Flexible thin film ferroic microstructures with a large recoverable deformation

Ferroelectric and ferroelastic materials exhibit reversible electromechanical responses via domain switching, even when they comprise just a few atomic layers. However, the maximum strain achievable in ferroelectics barely exceeds 1%. This limits the practical application of ferroelectric materials in nanoscale devices that require large deformation.

In this work, we have introduced the concept of topological engineering to freestanding ferroelectric thin films to amplify the domain-switching-induced strain, achieving a large deformation at a “macroscopic” architecture level. We strategically designed geometrically twisted freestanding BaTiO₃/CoFe₂O₄ thin film architectures, which show giant recoverable deformation over 10% along their longitudinal direction, far exceeding the maximum recoverable strain (usually <1%) in ferroelectric bulk materials or thin films clamped onto a substrate. We attribute the electron beam-induced shape memory effect to the interplay between the stress induced by the ferroelectric domain switching in the BaTiO₃ layer and the mechanical stress imposed by the CoFe₂O₄ layer. Our approach allows us to realize the shape-memory effect in ferroic nanocomposites with a thickness of ~20 nm, overcoming the size limitations in conventional shape-memory alloys.