Influence of native defects and their evolution on the ferromagnetic ordering of transition metals doped ZnO thin films.

We report on the correlation between the native defects including oxygen vacancies (V_O)  and Zinc vacancies (V_Zn) and their evolution under transition metal (TM) substitution on the ferromagnetic ordering of TM-doped ZnO thin films; TM = Co, Mn, Ni, and Cu. In addition, we report on the thermal annealing effects in different atmospheres including ambient air, vacuum, nitrogen, and hydrogen on the ferromagnetic ordering of TM-doped ZnO thin films. The TM-doped ZnO thin films were successfully synthesized using spray pyrolysis technique, and characterised using x-ray diffraction, x-ray photoelectron spectroscopy, superconducting quantum interference device magnetometry, photoluminescence spectroscopy, and ultraviolet-visible spectroscopy. All TM-doped ZnO films showed a single phase würtzite structure, indicating a successful incorporation of TM dopants into the ZnO lattice [1-3]. The doped films showed a clear ferromagnetic phase at room temperature, which is improved at low temperatures. The results clearly showed a direct correlation between the concentration of V_O and the observed ferromagnetism (FM). By increasing the Co, Mn, Ni, and Cu content, we observed a clear enhancement in the saturation magnetization and coercivity in parallel with a considerable enhancement in the concentration of V_O. The FM in the doped films became stronger after annealing in vacuum and weaker after annealing in air, in accordance with a considerable increase and decrease in the concentration of V_O that mediates the observed FM. In Cu doped ZnO, we observed a link between the concentration of V_Zn and the observed FM. The overall results provided a closer view on the ordering mechanism in TM-doped ZnO films and the role of the native defects that mediates this long-range ordering.