Role of H-bond along with oxygen and zinc vacancies in the enhancement of ferromagnetic behavior of ZnO films: An experimental and first principle-based study

Magnetic properties of H₂-ambience annealed pure ZnO films have been reported. ZnO films prepared by using DC magnetron sputtering show room temperature ferromagnetism (RT-FM) after annealing in hydrogen atmosphere at two different temperatures (350 °C and 400 °C). Enhancement in O-H concentration has been observed by analyzing core-level spectra recorded using x-ray photoelectron spectroscopy (XPS). The valence band spectra suggest the enhancement in carrier concentration after annealing in H₂-ambience. Enhancement in the vacancy type defects have been observed by positron annihilation spectroscopy and tailoring of defects have been observed by photoluminescence spectra. Electrical transport results also indicate rapid decrease in activation energies corresponding to thermally activated band and nearest neighbor hopping (NNH) conduction, i.e. increase in carrier concentration after annealing. Increase in saturation magnetization and Zn vacancy (evident from M-H and PL data) along with the O-H concentration, after annealing in H₂-ambience, suggest that Zn vacancies along with O-H concentration play a crucial role behind the ferromagnetic nature of the films. Surface morphology as examined by atomic force microscopy (AFM) shows that there is no significant variation of grain size and surface roughness of the films after annealing. The experimental results are further verified theoretically by performing the density functional theory calculations. Based on both experimental results and theoretical model, it has been explained that the enhancement of magnetic moment of H₂-ambience annealed ZnO is because of the increased carrier mediated exchange interaction among the localized magnetic moment at V_Zn sites.