d0 ferromagnetic surface in HfO2

Using first-principles simulations based on density functional theory, we demonstrate that surfaces of simple oxides can be ferromagnetic without involving magnetic transition metal ions, thus called d0 ferromagnetic surface. We take cubic HfO2 as an example to show that the surface electronic states depend critically on the surface termination and only O rich non-stoichiometric surfaces are ferromagnetic. Systematic studies of various surfaces with different surface indexes and terminations indicate that the O surface electronic states are spin polarized due to the large O 2p spin exchange energy. We argue that the mechanism proposed here is rather general and can be applied to other simple oxides such as SrO.