Room temperature ferromagnetism in defective TiO2 nanoparticles: Role of surface and grain boundary oxygen vacancies

Unexpected room temperature ferromagnetism (RTFM) in nonmagnetic metal oxide nanoparticles have been reported previously. This unconventional behaviour is primarily attributed to the presence of oxygen vacancies, and in this regard TiO2 is not an exception. This article, therefore, explores the effect of oxygen vacancies on the magnetic properties of TiO2 nanoparticles. TiO2 nanoparticles are prepared by sol-gel method and annealed in vacuum for 3 h and 8 h. These defective TiO2 nanoparticles are found to display RTFM and the highest magnetization is shown by the sample which is exposed for the maximum period of 8 h under vacuum. It is further observed that the RTFM is transformed to paramagnetism on air annealing of the defective TiO2 for 8 h. It is anticipated that oxygen vacancies on the surface and on the grain boundary triggers ferromagnetism in this otherwise nonmagnetic metal oxide system. The localized oxygen vacancies in these regions form oxygen vacancy clusters and undergo strong ferromagnetic exchange interaction. Switching of magnetism from ferro-to para is observed as disappearance of these oxygen vacancies set in.