Room-temperature ferromagnetism of Cu-doped ZnO films probed by soft X-ray magnetic circular dichroism

In this paper, we report the direct evidence of the room temperature ferromagnetism in O-deficient ZnO:Cu films using soft X-ray magnetic circular dichroism (SXMCD) and X-ray absorption (XAS). SXMCD has revealed two distinct features of Cu atoms associated with (1) magnetically ordered Cu ions present only in the oxygen deficient samples, and (2) magnetically disordered \textit{regular} Cu$^{2+}$ ions present in all the samples. These observations indicate that sufficient amount of both oxygen vacancies (V$_{O})$ and Cu impurities are essential to the observed ferromagnetism, and non-negligible portion of Cu impurities are uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic ``indirect double exchange'' model, in which alignment of localized magnetic moment of Cu in the vicinity of the V$_{O}$ are mediated by the large-sized vacancy orbitals.