Magnetism in undoped MgO studied by density functional theory

The origin of magnetism in undoped MgO has been studied based on density functional theory. It is shown that Mg vacancies can induce local moments in MgO while O vacancies cannot irrespective of the concentration. The origin of the local magnetic moments comes from the partially occupied e_g- and t_1u- (or only e_g-) orbitals and the through-bond spin polarization mechanism mediates the ferromagnetism in MgO. For the ground state configuration of bulk MgO with two Mg vacancies, the energy difference between the ferromagnetic and antiferromagnetic states (28 meV) is smaller than the thermal energy at room temperature (~30 meV), which means that the ferromagnetic state is not stable in the bulk. For MgO thin films, Mg vacancies tend to form at the surface region because of the lower formation energy at the surface site than at the subsurface site and in the bulk. Moreover, the formation energy of Mg vacancy in MgO quantum dot decreases much, allowing a larger concentration of Mg vacancies to appear. In conclusion, the magnetism can be sustained in the quantum dot when enough holes are introduced by the large concentration of Mg vacancies. This is consistent with the experimental result of magnetism observed in MgO nanocrystals.