Physical origin of the ferromagnetic ordering above room temperature in GaMnNnanowires

Abstract

We report a new method for large-scale production of GaMnN nanowires, by annealingmanganese–gallium oxide nanowires in flowing ammonia at high temperature.Microstructure analysis indicates that the GaMnN nanowires have wurtzite GaNstructure without Mn precipitates or Mn-related second phases. Magneticmeasurements reveal that ferromagnetic ordering exists in the GaMnN nanowires,whose Curie temperature is above room temperature. A mean-field model basedon 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 theincrease of the hole density and reveal a small ratio of the hole to the magneticdopant density in the samples. The low ratio of hole to Mn concentration and thesuperexchange antiferromagnetic interaction lead to the very concave temperaturedependence of the magnetization curve in contrast to conventional ferromagneticM–T behaviour. The theoretical calculation is in agreement with the experimental data.