Room temperature ferromagnetism in pristine TiO2 nanoparticles triggered by singly ionized surface oxygen vacancy induced via calcining in different air pressure

Thoroughly understanding the mechanism for the room temperature ferromagnetic performance (RTFM) in pristine TiO2 is crucial to the development of spintronic devices, and yet it is still under debated. To address this issue, pristine TiO2 nanoparticles were synthetized by a simple sol-gel technology and vacuum calcined under different air pressure in this work, and then the mechanism for the RTFM of the samples was systematically studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman scattering spectroscopy, electron paramagnetic resonance (EPR) and vibrating sample magnetometer (VSM), respectively. It is revealed from the results that surface oxygen vacancies were introduced into the TiO2 lattice via vacuum calcining, and the surface oxygen vacancies concentration increases with the decrease of calcining air pressure. The sample calcined at atmospheric pressure (1.01 × 105 Pa) is found to be diamagnetic, while the rest samples (those calcined under the pressure lower than 1.01 × 105 Pa) exhibit obvious ferromagnetism at room temperature. Moreover, the RTFM in these ferromagnetic pristine TiO2 nanoparticles tuned by the calcining air pressure shows similar variation trend to that of surface oxygen vacancies concentration. Surface oxygen vacancies are suggested to play the decisive role in inducing the RTFM of pristine TiO2 nanoparticles. Surface oxygen. vacancies in TiO2 lattice are prone to capture electrons to form singly ionized oxygen vacancies, which trigger ferromagnetic exchange coupling interactions, thus give rise to RTFM in the pristine TiO2 nanoparticles.