Defect types and room-temperature ferromagnetism in undoped rutile TiO₂ single crystals

Room-temperature ferromagnetism has been experimentally observed in annealed rutile TiO₂ single crystals when magnetic field is applied parallel to the sample plane. By combining X-ray absorption near edge structure spectrum and positron annihilation lifetime spectroscopy, Ti³⁺–V_O defect complexes (or clusters) have been identified in annealed crystals at high vacuum. We elucidate that the unpaired 3d electrons in Ti³⁺ ions provide the observed room-temperature ferromagnetism. Besides, excess oxygen ions in TiO₂ lattice could induce a number of Ti vacancies which increase magnetic moments obviously.