Topological transitions in epitaxial ultrathin ferroelectric heterostructures

Abstract

This thesis explores topological defects and topological defect transitions in epitaxial ultrathin ferroelectric heterostructures. Geometrical confinements in ultrathin films has enabled the realisation of several nontrivial topological polarisation arrangements in ferroelectrics, categorised as a range of topological defects such as bubble, meron, vortex, flux-closure domain, etc. These ferroelectric topological defects can be engineered by tuning depolarisation field, mechanical and electrical boundary conditions. Our model system is an ultrathin (001) oriented PbZrxTi1-xO3/ SrTiO3/ PbZrxTi1-xO3 heterostructure fabricated on La0.67Sr0.33MnO3 buffered SrTiO3 substrates. Several topological defects are realised under specific mechanical and electrical boundary conditions. Topological defect transitions are also achieved using different routes such as electric field, thickness variation, mechanical pressure and thin film milling. These topological defects have also gained immense technological interest on account of their emergent properties. This thesis further studies the functional properties in topological defects such as electrical conductivity in bubble domains. The motion of these bubble domains is also investigated. The results herein offer new insights on how to engineer topological defects and topological defect transitions in order to design multifunctional ferroelectric/multiferroic devices with enhanced operational speed, sensitivity and energy-efficiencies.