Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature

The realization of semiconductors that are ferromagnetic above room temperature will potentially lead to a new generation of spintronic devices with revolutionary electrical and optical properties. Transition temperatures in doped ZnO are high but, particularly for Mn doping, the reported moments have been small. We show that by careful control of both oxygen deficiency and aluminium doping the ferromagnetic moments measured at room temperature in n-type ZnMnO and ZnCoO are close to the ideal values of 5 mu(B) and 3 mu(B) respectively. Furthermore a clear correlation between the magnetization per transition metal ion and the ratio of the number of carriers to the number of transition metal donors was established as is expected for carrier-induced ferromagnetism for both the Mn and Co doped films. The dependence of the magnetization on carrier density is similar to that predicted for the transition temperature for a dilute magnetic semiconductor in which the exchange between the transition metal ions is through the free carriers. We observe a positive magnetoresistance but no anomalous Hall effect or anisotropic magnetoresistance in the ferromagnetic samples.