We have investigated electronic structures and magnetic properties of ZnO-based potential diluted magnetic semiconductors codoped with transition metals: Zn 1Ϫx (FeCo) x O and Zn 1Ϫx (FeCu) x O. We have found that the origin of the observed ferromagnetism in Zn 1Ϫx (FeCo) x O would be different from that in Zn 1Ϫx (FeCu) x O. Zn 1Ϫx (FeCo) x O does not have a tendency to form the Fe-O-Co ferromagnetic cluster, and so the doubleexchange mechanism will not be effective. In contrast, Zn 1Ϫx (FeCu)

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... x O has a tendency to form the Fe-O-Cu ferromagnetic cluster with the charge transfer between Fe and Cu, which would lead to the ferromagnetism via a type of the double-exchange mechanism. The ferromagnetic and nearly half-metallic ground state is obtained for Zn 1Ϫx (FeCu) x O.