High Curie temperature in Eu-doped GaN caused by volume-compensated Ga-vacancy

This study computationally demonstrates that room-temperature ferromagnetism, which has been experimentally observed in Eu-doped GaN, is induced by holes in N 2p states (i.e., Zener's double exchange interaction) that arise on the assumption that Ga vacancies appear as a result of the introduction of Eu ions (i.e., volume compensation). The calculated Curie temperature (T_C) suddenly increases over a certain range of Ga-vacancy concentrations and gradually increases with an increasing concentration of Eu ions. High T_C above room temperature is dominated by Zener's double exchange mechanism in partially occupied N 2p hole-states, which itinerate throughout the whole crystals, and low T_C is dominated by Zener's p-f exchange mechanism in Eu 4f and N 2p hybridization. We can reasonably explain the surprising experimental data of 4000 μ_B per Gd atom in Gd-doped GaN reported by Dhar et al. [Phys. Rev. Lett. 94, 037205 (2005)].