Controlling topological defect transitions in nanoscale lead zirconate titanate heterostructures

Varying thickness in ferroelectric heterostructures systematically changes both the strain and the electrical boundary conditions and thus the polarization screening. This has a direct result on the observed ferroelectric nanotopologies, from polar vortices, skyrmions, and bubbles to Kittel type stripe/labyrinthine domains. Here, a control of the topological defect transitions is reported in epitaxial (001)-oriented PbZr_0.4Ti_0.6O₃/SrTiO₃/PbZr_0.4Ti_0.6O₃ (PZT/STO/PZT) heterostructures. Piezoresponse force microscopy is exploited to capture various topological defect states, such as merons, skyrmions, dislocations, bimerons, and three- or fourfold junctions and hence to understand their transition pathways. The thickness of the dielectric spacer and/or ferroelectric layer is tuned during growth to manipulate the strength of the residual depolarization field; this consequently leads to a range of the abovementioned topological defect structures. This is further corroborated by effective Hamiltonian-based Monte Carlo simulations that provide insight into why and how altering the thickness of ferroelectric or dielectric layers triggers topological phase transitions. This controlled design of nanoscale ferroic topologies opens possibilities of engineering emergent transitions.