希土類窒化物半導体GdN薄膜の強磁性相転移とスピン秩序誘起のバンドギャップ減少

概要

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We performed ESR and magnetization measurements to make clear ferromagnetic properties of GdN thin films. We found two kinds of ferromagnetic phases ; one starts to develop below approximately 50K, and another one below approximately 30K. The Curie temperature estimated from the Arrott plot was 29.0K. The ratio of the 50-K-class-ferromagnetic phase to the 30-K-class-phase becomes high with the increase in the free carrier density, which suggests that the transition of the 50-K-class-ferromagnetic phase can be attributed to be the RKKY interaction. On the other hand, the Curie temperature depends on lattice expansion in the growth direction. This indicates that the 30-K-class-ferromagnetic phase obeys the superexchange interaction. Furthermore, we found that the band gap is dramatically reduced with the ferromagnetic spin ordering. The reduction of the band gap energy depends on magnetization, and the relationship between them was ruled by the Zeeman splitting.