Our first-principles calculations based on density functional theory predicted a new path for the formation of sp-ferromagnetic states: anion interstitials. The fluorine interstitials in MgO were found to be spin polarized and are more stable in the ferromagnetic state. This observation is attributed to the strong p-p interaction between the interstitial F and the neighboring O atoms. We demonstrate that F-doped MgO is potential p-type half-metallic ferromagnets, and the source of magnetism is

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... he spin triplet state splitting associated with p-p coupling, and ferromagnetism between interstitial atoms in these systems can be mediated by spin polarized holes.