Giant Uniaxial Strain Ferroelectric Domain Tuning in Freestanding PbTiO3 Films

Dimensionality and epitaxial strain have been recently utilized to engineer the interplay between the electrostatic and elastic energies to stabilize exotic ferroelectric domain structures and topological textures in epitaxial heterostructures and superlattices. As the strain state is fixed to the substrate lattice, the strain tunability is discrete and limited, which puts a hard constraint on the exploration and engineering of emergent ferroelectric properties in these thin films and heterostructures. Here, by using water‐soluble Sr3Al2O6 (SAO) as the sacrificial buffer layer, freestanding PbTiO3 (PTO) thin films are synthesized by reactive molecular beam epitaxy to provide ideal flexible systems for continuous strain engineering as they are free of substrate‐imposed clamping. With decreasing thickness below 30 unit cells, the freestanding PTO films show thickness‐dependent reduction of c/a ratio and ferroelectricity as a consequence of strong depolarization effect. Furthermore, continuous uniaxial tensile strain up to 6.4% is applied on these freestanding films, far exceeding the achievable value reported on epitaxial PTO films. Under large tensile strain, a c domain to a domain transition with the polarization flipped from the out‐of‐plane to in‐plane direction is observed. The present work highlights the exceptional tunability of strain and dimensionality in search of emergent phases in freestanding films. Here, by using water‐soluble Sr3Al2O6 as the sacrificial buffer layer, freestanding PbTiO3 thin films are synthesized to provide ideal flexible systems for continuous strain engineering. The freestanding films are used as a model to demonstrate a giant tensile strain of 6.4%, which leads to the ferroelectric rotation from c domain to a domain.