Rise and fall of ferromagnetism in O-irradiated Al₂O₃ single crystals

In dilute magnetic semiconductors studies, sapphire was usually used as non-magnetic substrate for films. We observed weak ferromagnetic component in Al₂O₃ single crystal substrate, and excluded the possibility of ferromagnetic contaminations carefully by inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy. The ferromagnetism rise and fall during the process of annealing-oxygen irradiation-annealing of the sapphire. The ferromagnetic changes are consistent with Al-vacancy related defects detected by positron annihilation spectroscopy. With first-principle calculations, we confirm that Al-vacancy can introduce magnetic moment for 3 μB in Al₂O₃ crystal and form stable V_Al-V_Al ferromagnetic coupling at room temperature