Localized Flexoelectric Effect Around Ba(CuNb) Nano-Clusters in Epitaxial BiFeO₃ Films for Enhancement of Electric and Multiferroic Properties

Room-temperature (RT) multiferroic materials have received significant research attention for various potential applications; however, their properties are not suitable for real-world implementation. In this study, a nano-scale localized flexoelectric effect is introduced to enhance the RT multiferroic performance of epitaxial bismuth iron oxide (BiFeO₃; BFO) thin films by embedding 10 mol% Ba(Cu_1/3Nb_2/3)O₃ (BCN) nano-clusters into the host BFO film, which originally has a rhombohedral crystal structure. By utilizing nano-clustering, a large out-of-plane coherent strain is localized around the nano-clusters, resulting in a highly strained tetragonality of the BFO structure; subsequently, the films exhibit peculiar types of domains and domain walls, such as nano-scale rotational vortices and antiparallel dipole configurations. These peculiar domain structures, which originate from the localized flexoelectric effect at the nano-scale, enable excellent ferroelectric, ferromagnetic, and RT multiferroic magnetoelectric coupling. This study reveals that the local variation in the localized flexoelectric field around nano-clusters considerably impacts the formation of unusual domain-wall structures. This suggests that the controlled introduction of nano-clusters with different crystal structures is promising for achieving the desired multiferroic properties.