Ferroelectric thin films have some specific properties such as low dielectric constant, high coercive force and presenting of polarization above Curie point, etc. These properties can be explained by the existence of surface layer and internal stresses in the films. The surface layer has lower dielectric constant and large spontaneous strain than in the bulk. The internal stresses will cause the shift of Curie point and create an internal bias field in the films.
Weiguo Liu, Weiguo Liu,
"Specific properties of ferroelectric thin films", Proc. SPIE 1519, International Conference on Thin Film Physics and Applications, (1 November 1991); doi: 10.1117/12.47333; https://doi.org/10.1117/12.47333