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

Abstract

We report first-principle atomistic simulations on the effect of local strain gradients on the nanoscale domain morphology of free-standing PbTiO3 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 • domain loops, whereas thicker ones develop, in addition, 180 • 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 o 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 o 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.

Novel high-temperature ferroelectric domain morphology in PbTiO3 ultrathin films

“…This is consistent with experimental XRD results for PbTiO 3 films of similar thickness [31,32] and PbTiO 3 /SrTiO 3 superlattices [19,33,34]. Morphologically, these are LandauLifshitz domains having 90 • closure domain caps at the surface as previously observed in theoretical studies of PbTiO 3 films using interatomic potentials [11,35].…”

“…A film thickness dependence has previously been identified by Belletti et al for the c d phase showing the 180 • domain walls (see Figure 2c) to vanish into a vertex as the film thickness is decreased [11]. Our simulations reproduce this dependence which further validates our model.…”

“…[100] p [11]. The angle variations for the other domain phases have not previously been reported, nor has direct explanation for the mechanism changing the rotation angle.…”

“…Since h is multivalued for dimensionally constrained systems, dependent upon the choice of origin used to mimic the periodic supercell, we sample the surface of h for a set of supercells centred on coordinates corresponding to an xy scan of the central (001) p plane of the film. A similar procedure was first used to investigate the surface of the hypertoroidal moment for PbTiO 3 thin films with Landau-Lifshitz domains, switching due to strain gradients [11], where a sinusoidal dependence in h z for the c d domain configuration was shown as is found here (Figure 7a). Similarly, the turning points along [100] p correspond to the centres of the domains (h z (centre) = ±|h z,max |) and the domain walls (h z (DW ) = 0) and therefore they have the same period as the underlying domain configuration.…”

“…Similarly, compensation of the intrinsic depolarising field through screening has been shown to induce the formation of novel ordered domain structures [10]. A rich variety of new closure domain morphologies, such as Landau-Lifshitz stripe domains [11,12], vortex and triclinic domains [13], have been predicted in PbTiO 3 , and BaTiO 3 thin films under open-circuit boundary conditions using density functional theory, effective Hamiltonian and interatomic potential models [14,15]. Tantalising experimental evidence for these closure domains in platlets and dots of BaTiO 3 has been presented in the form of 90 • stripe superdomains bifurcated by 180 • domain walls [16][17][18] and recent direct observation of vortices in PbTiO 3 /SrTiO 3 superlattices [19].…”