The effect upon ferromagnetism due to co-doping of ZnO with Co and Cu in the presence of variable numbers of O vacancies was reported. The co-doped nanoparticles Zn0.95-Co0.05CuyO (0.00 ≤ y < 0.009) were prepared via the chemical route with O vacancies introduced via annealing in a reducing atmosphere for variable amounts of time. In addition to the magnetization, the particles were characterized by X-ray diffraction, X-ray photoemission spectroscopy and X-ray absorption near-edge spectroscopy. The Co ions were determined to be in the +2 state in a tetrahedral symmetry, with no evidence for metallic Co or Cu. However, the ionic state of Cu was found to change from +2 to +1 state with increasing Cu concentration, which appeared to strongly decrease the concentration of O vacancies. It was found that the ferromagnetic moment initially increased with the addition of Cu but decreased above a typical concentration that coincided with the appearance of the Cu+ state and the decrease of O vacancy concentration. It was concluded that the effect of Cu in the very low range of concentrations, where it appeared to go in as Cu2+, was to stabilize ferromagnetism indirectly via generation of O vacancies. The effects of O vacancy concentration on the ferromagnetism were interpreted in the light of the F-center exchange model.

Competing Effects of Cu Ionic Charge and Oxygen Vacancies on the Ferromagnetism of (Zn,Co)O Nanoparticles. Naeem, M., Hasanain, S.K., Afgan, S.S., Rumaiz, A.: Journal of Physics - Condensed Matter, 2008, 20[25], 255223