Probing Real-Space Topological Textures in Multiferroic Heterostructures with Multi-Modal Electron Microscopy

Real-space polar topological textures such as polar vortices and skyrmions in ferroelectric heterostructures emerge resulting from the interplay of elastic, electrostatic and gradient energies. While recent advancements have largely focused on proper ferroelectric systems like PbTiO3/SrTiO3 (PTO/STO) heterostructures, multiferroic BiFeO3-based systems offer a largely unexplored phase space. These materials introduce additional order parameters, such as oxygen octahedral tilts and spin, which unlock a wealth of emergent functionalities that remain to be fully understood.
To meet the challenge of characterizing these three-dimensional (3D), nanometer-sized objects, we have developed methods based on four-dimensional scanning transmission electron microscopy (4D-STEM). These approaches enable robust imaging of polar order and quantitative mapping of detailed atomic structures. In this talk, I will first present our discovery of a 3D dipolar wave phase in BiFeO3/TbScO3 (BFO/TSO) superlattices, its unique properties and the incommensurate, picometer-scale antiferrodistortive modes captured using multi-slice electron ptychography. I will then discuss recent advancements in electron imaging that enable non-destructive study of polar order in epitaxial thin films, opening exciting opportunities for in-situ and/or operando observations of potential topological phase transitions.
This research was supported by the DOE (Grant No. DE-SC0025423) and USC Viterbi Start-up Fund. Electron microscopy data were acquired at the USC Core Center of Excellence in Nano Imaging.