Study on the finite-size effects in ferroelectric thin films and fine-particles

Finite size effects on the dielectric properties in ferroelectric materials were studied.Recently，the thickness of ferroelectric films is less than several tens of nanometer.Accordingly，the diameter of the particles in the film may be less than 10 nm。It is known that some dielectric properties differ from that of bulk materials when the size of crystallites approaches nano-meter size.However，the microscopic understanding of the size effect has not been obtained。In this research，we deal with the size effects on the phase transition in several perovskite type ferroelectrics.The first result concerns with the origin of the size effect on the phase transition.The size effects are successfully described by phenomenological theories based on the Landau-Ginzburg@equation。The theory assumes the electric polarization near the surface is not uniform and differs from the bulk value。We found that the crystal lattices of lead titanate and barium titanate relaxes towards outside by X-ray measurements with the use of a simple model。We pointed out that the size effect should be caused by the non-homogenous polarization caused by the surface relaxation.We also studied the low-temperature growth of perovskite type ferroelectric films.If we can crystallize the film at low temperatures，we can obtain film composed of very small particles。The phase transition temperatures of such small particle should be lower than that of bulk crystals by the size effect。The dielectric constant of the film is expected to be larger than that of bulk material.We proposed a technique to insert a seeding layer between the film and the substrate。For example，insertion of PbTiO I D23个D2 seeding layer made it possible to obtain a well crystallized film at very low temperature such as 450℃.

Finite size effects on the dielectric properties in ferroelectric materials were studied. Recently, the thickness of ferroelectric films is less than several tens of nanometer. Accordingly, the diameter of the particles in the film may be less than 10 nm. It is known that some dielectric properties differ from that of bulk materials when the size of crystallites approaches nano-meter size. However, the microscopic understanding of the size effect has not been obtained. In this research, we deal with the size effects on the phase transition in several perovskite type ferroelectrics.The first result concerns with the origin of the size effect on the phase transition. The size effects are successfully described by phenomenological theories based on the Landau-Ginzburg equation. The theory assumes the electric polarization near the surface is not uniform and differs from the bulk value. We found that the crystal lattices of lead titanate and barium titanate relaxes towards outside by X-ray measurements with the use of a simple model. We pointed out that the size effect should be caused by the non-homogenous polarization caused by the surface relaxation.We also studied the low-temperature growth of perovskite type ferroelectric films. If we can crystallize the film at low temperatures, we can obtain film composed of very small particles. The phase transition temperatures of such small particle should be lower than that of bulk crystals by the size effect. The dielectric constant of the film is expected to be larger than that of bulk material. We proposed a technique to insert a seeding layer between the film and the substrate. For example, insertion of PbTiOィイD23ィエD2 seeding layer made it possible to obtain a well crystallized film at very low temperature such as 450℃.