Effects of Co concentration and annealing on the magnetic properties of Co-doped ZnO films: Role of oxygen vacancies on the ferromagnetic ordering

Abstract

Abstract Zn1-xCoxO thin films with different Co concentrations were deposited using ultrasonic spray pyrolysis technique. Comprehensive structural, elemental and optical investigations confirmed the single phase polycrystalline hexagonal wurtzite structure of all films and proved the absence of secondary phases like Co clusters or Co oxides. A considerable enhancement in the concentration of oxygen vacancy by increasing the Co content was observed from the XPS results. Increasing the Co concentration induced a slight decrease in the optical energy gap due to a weak sp-d exchange interaction between free charge carriers and the localized d electrons associated with the doped Co2+ cations. A clear room temperature ferromagnetic ordering was observed in all Co doped ZnO films with an increase in the saturation magnetization and coercivity with increasing the Co content. Photoluminescence and XPS results showed a clear decrease in the oxygen vacancy concentration after annealing the films in air at 600 °C, and this decrease is accompanied by a clear decrease in the saturation magnetization and coercivity. The overall results indicate a correlation between the concentration of oxygen vacancies and the ferromagnetic ordering in Co doped ZnO films.