Room temperature ferromagnetism in D-D neutron irradiated rutile TiO₂ single crystals

Room temperature ferromagnetism (RTFM) was observed in unirradiated rutile TiO₂ single crystals prepared by the floating zone method due to oxygen vacancy (V_O) defects. D-D neutrons mainly collide elastically with TiO₂, producing V_O, titanium vacancies (V_Ti) and other point defects; the density and kind of defect is related to the neutron irradiation fluence. D-D neutron irradiation is used to regulate the concentration and type of defect, avoiding impurity elements. As the irradiation fluence increases, the saturation magnetization (M_s) first increases, then decreases and then increases. To verify the origin of RTFM, the CASTEP module was used to calculate the magnetic and structural properties of point defects in TiO₂. V_O induces a 2.39 μ _B magnetic moment, Ti³⁺ and F⁺ induce 1.28 μ _B and 1.70 μ _B magnetic moments, respectively, while V_Ti induces a magnetic moment of ∼4 μ _B. Combining experimental and theoretical results, increases in V_O concentration lead to M_s increases; more V_O combine with electrons to form F⁺, inducing a smaller magnetic moment. V_O and V_Ti play a key role and M_s changes accordingly with larger fluence. V_O, F⁺ and V_Ti are the most likely origins of RTFM.