Ferroelectric Metal in Tetragonal BiCoO3/BiFeO3 Bilayers and Its Electric Field Effect

By first-principles calculations we investigate the electronic structure of tetragonal BiCoO 3 /BiFeO 3 bilayers with different terminations. The multiferroic insulator BiCoO 3 and BiFeO 3 transform into metal in all of three models. Particularly, energetically favored model CoO 2 -BiO exhibits ferroelectric metallic properties, and external electric field enhances the ferroelectric displacements significantly. The metallic character is mainly associated to e g electrons, while t 2g electrons are responsible for ferroelectric properties. Moreover, the strong hybridization between e g and O p electrons around Fermi level provides conditions to the coexistence of ferroelectric and metallic properties. These special behaviors of electrons are influenced by the interfacial electronic reconstruction with formed Bi-O electrovalent bond, which breaks O A -Fe/Co-O B coupling partially. Besides, the external electric field reverses spin polarization of Fe/Co ions efficiently, even reaching 100%.