Polarization and Dielectric Properties of BiFeO₃-BaTiO₃ Superlattice-Structured Ferroelectric Films

Superlattice-structured epitaxial thin films composed of Mn(5%)-doped BiFeO₃ and BaTiO₃ with a total thickness of 600 perovskite (ABO₃) unit cells were grown on single-crystal SrTiO₃ substrates by pulsed laser deposition, and their polarization and dielectric properties were investigated. When the layers of Mn-BiFeO₃ and BaTiO₃ have over 25 ABO₃ unit cells (N), the superlattice can be regarded as a simple series connection of their individual capacitors. The superlattices with an N of 5 or less behave as a unified ferroelectric, where the BaTiO₃ and Mn-BiFeO₃ layers are structurally and electronically coupled. Density functional theory calculations can explain the behavior of spontaneous polarization for the superlattices in this thin regime. We propose that a superlattice formation comprising two types of perovskite layers with different crystal symmetries opens a path to novel ferroelectrics that cannot be obtained in a solid solution system.