PHASE-TRANSITION BEHAVIOR OF THE SPONTANEOUS POLARIZATION AND SUSCEPTIBILITY OF FERROELECTRIC THIN-FILMS

Phase-transition behavior of the spontaneous polarization and dielectric susceptibility of ferroelectric thin films was studied based on the Ising model in a transverse field. The modification of the exchange constant and the transverse field in the surface layer might lead to the spontaneous polarization and the Curie temperature changing in a different direction as well as in the same direction. When the Curie temperature of the films is enhanced, the temperature dependence of the spontaneous polarization shows a tail-like structure; and there exist two peaks in the susceptibility-temperature curve. One is located at the Curie temperature of the films, the intensity of which decreases with increasing film thickness. The other is around the bulk Curie point, the intensity of which increases with increasing film thickness. The theory gives a reasonable description of the experimental facts of triglycine sulfate (TGS) thin films in the literature.