Domain-Wall Tunneling Electroresistance Effect

Ferroelectric tunnel junctions (FTJs) utilizing an in-plane head-to-head ferroelectric domain wall (DW) have recently been realized, showing interesting physics and new functionalities. However, the DW state in these junctions was found to be metastable and not reversible after applying an electric field. In this work, we demonstrate that a stable and reversible head-to-head DW state can be achieved in FTJs by proper engineering of polar interfaces. Using density functional theory (DFT) calculations and phenomenological modeling, we explore the DW stability by varying stoichiometry of the ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x\mathrm{O}/{\mathrm{TiO}}_2$ interfaces in FTJs with ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_3$ electrodes and a ferroelectric ${\mathrm{BaTiO}}_3$ tunnel barrier. For, $x\le 0.4$ we find that the DW state becomes a global minimum and the calculated hysteresis loops exhibit three reversible polarization states. For such FTJs, our quantum transport calculations predict the emergence of a DW tunneling electroresistance effect—reversible switching of the tunneling conductance between the highly conductive DW state and two much less conductive uniform polarization states.

Figure 1

Relaxed atomic structure of a DW-FTJ for UP (a) and head-to-head DW (b) states for $x=0.1$. (c) Energy difference (dots) between DW state ($E_{\mathrm{DW}}$) and UP state ($E_{\mathrm{UP}}$) as a function of interface Sr concentration $x$. Circles: DFT results; line: fitting results using a model described in Supplemental Material [36], Sec. S3.

Figure 2

Total energy (with respect to the DW state) as a function of interpolation parameter $\lambda$ calculated from DFT calculations (symbols) and fitted using the phenomenological model (lines). Black, red, and blue plots correspond to $x=0.1$, $x=0.3$ (with an offset of 0.3 eV) and $x=0.5$ (with an offset of 0.6 eV), respectively. Notice that $\lambda =0$ and $\lambda =\pm 1$ correspond to the DW and UP states, respectively.

Figure 3

(a) Polarization profile assumed within the phenomenological model for the DW state (red) and in the UP state (blue). (b) Simulated hysteresis loop for $x=0.1$ based on the phenomenological model. Red dots indicate the DW state. Insets: schematic representation of different polarized states. $V_{C1}$ and $V_{C2}$ denote coercive voltages.

Figure 4

Calcuated ${\mathbf{k}}_{||}$-resolved transmission in the two-dimensional Brillouin zone of asymmetric FTJ with $x=0.1$ at the left interface and $x=0.3$ at the right interface in the DW state (a), and right UP (b) and left UP (c) states.