Room-temperature magnetoelectric coupling in nanocrystalline Na_(0.5)Bi_(0.5)TiO_3

摘要

Nanocrystalline Na_(0.5)Bi_(0.5)TiO_3 was prepared by sol-gel method. The nanocrystalline Na_(0.5)Bi_(0.5)TiO_3 plates present room temperature ferromagnetism (FM). The reduction of FM for the plate with subsequent long time and high temperature air-annealing indicating that the observed ferromagnetism is connected with the vacancies atear the surface of nanograins. For Na_(0.5)Bi_(0.5)TiO_3 powders after annealing at 900℃ in air, a subsequent annealing in vacuum at 900℃ for 20min weakens the room-temperature FM, but a subsequent annealing in oxygen atmosphere at 900℃ for 20min enhances the room-temperature FM, which means that the observed FM in nanocrystalline Na_(0.5)Bi_(0.5)TiO_3 may originate from cation vacancy atear the surface of nanograins. The results of density functional theory calculation with the local density approximation plus on-site effect (LDA + U) method on the magnetism of Na_(0.5)Bi_(0.5)TiO_3 (100) surface show that Na vacancies can introduce a nonzero magnetic moment. The Na_(0.5)Bi_(0.5)TiO_3 plates annealed at 900℃ for 1 h present d~0 multiferroicity with the coexistence of ferromagnetism and ferroelectric-ity at room temperature. The room-temperature magnetodielectric effect was observed in Na_(0.5)Bi_(0.5)TiO_3 plates. In addition, the electric field treatment leads to an enormous enhancement of saturation magnetization for Na_(0.5)Bi_(0.5)TiO_3 multiferroic plate, showing a strong magnetoelectric coupling.