysteresis loops are distinctly weaker than the magnetization hysteresis loops ($M-H)$ measured by SQUID. Therefore we conclude that both intrinsic and extrinsic ferromagnetism co-exist in the samples.