Reversible Polarization Rotation in Epitaxial Ferroelectric Bilayers

Polarization rotation engineering is a promising path to giant dielectric and electromechanical responses in ferroelectric materials and devices. This work demonstrates robust and reversible in- to out-of-plane polarization rotation in ultrathin (nanoscale) epitaxial (001) tetragonal PbZr_0.3Ti_0.7O₃ (PZT-T)/rhombohedral PbZr_0.55Ti_0.45O₃ (PZT-R) ferroelectric bilayers. An underlying 20 nm thick PZT-R layer reduces the symmetry in a 5 nm thick PZT-T layer by imposing an in-plane tensile strain while simultaneously decoupling the PZT-T layer from the substrate. This prevents clamping and facilitates large-scale polarization rotation switching (≈60 μC cm⁻²) and an effective d ₃₃ response 500% (≈250 pm V⁻¹) larger than the PZT-R layer alone. Furthermore, this enhancement is stable for more than 10⁷ electrical switching cycles. These bilayers present a simple and highly controllable means to design and optimize rotational polar systems as an alternate to traditional composition-based approaches. The precise control of the subtle interface-driven interactions between the lattice and the external factors that control polarization opens a new door to enhanced—or completely new—functional properties.