Impurity Clustering and Ferromagnetic Interactions that are not Carrier-Induced in Dilute Magnetic Semiconductors: The Case of Cu2O:Co

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.