Competing effects of Cu ionic charge and oxygen vacancies on the ferromagnetism of (Zn,Co)O nanoparticles

Abstract

We report the effects on the ferromagnetism due to co-doping of ZnO with Co and Cu inthe presence of variable numbers of oxygen vacancies. The co-doped nanoparticlesZn0.95−Co0.05CuyO (0.00≤y<0.009) were prepared via the chemical route with oxygen vacancies introduced viaannealing in a reducing atmosphere for variable amounts of time. In additionto the magnetization, the particles were characterized by x-ray diffraction(XRD), x-ray photoemission spectroscopy (XPS) and x-ray absorption nearedge spectroscopy (XANES). The Co ions were determined to be in the+2 state in a tetrahedral symmetry, with no evidence for metallic Coor Cu. However, the ionic state of Cu is found to change from+2 to+1 state with increasing Cu concentration, which appears to strongly decrease the concentration ofoxygen vacancies. It is found that the ferromagnetic moment initially increases with the additionof Cu but decreases above a typical concentration that coincides with the appearance of theCu+1 state and the decrease of O vacancy concentration. It is concluded that the effectof Cu in the very low range of concentrations, where it appears to go in asCu+2, is to stabilize ferromagnetism indirectly via generation of O vacancies. The effects of Ovacancy concentration on the ferromagnetism are interpreted in the light of the F-centerexchange (FCE) model.