Resonant valence bond states in zinc vacancies induce the ferromagnetism of ZnO

•The spin moments of ZnO arise from the electrons trapped in oxygen vacancies.•The itinerant electrons residing in zinc vacancies fall into resonant valence bond states.•The ferromagnetism of ZnO is present as both oxygen and zinc vacancies are coexisting.•The couplings between the conduction band and both vacancy states both make the ferromagnetism decrease.

A theoretical model was proposed to investigate the mechanism of ferromagnetism in ZnO as well as to simulate the experimental result that the ferromagnetism can be enhanced by UV irradiation as UV photon energy is equivalent to the band gap. In the model, the spin moments arise from the trapped electrons in oxygen vacancy states and coexist with the itinerant electrons which reside in zinc vacancy states and fall into resonant valence bond states. Charge exchange between the conduction band of ZnO and both vacancy states makes electrons on both vacancy states delocalized and results in a decrease of the ferromagnetism as well.