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Spin-Polarized Ground States and Ferromagnetic Order Induced by Low-Coordinated Surface Atoms and Defects in Nanoscale Magnesium Oxide

We investigate the effect of 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 corners of cube-like MgO clusters, a magnetic state becomes lower in total energy than the nonmagnetic singlet state by 1-2 eV, resulting in the spin-polarized ground state. The spin density is not only located at the surrounding O atoms neighbor to the corner Mg vacancy site but is also extended to the distant (1 nm or longer) low-coordinated surface O atoms along the <110> directions. This directional spin delocalization allows a remote Mg vacancy-Mg vacancyinteraction, eventually leading to a spontaneous long-range ferromagnetic interaction.