Effects of shear and bending strains on domain structures in freestanding ferroelectric thin films from phase-field simulations

Freestanding ferroelectric thin films, free from substrate constraints, present a platform for advanced strain engineering owing to their exceptional mechanical flexibility. The strain state in freestanding ferroelectric thin films can be modulated through various mechanical deformations, enabling precise control over the physical properties and performance of the ferroelectric films. Here, we utilized phase-field simulations to explore the polarization evolution and switching behavior of freestanding BaTiO₃ ferroelectric thin films under bending and shear strains. Our findings reveal that shear strain transforms flux-closure domains into a monoclinic phase, increasing the coercive field, maximum polarization, and remanent polarization, thereby broadening the ferroelectric polarization-electric field hysteresis loop. The underlying mechanism involves the competition between elastic and electrostatic energies, which becomes more pronounced with increasing shear strain. Additionally, this contrasts with the modulation of domain structures by bending strain, which causes a rightward shift in the ferroelectric polarization-electric field hysteresis loop due to the flexoelectric fields generated by bending deformation. These findings provide profound insights into the strain effects in ferroelectrics, highlighting the complex interplay between mechanical deformation and electrical response. The ability to manipulate domain structures and polarization behaviors through controlled mechanical strains paves the way for designing high-performance, flexible ferroelectric devices.