Contributions of magnetic properties in epitaxial copper-doped ZnO

Diluted magnetic semiconductors have great potential in applications for biological detection and spintronics. However, the origin of magnetism is complex and it is of significant importance to clarify the contributions from various origins. We prepared epitaxial copper-doped ZnO films and investigated the origin of ferromagnetism by combining various characterization methods. The results show that, with nominal Cu concentrations of up to 7.3 at.%, the Cu atoms substitute for the Zn atoms and form strong covalence bonds (Cu-Zn-O), which show a property commensurate with that of the Zn-O bonds in the ZnO host. With further increases in Cu concentrations, the substitutional Cu-Zn effect is obscured, and the [CuZnO4] clusters, regulated by the wurtzite ZnO host, segregate into CuO phase after annealing in air. Magnetization in volume increases with increasing Cu content up to about 7.3 at.% and then decreases with further increase, while the magnetic moment per Cu atom decreases monotonically with the increase in Cu content. We have demonstrated that the substitution of Cu for Zn and the presence of strong Cu-Zn-O bonds are necessary for ferromagnetism while the [CuZnO4] clusters are detrimental to the ferromagnetism. The enhancement of ferromagnetism in volume is strongly correlated with the moderate oxygen vacancy mediated Cu ions.