The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA plus U studies

The electronic structure and ferromagnetic stability of Co-doped SnO2 are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U-Co transforms the ground state of Co-doped SnO2 to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U-Co calculations show that the pure substitutional Co defeats in SnO2 cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U-Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U-Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U-O on O 2s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U-Co calculations are not changed by the correction of band gap. (C) 2009 Elsevier B.V. All rights reserved.