Monodomain to polydomain transition in ferroelectric PbTiO₃ thin films with La_0.67Sr_0.33MnO₃ electrodes

Finite size effects in ferroelectric thin films have been probed in a series of epitaxial perovskite c-axis oriented PbTiO₃ films grown on thin La_0.67Sr_0.33MnO₃ epitaxial electrodes. The film thickness ranges from 480 down to 28 Å (seven 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.