Surface effects on phase transitions in ferroelectrics and dipolar magnets

A phenomenological theory is developed to describe the change of the local spontaneous polarization in the vicinity of a free surface of a ferroelectric thin film which is kept between metallic electrodes. It is shown that depolarizing field effects reduce the deviation of this local polarization from its bulk value, as compared to surface effects on phase transitions in other systems. In particular, the critical exponents describing the behavior of the local polarization in the vicinity of the Curie temperature $T_C$ are the same as the bulk exponents and only critical amplitudes are changed. This behavior contrasts to phase transitions in other systems (antiferroelectrics, ferro- and antiferromagnets, ordering alloys etc.) where different exponents are predicted. In order to improve upon this Landau-type theory by taking into account the effects of statistical fluctuations near $T_C$, recent results of renormalization-group theory are used to estimate logarithmic correction factors which should modify the critical behavior of the local polarization. Finally the experimental implications of our results are briefly discussed, and also a discussion of surface effects on the phase transition of dipolar magnets is given.