Surface State Induced Ferromagnetism in Co- and Mn-Doped ZnO Surfaces

We studied the magnetic coupling between magnetic atoms at the O-terminated ZnO(000 (1) over bar) surface using first-principles total energy calculations. It was found that the substitutional Co and Mn in the O-terminated ZnO(000 (1) over bar) surface tend to occupy the outermost cation layer. Different from the magnetic behaviors between substitutional Co and Mn atoms in bulk ZnO, at O-terminated ZnO(000 (1) over bar), due to the presence of surface states, the magnetic atoms display ferromagnetic coupling. On the basis of the analysis of electron density of states, n- and p-type doping could enhance the ferromagnetic coupling between Co and Mn atoms at O-terminated ZnO(000 (1) over bar), respectively, in good agreement with previous experimental observations. Our results complement the understanding of the origin of ferromagnetism in ZnO-based diluted magnetic semiconductors.