Ferromagnetic spin-order in p-type N-doped SnO2 films prepared by thermal oxidation of SnNx

N-doped SnO2 films were synthesized by oxidative annealing of sputtered SnNx films with various temperatures. The content of the residual N in SnO2:N films decreases with the increase of temperature. N atom is incorporated well at the O lattice site and red-shifts the band-gap by the formation of impurity bands. All the films show p-type conduction and the highest hole concentration is 2.08×1019 cm−3 in the sample obtained at 400 °C, indicating an effective way to alleviate the self-compensation effect by thermal oxidation. The substituted incorporation of N can also enhance the ferromagnetism and the largest saturation magnetization is 10.1 emu/cm3. Theoretical calculations based on the density-functional theory suggest that the ferromagnetism is most likely due to the double exchange mechanism through the p-p interaction. Meanwhile, the ferromagnetic coupling competes with antiferromagnetic coupling as the N-N distance changes, causing the nonmonotonic variation of saturation magnetization.