Coercive field of ultrathin Pb(Zr0.52Ti0.48)O3 epitaxial films

Abstract

The polarization reversal in single-crystalline ferroelectric films has been investigated experimentally and theoretically. The hysteresis loops were measured for Pb(Zr0.52Ti0.48)O3 films with thicknesses ranging from 8 to 250 nm. These films were grown epitaxially on SrRuO3 bottom electrodes deposited on SrTiO3 substrates. The measurements using Pt top electrodes showed that the coercive field Ec increases drastically as the film becomes thinner, reaching values as high as Ec≈1200 kV/cm. To understand this observation, we calculated the thermodynamic coercive field Eth of a ferroelectric film as a function of the misfit strain Sm in an epitaxial system and showed that Eth strongly depends on Sm. However, the coercive field of ultrathin films, when measured at high frequencies, exceeds the calculated thermodynamic limit. Since this is impossible for an intrinsic coercive field Ec, we conclude that measurements give an apparent Ec rather than the intrinsic one. An enormous increase of apparent coercive field in ultrathin films may be explained by the presence of a conductive nonferroelectric interface layer.