Unusual Behavior of the Ferroelectric Polarization inPbTiO3/SrTiO3Superlattices

Artificial ${\mathrm{PbTiO}}_3/{\mathrm{SrTiO}}_3$ 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 ${\mathrm{PbTiO}}_3$ to ${\mathrm{SrTiO}}_3$ was changed. For ${\mathrm{PbTiO}}_3$ layer thicknesses larger than the 3-unit cell ${\mathrm{SrTiO}}_3$ thickness used in the structure, the polarization is found to be reduced as the ${\mathrm{PbTiO}}_3$ 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 ${\mathrm{PbTiO}}_3$ to ${\mathrm{SrTiO}}_3$ of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.