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

TL;DR

This study investigates how nickel incorporates into Zn/Co-based magnetic oxides, revealing its valence states, lattice positions, and effects on magnetic properties through various doping methods.

Contribution

It provides detailed insights into Ni's valence states and lattice sites in different oxide structures, and how these influence magnetic behavior and phase separation.

Findings

Ni in ZnO is in 2+ state on tetrahedral sites, acts as a deep donor.

Ni in ZnCo2O4 is in 3+ state on octahedral sites, induces ferrimagnetism.

High Ni concentrations lead to NiO phase separation and exchange bias effects.

Abstract

Ni incorporation has been studied in a comprehensive range of Zn/Co-based magnetic oxides to elucidate it valence state and lattice incorporation. The resulting structural and magnetic properties are studied in detail. To the one end Ni in incorporated by in-diffusion as well as reactive magnetron co-sputtering in wurtzite ZnO where only the Ni-diffused ZnO exhibits significant conductivity. This is complemented by Ni and Co codoping of ZnO leading. To the other end, the ZnCo$_2$O$_4$ spinel is co-doped 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 about 10\% is found. Due to its smaller magnetic moment it can induce partial uncompensation of the Co magnetic moments due to antiferromagnetic coupling. In the spinel Ni is found to be incorporated in its formal 3+ oxidation state on octahedral sites and couples antiferromagnetically to the Co moments leading again to magnetic uncompensation of the otherwise antiferromagnetic ZnCo$_2$O$_4$ spinel and to ferrimagnetism at higher Ni concentrations. Increasing Ni even further leads to phase separation of cubic NiO resulting in an exchange-biased composite magnetic oxide.