Room-temperature ferromagnetism in Zn1−xCoxO magnetic semiconductors prepared by sputtering

We have used magnetron cosputtering to grow Zn1−xCoxO magnetic dilute semiconductors. The growth has been performed on SiO2∕Si and Al2O3(0001) substrates. The Co concentration has been varied between 0.1 and 0.25 and the substrate temperature between room temperature and 600°C. X-ray diffraction analysis has shown that for the films grown on Si substrates the structural quality of the film is improved by increasing the growth temperature and/or postgrowth annealing. The films are textured with c axis of the wurtzite structure along the growth direction. However, for the films grown on Al2O3 substrate quasi-epitaxial films have been obtained for 600°C substrate temperature. Magnetization measurements have shown that the ferromagnetism is directly correlated to the structural quality and appears by increasing the growth temperature and/or postgrowth annealing. Moreover, for the highly textured film a clear magnetic perpendicular anisotropy has been evidenced with the easy magnetization axis along the growth direction. To evidence the intrinsic nature of the ferromagnetism in the films, transmission optical measurements have been used. They show three absorption bands that are characteristics of d-d transitions of tetrahedrally coordinated Co2+. This has been supported by nuclear magnetic resonance and magnetic thermal variation.