In Situ Heating STEM of the Thickness-Dependent Evolution of Ferroelastic Domains

Ferroelastic domain walls (DWs) underpin key functionalities in complex oxides, where their behavior is governed by elastic interactions and boundary conditions. In ferroic thin films, these interactions are strongly thickness dependent, characterized by a monopolar-dipolar crossover in DW behavior. However, how temperature influences DW dynamics within these size-controlled interaction regimes remains unexplored. Here, LaAlO₃ thin films spanning the dipolar (< 200 nm) and crossover (200-300 nm) regimes are investigated using in situ heating scanning transmission electron microscopy (STEM) and a machine learning-driven image analysis approach. By tracking DW curvature and area fraction during cooling from above T_C (∼550°C) to room temperature (RT), we reveal a distinct interplay between temperature and thickness. In the dipolar regime, DWs exhibit large curvature and significant reconfiguration near T_C, which reduces upon cooling, consistent with the well-known temperature freezing regime. In contrast, within the crossover regime, DWs remain nearly immobile with minimal reconfiguration through cooling and curvature values an order of magnitude lower at RT. These results map the evolution of DWs across the thermally driven super-elastic to freezing regimes, revealing the interplay between elastic interaction regimes and thermal activation, providing insights for domain engineering in free-standing oxide thin films.