Spin-polarized ground states and ferromagnetic order induced by low-coordinated surface atoms and defects in nanoscale magnesium oxide

Abstract

We investigate the effect of surface defects and the related low-coordinated surface atoms on the defect-induced magnetism in MgO nanocrystallites using hybrid density functional theory calculations. It has been demonstrated that when Mg vacancies are introduced at the surface or near surface of cubelike MgO clusters, a magnetic state ($S$ $\ensuremath{\ge}$ 1) becomes lower in total energy than the nonmagnetic singlet state ($S$ = 0) by several electron volts, resulting in the robust spin-polarized ground state. The total spin $S$ of the clusters in their ground state is equal to the number of the surface Mg vacancies introduced. The resulting spin density is not only located at the surrounding O atoms neighbor to the Mg vacancy site but is also extended to the low-coordinated surface O atoms along the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction, forming ferromagneticlike domains. This directional spin delocalization allows a remote (\ensuremath{\sim}1 nm or longer) vacancy-vacancy interaction, eventually leading to a long-range ferromagnetic interaction.