Thermal effect on domain wall roughness in epitaxial PbZr_0.2Ti_0.8O₃ thin films with different as-grow polarization orientations

In epitaxial ferroelectric films, the asymmetric boundary condition can lead to strong polarization asymmetry that affects the thermal stability of different polar states. We have carried out piezo-response force microscopy (PFM) studies on the thermal evolution of domain walls (DW) in epitaxial 50 nm PbZr_0.2Ti_0.8O₃ (PZT) films grown on (La,Sr)MnO₃ and LaNiO₃ buffered SrTiO₃, which show different as-grown polarization. We wrote stripe-domains and identified the DWs from the PFM images. The correlation function of the DW roughness depends on the distance L along the DW as L^2ζ. At room temperature, the roughness exponent ζ is 0.2-0.3, consistent with 2D DWs in weak random bond disorder. The films were then heated at progressively higher temperatures till approaching T_C. The subsequent PFM images show that ζ gradually increases to ~0.6, evolving to dominant thermal roughening. Both the bottom electrode type and the as-grown polarization orientation affect the T-dependence of ζ. We also compare the results obtained from in situ high temperature measurements and rapid thermal quench. Our study provides critical information about the thermal stability and size scaling limit of epitaxial thin films-based ferroelectric devices.