Domain Motion of Ferroelectricity of Bi 2 SrTa 2 O 9 Single Crystals under an AC-Voltage Electric Field

A novel phenomenon, which increases the remanent polarization of Bi 2 SrTa 2 O 9 single crystals, a promising candidate for ferroelectric random access memories (FeRAM), has been identified. The single crystals, grown in vapor phases using the self-flux method, have a composition characterized as Bi x Sr y Ta 2 O 9 ( x =2.08±0.09, y =1.04±0.06). In contrast to Bi x Sr y Ta 2 O 9 ( x =1.91±0.05, y =1.27±0.08) single crystals grown by the self-flux method, the coercive field of the present single crystals is smaller. Observing optical anisotropy in the c-plane, we found that this material has a paraelectric phase, which might originate from the partial distortion of the crystal. After voltage was applied, the paraelectric phase disappeared and the crystal became a ferroelectric domain structure. Measuring the electrical properties in the c-plane, the remanent polarization of the Bi 2 SrTa 2 O 9 single crystal was increased by applying ac-voltage. One-hour annealing over the Curie temperature also produced a paraelectric phase in the crystal but it was confirmed that this paraelectric phase can also be decreased by applying ac-voltage. Using this ac-voltage application, we can clearly observe the domain structure of BiSTa single crystal for the first time.