Polarization Scaling in Ultra-thin Epitaxial Ferroelectric Heterostructures: Experimental Results

Scaling of the structural order parameter and the polarization was investigated in ultra-thin epitaxial PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ /SrRuO$_{3}$/SrTiO$_{3}$ model ferroelectric heterostructures. High Resolution Electron Microscopy and Synchrotron X-Ray studies show that a high tetragonality (c/a$\sim $1.06) is maintained down to 40 {\AA} thick films, suggesting indirectly that ferroelectricity is fully preserved at such ultrathin thicknesses. However, measurement of the switchable polarization ($\Delta P)$ using a novel pulsed probe setup revealed a systematic drop from $\sim 140\mu C/cm^2$ for a 150 {\AA} thick film to $11\mu C/cm^2$ for a 40 {\AA} thick film. This contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180$^{o}$ polydomain state for the thinnest films. This work was supported by MRSEC Grant {\#} 00-8008, DOE Grant DE-FG02-01ER45937 and NSF-DFG Grant 02-44288.