We report here on the magnetic properties of ZnO:Mn- and ZnO:Co-doped nanoparticles. We have found that the ferromagnetism of ZnO:Mn can be switched on and off by consecutive low-temperature annealings in ${\mathrm{O}}_2$ and ${\mathrm{N}}_2$, respectively, while the opposite phenomenology was observed for ZnO:Co. These results suggest that different defects (presumably $n$-type for ZnO:Co and $p$-type for ZnO:Mn) are required to induce a ferromagnetic coupling in each case. We will argue that ferromagnetism is likely to be restricted to a very thin, nanometric layer at the grain surface. These findings reveal and give insight into the dramatic relevance of surface effects to the occurrence of ferromagnetism in ZnO-doped oxides.

• Received 3 August 2006

DOI:https://doi.org/10.1103/PhysRevB.75.155322