Na0.5Bi0.5TiO3 (NBT) nanocrystalline powders prepared by sol-gel method with annealing at 900 degrees C in air 1 h present room-temperature ferromagnetism (FM). The subsequent annealing in vacuum at 900 degrees C for 30 min weakens the room-temperature FM, while subsequent treatments in oxygen atmosphere at room-temperature enhances the room-temperature FM, indicating that the room-temperature FM may be induced by the adsorbed oxygen on the NBT nanoparticle surface. The adsorption of O-2 molecule on the NBT (1 0 0) surface is studied by using density functional theory within local density approximation plus on-site effect method. The physisorption of configuration R5 is the most stable, whereas the chemisorption of O-2 is unfavorable at all adsorption sites. The physisorbed O-2 molecule on the NBT (1 0 0) surface with a magnetic moment (MM) closes to that for an isolated O-2 molecule. The magnetism of configuration R5 is mainly from the O p orbitals. The stable ferromagnetic coupling mechanism is the direct exchange interaction between the nearest-neighbor O-2 molecules adsorbed on the surface. The adsorption of 02 molecule on ferroelectric materials may be a promising approach to achieve multiferroic materials. (C) 2017 Elsevier B.V. All rights reserved.