Flexosensitive polarization vortices in thin ferroelectric films

Abstract

We consider theoretically the influence of the flexoelectric coupling on the spatial distribution and temperature behavior of spontaneous polarization for several types of stable domain structure in thin ferroelectric films, such as stripe domains and vortices. Finite element modelling (FEM) for BaTiO3 films and analytical calculations within the Landau-Ginzburg-Devonshire approach reveals that an outof-plane polarization component can be very sensitive to the flexoelectric coupling for periodic quasi-2D stripe domains and 3D vortex-antivortex structures. However, the influence is rather different for these structures. The flexoelectric coupling increases significantly the amplitude of a small out-of-plane polarization component in the stripe domains, and the "up" or "down" direction of the component is defined by the sign of the flexoelectric coefficients. Concerning the vortex-antivortex pairs, their antivortices with in-plane anti-сirculation have smooth wide dipolar cores through the entire film, whose shape and other features are almost insensitive to the coupling. The vortices with in-plane vorticity have spike-like cores with an out-of-plane quadrupolar moment induced by the flexoelectric coupling. The cores are located near the film-dead layer interfaces. FEM results corroborated by analytical calculations prove that a change of the flexoelectric coefficient sign leads to a reorientation of the core axial polarization, making the flexo-sensitive 3D vortices similar to the recently introduced "flexons" in cylindrical nanoparticles. The relatively wide temperature range (from 200 to 400 K) of the flexosensitive vortices' existence gives us the hope that they can be observed experimentally in thin ferroelectric films by scanning probe and nonlinear optical microscopy methods.

Thickness-Dependent Polar Domain Evolution in Strained, Ultrathin PbTiO3 Films

“…Moreover, according to a previous study, large tensile strains facilitate the formation of in-plane polarizations, which promotes the formation of meron polar structures. Particularly, we note that the flexoelectric effect is usually involved in ultrathin ferroelectric films because of the possible coexisting, low-symmetry phases, which may also be associated with the meron topologies according to the recent literature. , In addition, there are CDW as marked by a pair of green arrows in Figure a.…”

“…Particularly, we note that the flexoelectric effect is usually involved in ultrathin ferroelectric films because of the possible coexisting, lowsymmetry phases, which may also be associated with the meron topologies according to the recent literature. 46,47 In addition, there are CDW as marked by a pair of green arrows in Figure 5a.…”