Valence state, lattice incorporation, and resulting magnetic properties of Ni in Zn/Co-based magnetic oxides

Abstract

Ni incorporation has been studied in a comprehensive range of Zn/Co-based magnetic oxides to elucidate its valence state and lattice incorporation. The resulting structural and magnetic properties of a range of related types of samples are studied in detail. On the one hand, Ni doping is studied in wurtzite ZnO which is either done by in-diffusion of the Ni into bulk ZnO or by reactive magnetron sputtering for Ni-doped thin films of ZnO. The latter is complemented by Ni and Co codoping of ZnO leading to altered magnetic properties which are then dominated by Co. On the other hand, the ZnCo2O4 spinel is codoped with varying amounts of Ni. In the wurtzite oxides, Ni is exclusively found on tetrahedral lattice sites in its formal 2+ oxidation state as deep donor. It behaves as an anisotropic paramagnet, and a limited solubility of Ni below 10% is found. Furthermore, the partial compensation of the antiferromagnetically coupled Co magnetic moments is induced by the Ni due to its smaller magnetic moment. In the ZnCo2O4 spinel, Ni is found to be incorporated in its formal 3+ oxidation state on octahedral sites and also couples antiferromagnetically to the Co moments. At low Ni concentrations, this leads to a lifting of the partial magnetic compensation of the antiferromagnetic ZnCo2O4 spinel and to ferrimagnetism at higher Ni concentrations. Increasing the Ni concentration even further leads to phase separation of cubic NiO resulting in a structurally less defined, exchange-biased composite magnetic oxide.