Structural and magnetic properties of Cu-doped ZnO epitaxial films at the coalescence limit - A superparamagnetic CuO-ZnO nanocomposite

Abstract

A series of Cu-doped ZnO epitaxial films has been grown on sapphire substrates to investigate the possibilities of preparing a doped magnetic oxide at the coalescence limit. The growth was performed using reactive magnetron sputtering by varying the oxygen partial pressure to tune the incorporation of the Cu dopant and the resulting valence state. At low oxygen pressures, metallic Cu precipitates are formed, while at high oxygen partial pressures, the formation of CuO as a secondary phase could be evidenced. In-between, only a small fraction of the Cu can be substitutionally incorporated into the ZnO host matrix. The resulting magnetic properties are predominantly paramagnetic. However, the formation of the secondary CuO phase is accompanied by a small field imprinted magnetic component, which suggests that the CuO secondary phase consists of small and, thus, magnetically unblocked nanoprecipitates which are dispersed in the ZnO host matrix.