Defect-Induced High-$T_{C}$ Ferromagnetism and Spinodal Nanodecomposition in MgO

Based on a first-principles study of the magnetic properties of MgO with Mg vacancies [1,2], we calculated the electronic structures and exchange coupling constants for Monte Carlo Simulation (MCS) of the Curie temperature (\textit{Tc}). We also performed MCS of the spinodal nanodecomposition based on the calculated chemical pair interactions between the vacancies. In this study, it was found that hole-doping by Mg vacancies leads to a ferromagnetic ground state, invoking long-range magnetic interaction, and that \textit{Tc} can reach room temperature at sufficient vacancy concentrations of 15 at.{\%} under a homogeneously distributed condition. However, it was also found that the chemical pair interactions between vacancies are significantly attractive and that the system can form super-paramagnetic clusters of vacancies with strong ferromagnetic coupling in the clusters. These results suggest that, by the spinodal nanodecomposition, the \textit{Tc} or blocking temperature ($T_{B})$ can be enhanced and reach room temperature at smaller vacancy concentrations compared with those estimated for room-temperature ferromagnetism under the homogeneous distribution condition.\\[4pt] [1] M. Seike, et al. Jpn. J. Appl. Phys. 50, 090204 (2011).\\[0pt] [2] K. Sato, et al. Rev. Mod. Phys. 82, 1633 (2010).