Magnetic, magnetotransport, and optical properties of Al-doped Zn0.95Co0.05O thin films

Thin films of 5% Co-doped ZnO with a range of Al codoping exhibit a band-edge shift, which varies with carrier concentration as n2∕3. Carrier effective mass is 0.26me and mobility is ∼10cm2V−1s−1. The doped films, which contain coherent Co clusters of 4–8nm in size, exhibit a ferromagnetic moment of 0.3–1.0μB per cobalt. The magnetism is progressively destroyed by Al doping due to a reduction in Co-cluster formation. Magnetoresistance appears below 30K, but these materials cannot be regarded as dilute magnetic semiconductors.