Ferroelectric tunnel junction (FTJ) is a tunnel junction in which two metal electrodes are separated by a thin ferroelectric layer. The spontaneous polarization of the ferroelectric layer can be switched by an applied electric field. The electrical resistance of a FTJ strongly depends on the orientation of the electric polarization. This phenomenon is known as tunneling electroresistance (TER). There are at least three mechanisms which are responsible for the TER effect resulting from the switching of ferroelectric polarization: (i) change in the electrostatic potential profile across the ferroelectric barrier: (ii) change in the transmission coefficient across interfaces; (iii) change in the attenuation constant of the barrier.
Using a ferroelectric barrier in a magnetic tunnel junction makes a multiferroic tunnel junction (MFTJ), whose transport properties depends on both the ferroelectric polarization of the barrier and the magnetization orientation of the electrodes. In MFTJs the spin polarization of tunneling electrons is affected by the ferroelectric polarization and thus the TER effect coexists with tunneling magnetoresistance (TMR) making MFTJ a four-state resistance device. Our first-principles and model calculations demonstrate the significance of these phenomena and pave the way for the experimental realization of FTJs and MFTJs.
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Schematic band diagram of a ferroelectric tunnel junction.
Demonstration of tunneling electroresistance effect: Ferroelectric polarization pattern of a thin ferroelectric film in the form of the MRSEC logotip (top) and the same pattern imaged by detecting local tunneling current across the film (bottom) (courtesy A. Gruverman).
Magnetic (a), ferroelectric (b) and multiferroic (c) tunnel junctions, and resistance response of these junctions to magnetic and electric fields.
Four-state resistance states in SrRuO3/BaTiO3/SrRuO3 multiferroic tunnel junction: Calculated k||-resolved transmission across the for different polarization P of the barrier and magnetization M of the electrodes.