  We have investigated electronic structures and magnetic properties of
potential ZnO based diluted magnetic semiconductors: (Fe, Co) and (Fe, Cu)
codoped ZnO. The origins of ferromagnetism are shown to be different between
two. (Fe, Co) codoped ZnO does not have a tendency of Fe-O-Co ferromagnetic
cluster formation, and so the double exchange mechanism will not be effective.
In contrast, (Fe, Cu) codoped ZnO has a tendency of the Fe-O-Cu ferromagnetic
cluster formation with the charge transfer between Fe and Cu, which would lead
to the ferromagnetism through the double-exchange mechanism. The ferromagnetic
and nearly half-metallic ground state is obtained for (Fe, Cu) codoped ZnO.
