Large linear high-frequency strain by interlocked monoclinic polar nanoregions.

Ferroelectric films with large and linear strains are crucial for precision microactuator applications, especially at high frequencies. However, existing strategies that rely on frequency- and temperature-dependent dynamics have had limited success in enhancing strain response under such conditions. Here, through promoted local strain fluctuation, we achieve an interlocked polar configuration in spin-coated epitaxial (K,Na)NbO3-based ferroelectric films. The films demonstrate high-frequency strains exceeding 1.1% with high linearity and stability even when measured at 105 Hz. The presence of interlocked monoclinic and tetragonal polar nanoregions boosts piezoelectric response by promoting polarization dynamics across a broad frequency range. Additionally, the interplay between two distinct polarization switching mechanisms, arising from different symmetries and boundary conditions, mutually compensates, contributing to the observed overall linearity. This approach presents a promising yet facile strategy for achieving ferroelectric films with reliable, large and linear strain across a wide high-frequency range.