Strain-gradient-induced switching of nanoscale domains in free-standing ultrathin films

TL;DR

This study uses atomistic simulations to show how local strain gradients can switch nanoscale ferroelectric domains in ultrathin PbTiO₃ films, revealing thickness-dependent switching mechanisms and the role of flexoelectricity.

Contribution

It demonstrates the first detailed atomistic insight into strain-gradient-induced domain switching in free-standing ultrathin ferroelectric films, highlighting thickness-dependent behaviors.

Findings

Strain gradients can switch polarization in nanoscale domains.

Thinner films show domain wall migration during switching.

Thicker films exhibit progressive switching without domain-wall motion.

Abstract

We report first-principle atomistic simulations on the effect of local strain gradients on the nanoscale domain morphology of free-standing PbTiO$_3$ ultrathin films. First, the ferroelectric properties of free films at the atomic level are reviewed. For the explored thicknesses (10 to 23 unit cells), we find flux-closure domain structures whose morphology is thickness dependent. A critical value of 20 unit cells is observed: thinner films show structures with 90$^\circ$ domain loops, whereas thicker ones develop, in addition, 180$^\circ$ domain walls, giving rise to structures of the Landau-Lifshitz type. When a local and compressive strain gradient at the top surface is imposed, the gradient is able to switch the polarization of the downward domains, but not to the opposite ones. The evolution of the domain pattern as a function of the strain gradient strength consequently depends on the film thickness. Our simulations indicate that in thinner films, first the 90$^\circ$ domain loops migrate towards the strain-gradient region, and then the polarization in that zone is gradually switched. In thicker films, instead, the switching in the strain-gradient region is progressive, not involving domain-wall motion, which is attributed to less mobile 180$^\circ$ domain walls. The ferroelectric switching is understood based on the knowledge of the local atomic properties, and the results confirm that mechanical flexoelectricity provides a means to control the nanodomain pattern in ferroelectric systems.