Effect of Defects on Magnetic Properties of O⁺-implanted AlN Films by Positron Annihilation Spectroscopy

Room-temperature ferromagnetism is observed in the O⁺-implanted AlN films with O⁺ doses of 5\times10^16 cm⁻² (AlN: \rmO_5\times10^16 ) and 2\times10^17 cm⁻² (AlN: \rmO_2\times10^17 ). The observed magnetic anisotropy indicates that the ferromagnetism is attributed to the intrinsic properties of O⁺-implanted AlN films. The out-of-plane saturation magnetization (M_\rmS) of the AlN: \rmO_5\times10^16 is about 0.68 emu/g, much higher than that of AlN: \rmO_2\times10^17 , 0.09 emu/g, which is due to the excessively high O⁺ dose made more O⁺ ions occupy adjacent Al³⁺ positions in forms of antiferromagnetic coupling. Doppler broadening of positron annihilation radiation measurements demonstrate the existence of Al vacancies in the O⁺-implanted AlN films. The first-principles calculations suggest that the ferromagnetism originates mainly from the Al vacancies. Meanwhile, the formation of divacancies or vacancy clusters by high concentrations of Al vacancies will lead to the transformation of V_Al–V_Al coupling from ferromagnetim to antiferromagnetism, ultimately weakening the ferromagnetism of the sample.