Ferromagnetism of Na0.5Bi0.5TiO3 (1 0 0) surface with O2 adsorption

Abstract

Na0.5Bi0.5TiO3 (NBT) nanocrystalline powders prepared by sol-gel method with annealing at 900°C in air 1h present room-temperature ferromagnetism (FM). The subsequent annealing in vacuum at 900°C for 30min 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 O2 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 O2 is unfavorable at all adsorption sites. The physisorbed O2 molecule on the NBT (1 0 0) surface with a magnetic moment (MM) closes to that for an isolated O2 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 O2 molecules adsorbed on the surface. The adsorption of O2 molecule on ferroelectric materials may be a promising approach to achieve multiferroic materials.