Polaron-induced intrinsic ferromagnetic ordering in Cu-doped ZnO thin films

We prepared high-quality Cu-doped ZnO films by the spray pyrolysis technique and explored the correlations between their magnetic ordering and the existing native defect states in the ZnO lattice. The magnetization curves showed a paramagnetic phase, which coexists with the ferromagnetic phase at room temperature in all doped films. The results indicated that the detected ferromagnetic ordering is intrinsic and induced by the Cu dopants and correlated with the existing defect states and vacancies in the lattice. By increasing the Cu dopant in the films, we detected a clear improvement in the ferromagnetic ordering. This is correlated with an obvious enhancement in the oxygen vacancy (V_O) and zinc interstitial (Zn_i) concentrations and a slight reduction in the concentration of zinc vacancies and oxygen interstitial levels. The magnetic ordering mechanism in these films is associated with the long-range ferromagnetic coupling between Cu ions mediated by the intrinsic defects of V_O and Zn_i, through the reaction defect complexes networks of Cu⁺²-V_O-Cu⁺ and Cu⁺²-Zn_i-Cu⁺. The bound-magnetic polarons (BMPs) are formed through these indirect exchange interactions. The magnetization curves are closely fitted with the BMPs model. The concentration of these BMPs is found to be above the reported percolation threshold of BMPs to construct long-range ferromagnetic coupling in Cu-doped ZnO.