Correlation of Interface Impurities and Chemical Gradients with High Magnetoelectric Coupling Strength in Multiferroic BiFeO-BaTiO Superlattices.

The detailed understanding of magnetoelectric (ME) coupling in multiferroic oxide heterostructures is still a challenge. In particular, very little is known to date concerning the impact of the chemical interface structure and unwanted impurities that may be buried within short-period multiferroic BiFeO-BaTiO superlattices during growth. Here, we demonstrate how trace impurities and elemental concentration gradients contribute to high ME voltage coefficients in thin-film superlattices, which are built from 15 double layers of BiFeO-BaTiO. Surprisingly, the highest ME voltage coefficient of 55 V cm Oe at 300 K was measured for a superlattice with a few atomic percent of Ba and Ti that diffused into the nominally 5 nm thin BiFeO layers, according to analytical transmission electron microscopy. In addition, highly sensitive enhancements of the cation signals were observed in depth profiles by secondary ion mass spectrometry at the interfaces of BaTiO and BiFeO. As these interface features correlate with the ME performance of the samples, they point to the importance of charge effects at the interfaces, that is, to a possible charge mediation of ME coupling in oxide superlattices. The challenge is to provide cleaner materials and processes, as well as a well-defined control of the chemical interface structure, to push forward the application of oxide superlattices in multiferroic ME devices.