Microstructure and magnetic properties of Co-doped ZnO films deposited by gas flow sputtering

Co-doped ZnO films with a Co concentration of 8–20 at.% were fabricated using the low-energy process of gas flow sputtering. X-ray diffraction, X-ray photoelectron spectroscopy, and optical absorption measurements revealed that the Co ions replace Zn ions in the ZnO matrix and that the Co ions have an oxidation state of 2+. The magnetic properties of the film depend on the Co concentration. The plots of magnetization and inverse susceptibility vs. temperature indicate that the film with a high Co concentration (20 at.%) contains a ferromagnetic component, while that with a low Co concentration (8 at.%) contains an antiferromagnetic component. The film with an intermediate Co concentration (10 at.%) contains a ferromagnetic component with a low Curie temperature. Hysteresis was not found in magnetization curves for all the samples, including the sample at 5 K. The films exhibited a high resistivity of 4 × 107–2 × 108 Ω cm at room temperature, and carrier-mediated magnetism is not likely to be applicable for the mechanisms of the magnetism in the films.