Density functional theory description of origin of ferromagnetism in Cu doped SnO2

• The formation and ionization of native defects in Cu doped SnO2 are investigated.
• FM and AFM coupling between Cu atoms are investigated.
• The FM stability is explained by the interaction of Cu 3d energy level.
• It is found that the magnetic moment of Cu doped SnO2 can be mediated by VO and VSn.

The defect formation and ferromagnetic stability in Cu doped SnO2 are studied using the first-principle density functional method within the local density approximation (LDA) and LDA+U schemes. Firstly, we investigate formation energies of oxygen vacancy (VO), tin vacancy (VSn), oxygen interstitial (Oi), and tin interstitial (Sni), Cu replacing Sn (CuSn) and Cu replacing O (CuO). The formation energy calculation by LDA and LDA+U shows that CuSn formation is more beneficial than CuO. Secondly, ferromagnetic (FM) and antiferromagnetic (AFM) couplings in Cu doped SnO2 are studied. The result by LDA and LDA+U shows that FM state in energy is lower than AFM for all configurations, which indicates that Cu doped SnO2 possesses FM stability. The FM stability can be explained by the interaction of Cu 3d level. Thirdly, impact of VO and VSn on magnetic moment is also analyzed. It is found that magnetic moment can be mediated by the defects.