Unusual Behavior of the Ferroelectric Polarization in PbTiO₃/SrTiO₃ Superlattices

Artificial PbTiO₃/SrTiO₃ superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO₃ to SrTiO₃ was changed. For PbTiO₃ layer thicknesses larger than the 3-unit cell SrTiO₃ thickness used in the structure, the polarization is found to be reduced as the PbTiO₃ thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO₃ to SrTiO₃ of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.