  Finite size effects in ferroelectric thin films have been probed in a series
of epitaxial perovskite c-axis oriented PbTiO3 films grown on thin
La0.67Sr0.33MnO3 epitaxial electrodes. The film thickness ranges from 480 down
to 28 A (7 unit cells). The evolution of the film tetragonality c/a, studied
using high resolution x-ray diffraction measurements, shows first a decrease of
c/a with decreasing film thickness followed by a recovery of c/a at small
thicknesses. This recovery is accompanied by a change from a monodomain to a
polydomain configuration of the polarization, as directly demonstrated by
piezoresponse atomic force microscopy measurements.
