Magnetic interactions in the Zn-Co-O system: tuning local structure, valence and carrier type from extremely Co doped ZnO to ZnCo2O4.

We have investigated the relation between local structure, valence and carrier type with magnetism in the Zn-Co-O system. Thin films ranging from wurtzite Zn(1-x)Co(x)O (Co:ZnO) to ZnCo2O4 spinel were grown on c-sapphire substrates. On the one hand, the unprecedented doping of x = 0.6 Co in ZnO enables to study the structural and magnetic properties well-above the coalescence limit. On the other hand, the ZnCo2O4 spinel provides a p-type environment. We find a strong correlation between local structure, valence and carrier type throughout the Zn-Co-O system. In contrast to earlier publications neither 60% Co:ZnO nor ZnCo2O4 exhibit any sign of ferromagnetic order despite of the high concentration of magnetic ions and a p-type carrier background. Instead, antiferromagnetic exchange is found to be the predominant magnetic interaction in the Zn-Co-O system.