Evidence of oxygen and Ti vacancy induced ferromagnetism in post-annealed undoped anatase TiO2 nanocrystals: A spectroscopic analysis

Highlights: • Evidence of vacancy-induced room-temperature (RT) ferromagnetism (FM) is observed in TiO₂ nanocrystals. • Inherent crystal defects are analyzed using XPS, PL, Raman, CDB and PAS Spectroscopy. • Correlation between defects and RTFM are investigated carefully. • Defect-induced-FM provides opportunity to develop dilute ferromagnetic oxides for new spin-functionalities. -- Abstract: The evidence of oxygen and titanium (Ti) vacancy induced room-temperature ferromagnetism (RTFM) is observed in anatase TiO₂ nanocrystals investigated by spectroscopic techniques. RTFM significantly depends on the annealing temperature due to modification of intrinsic defects. Magnetic moment (M_S) and Curie temperature (T_C) are found to increase initially with increase of annealing temperature and then decreases on further annealing. The samples annealed at lower temperature, is found to possess significant amount of singly ionized oxygen vacancy (V_O⁺) defects. In addition, coincident doppler broadening and positron annihilation spectroscopic analysis provides an indication that such oxygen vacancy may merge with one or two Ti vacancy (V_Ti) and thereby forming larger-sized defect-combinations like divacancy V_Ti+O and trivacancy V_Ti+O+Ti which act as dominant positron trapping centre within the nanocrystalline TiO₂. Hence, the combination of V_O and V_Ti defects play the critical role in inducing RTFM in TiO₂ nanocrystals which can yield promising spintronic applications.