Electron engineering of metallic multiferroic polarons in epitaxial BaTiO 3

The coexistence of ferroelectricity, conductivity, and magnetism in a single-phase material has attracted considerable attention due to fundamental interest and tremendous technological potential. However, their mutually exclusive mechanisms hinder the discovery of multifunctional conducting multiferroics. Here, we propose a new material design approach for electron engineering to enable these conflicting properties to coexist. We use first principles calculations to demonstrate that appropriate mechanical strain can turn the excess electrons in doped BaTiO3 from a free-carrier configuration to a localized polaronic state by modulating the electron–phonon coupling. The resulting localized spin-polarized electron survives the host ferroelectricity and consequently manifests as a multiferroic polaron. The multiferroic properties coexist with the electronic conductivity arising from the high-hopping mobility of the polaron, which enables the doped epitaxial BaTiO3 to act as a multiferroic conducting material. This mechanical control over the electron configuration is a potential path toward unusual coexisting properties.