Giant ferroelectric modulation of barrier height and width in multiferroic tunnel junctions

The high tunneling electroresistance (TER) effect, generally caused by ferroelectric (FE)-modulated barrier height or width, is essential for the applications of multiferroic tunnel junctions in data storage. It is traditionally obtained by distinct electrical screening lengths of electrodes. Interface engineering can enhance the TER effect further. In this work, taking $\mathrm{Co}\text{\ensuremath{-}}{({\mathrm{TiO}}₂\text{\ensuremath{-}}\mathrm{BaO})}_N\text{\ensuremath{-}}\mathrm{Co}$ tunnel junctions as examples, we demonstrate a distinct principle than the screening lengths for designing extraordinary TER effect. We reveal that when the interfacial FE displacement is much larger than that of the FE bulk, it will bend the barrier band near the interface violently, and the interfacial polarization direction pointing to or away from the interface determines whether the energy band rises or falls. The large interfacial Ba-O displacement and its corresponding polarization direction in $\mathrm{Co}\text{\ensuremath{-}}{\mathrm{BaTiO}}₃\text{\ensuremath{-}}\mathrm{Co}$ tunnel junctions can be significantly modulated by the direction of FE polarization, resulting in a metallic-insulating transition of the entire thin ${\mathrm{BaTiO}}₃$ barrier. For thick ${\mathrm{BaTiO}}₃$ barrier ($N=25$, $\ensuremath{\sim}10$ nm), the effective tunnel barrier width shifts between about 2 nm and 6.5 nm as the polarization of ${\mathrm{BaTiO}}₃$ switches direction, which can dramatically modulate the tunneling efficiency. This effect shed light on a novel route for enhancing TER through the interface engineering.