Physical origin of the ferromagnetic ordering above room temperature in GaMnN nanowires

We report a new method for large-scale production of GaMnN nanowires, by annealing manganese-gallium oxide nanowires in flowing ammonia at high temperature. Microstructure analysis indicates that the GaMnN nanowires have wurtzite GaN structure without Mn precipitates or Mn-related second phases. Magnetic measurements reveal that ferromagnetic ordering exists in the GaMnN nanowires, whose Curie temperature is above room temperature. A mean-field model based on the exchange coupling of the nondegenerate carrier-localized impurity band, together with the consideration of the superexchange antiferromagnetic interaction, is used to explain the physical origin of the observed ferromagnetic ordering. Theoretical calculations indicate that the Curie temperature increases with the increase of the hole density and reveal a small ratio of the hole to the magnetic dopant density in the samples. The low ratio of hole to Mn concentration and the superexchange antiferromagnetic interaction lead to the very concave temperature dependence of the magnetization curve in contrast to conventional ferromagnetic M-T behaviour. The theoretical calculation is in agreement with the experimental data