Defect induced ferromagnetic ordering in epitaxial Zn_0.95Mn_0.05O films on sapphire (0 0 0 1)

We report the influence of Mn dopant on magnetic properties of Zn_0.95Mn_0.05O (ZMO)/Al₂O₃(0 0 0 1) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L _2,3 edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0 0 0 5) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L ₃-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (V_Zn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous V_Zn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density V_Zn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.