Phase transitions in ferroelectric thin films

Abstract

We discuss theory and experiment for phase transitions in ferroelectric films, 70–400 nm thick. It is found that the Curie temperature T_c can be shifted up or down by temperatures as great as 70 K. In addition, transitions that are strongly first-order in bulk may become continuous in sufficiently thin films. Finally, the shape of the spontaneous polarization curve, plotted versus temperature, can become complicated, dropping to zero at an elevated T_c in two discrete jumps, with the higher temperature discontinuity arising from a residual ferroelectricity in a thick surface layer. These points are illustrated experimentally with data on PZT (PhZr_1−x Ti _x O₃) and KNO₃.