Flexoelectric Polarization Control via Defect-Driven Strain Gradients in Complex Oxide Thin Films

Electromechanical coupling in oxide thin films can be realized through strain-gradient-driven flexoelectricity, but the lack of directional control has hindered its practical implementation in devices. Here, we demonstrate that defect-gradient engineering through spatial redistribution of oxygen vacancies provides a device-compatible route to control flexoelectric polarization in single-crystalline LaAlO₃ (LAO) thin films. Vacancy-rich lines patterned on the LAO surface induce local lattice expansion and converging strain gradients, which yield a pronounced in-plane electromechanical response as revealed by lateral piezoresponse force microscopy. Remarkably, when the spacing between vacancy-rich patterns is reduced below ∼100 nm, the response evolves from alternating to unidirectional, ultimately producing uniform in-plane poling. Finite-difference drift-diffusion and Poisson simulations corroborate that asymmetric defect distributions drive a robust in-plane electromechanical response. This electrically writable defect-gradient strategy enables deterministic modulation of flexoelectric polarization at microscopic and mesoscopic scales, without mechanical deformation, opening pathways to defect-gradient-based functionalities in rewritable oxide electronics, nanoscale sensing, actuation, and energy harvesting.