Emergence of critical thickness and multifaceted role of stacking faults in high misfit hexagonal ferrites films

Polarization in improper ferroelectrics is believed to be robust against the depolarizing field. However, this characteristic appears to be disrupted in hexagonal ferrite and manganite, leading to a controversial debate regarding the origin of the critical thickness. In this study, we design hexagonal TbFeO3 (h-TFO) films which exhibits a critical thickness exceeding 5 nm at room temperature and an exceptionally long suppression layer in thicker films. It is explained that the critical thickness effect originates from the synergistic interplay of the interface, strain, and vacancies, which is confirmed by combining scanning transmission electron microscopy (STEM), electron energy-loss spectroscopy (EELS) and density functional theory (DFT). This explanation is equally applicable to the polarization suppression near the stacking fault (SF). Finally, we find that the SF reduces the suppression length to an extent, and contributes to changes in the topological order of the domain in h-TFO film. Our study provides a more comprehensive understanding of the origin of critical thickness and the multifaceted role of the SF in ferroelectrics.