Impurity clustering and ferromagnetic interactions that are not carrier induced in dilute magnetic semiconductors:: The case of Cu2OCo

Current models for ferromagnetism in diluted magnetic semiconductors, such as p-d exchange or double-exchange, rely on the presence of partially filled gap states. We point out a new mechanism, not requiring partially filled states, in which ferromagnetic coupling arises from the occupation of previously unoccupied levels when two transition metal impurities form a close pair. We find from first-principles calculations that this mechanism explains strong ferromagnetic coupling between Co impurities in Cu2O, and at the same time gives rise to Co clustering.