Uniform arrays of centre-type topological domains in epitaxial ferroelectric thin films†
Topological domains in ferroelectric materials have attracted considerable interest owing to their exotic functionalities. In this study, using vector piezoresponse force microscopy (PFM), we observe spontaneous ferroelectric topological domains in epitaxial BiFeO3 thin films with two different intrinsic domain structures (‘mosaic-like’ and ‘stripe-like’). These domains contain three typical topological arrangements: centre-convergent domains, centre-divergent domains, and double-centre domains. These spontaneous centre domains in ferroelectrics can be explained via the ‘Ashkin–Teller’ model, in which very large interfacial lattice mismatch strains and local electric fields play a role. The uniform arrays of centre domain structure and reversible switching of these centre domains are demonstrated utilizing the radial electric field generated by the PFM tip bias. In the film with ‘mosaic-like’ intrinsic domains, the domain diameter increases significantly from 118 to 306 nm and from 158 to 300 nm for −8 V and +8 V tip voltage respectively, as the pulse increases from 0.3 to 10 s. In the film with ‘stripe-like’ intrinsic domains, the domain diameter increases slowly from 79 to 174 nm and from 98 to 149 nm for −9 V and +9 V tip voltage respectively, as the pulse increases from 0.3 s to 25 s. Measurements of retention characteristics indicate that most of these topological domain states are stable for the entire 15000 min duration in the atmosphere, assisted by the movement and enrichment of compensating charges from defect ions in the films and environment.