Ferroelectric Metal in Tetragonal BiCoO(3)/BiFeO(3) 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%.