  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.
