| Abstract: | Room-temperature ferromagnetism in WO3 films was enhanced by 130 keV Ar+ ion irradiation. The X-ray diffraction (XRD) and Raman measurements not only confirmed the monoclinic phase of the irradiated WO3 films, but also showed that oxygen vacancy (VO) defects were formed. The analysis of photoluminescence spectra strongly reconfirmed the presence of oxygen vacancy. X-ray photoelectron spectroscopy (XPS) measurements revealed that the contents of VO and induced W5+ ions increase with increasing irradiation fluence and rich W5+-VO defect complexes in the irradiated WO3 films were formed. Further, the magnetic measurements exhibited a 2-fold enhancement in the saturation magnetization at the largest fluence of 3 × 1016 ions/cm2. At lower irradiation fluence, a bound magnetic polaron model was proposed to reveal the ferromagnetic exchange coupling resulting from overlapping of VO+ and VO++ defect states, and 5d1 states of W5+. At high irradiation fluence, the carrier concentration reaches 1.02 × 1020/cm3 and carrier-mediated exchange interactions result in the film's ferromagnetism. |
