Robust highly-spin-polarized ferromagnetism in hexagonal CrGa7N8 and MnGa7N8

Abstract

Motivated by high-temperature ferromagnetism in transition-metal doped gallium nitrides, we study 12.5% transition-metal-substituted wurtzite GaN, XGa7N8 (X = Cr, Mn, Fe, Co, Ni), using a full-potential density-functional method. Our calculated results show that CrGa7N8 is a metallic ferromagnet and MnGa7N8 is a typical half-metallic ferromagnet with a large half-metallic gap of 0.75 eV. They both have high spin polarization at the Fermi levels, 96.8% for CrGa7N8 and 99.9% for MnGa7N8 with spin–orbit coupling taken into account. Their ferromagnetism is very robust against the corresponding antiferromagnetic fluctuations and is attributed to the right positions of the d levels and the large spin exchange splitting driven by strong p–d hybridization. Antiferromagnetic orders are shown to be favourable in the cases of Fe, Co and Ni because their d levels are much lower in energy than that of both Cr and Mn and the majority-spin parts of them are merged in the valence bands of wurtzite GaN. The two highly-spin-polarized ferromagnetic phases should play some important roles in the ferromagnetism in GaN-based diluted magnetic semiconductors and could be realized in future high-quality samples.