Epitaxial Coherence at Interfaces as Origin of High Magnetoelectric Coupling in Multiferroic BaTiO3–BiFeO3 Superlattices

Abstract

Multiferroic superlattices consisting of 15 double layers BaTiO3–BiFeO3 show a very high magnetoelectric voltage coefficient αME of up to 49 V cm−1 Oe−1 at 300 K, measured at 1 kHz with zero direct current (DC) bias magnetic field. However, the microscopic origins of such high αME values, and the temperature and DC magnetic field dependencies of αME are not understood up to now. Therefore, in this study two superlattices grown at high/low oxygen partial pressures having high/low αME values, respectively, are compared. While at high growth pressure the strain contrast in high‐resolution transmission electron microscopy images is limited to few layers at the substrate interface, the low‐pressure sample shows much more pronounced microstrain. This is additionally visualized in Fourier transformed images, and the out‐of‐plane lattice parameter of the single BaTiO3 layer is increased near the interface. In addition to a low density of oxygen vacancies, it seems to be important to avoid micromechanical clamping of the 2D single‐phase nanolayers of the multiferroic BaTiO3–BiFeO3 superlattices to achieve high magnetoelectric coupling.