Ferromagnetism in Cu-doped ZnO from first-principles theory

Using the first-principles method based on the density functional theory, we have studied the electronic structure and the ferromagnetic stability in Cu-doped ZnO. The system shows the half-metallic ground state and the high ferromagnetic stability for all the calculated Cu concentrations within the generalized gradient approximation (GGA). The delocalized holes induced by O 2p and Cu 3d hybridization are found to be very efficient to mediate the ferromagnetic exchange interaction. While going beyond the GGA, the ferromagnetic stability as a function of the Coulomb correlation U shows a sudden decrease when U=3 eV where the system becomes an insulating ground state. By doping the p-type or n-type defects, the holes can be increased or compensated. The n-type defect, such as O vacancy, Zn interstitial, or H interstitial decreases the ferromagnetic stability, while the p-type defect, such as Zn vacancy, increases the ferromagnetic stability. By keeping the system to be the metallic ground state, the defect of the Zn vacancy could retain the high ferromagnetic stability of the system even in the case of U.