van der Waals multiferroic tunnel junctions based on sliding multiferroic layered ${\mathrm{VSi}}_2{\mathrm{N}}_4$

Multiferroic tunnel junctions (MFTJs) have attracted considerable attention due to their multifunctional properties, which are valuable for nonvolatile memory devices. The recent advancements in van der Waals (vdW) multiferroic materials, combining ferromagnetic and ferroelectric properties, provide an excellent platform for exploring MFTJs at the atomic scale. In this study, we employ a combination of nonequilibrium Green's function and density functional theory to theoretically investigate the spin-dependent transport properties of vdW MFTJs, which consist of metal electrodes and sliding multiferroic layered ${\mathrm{VSi}}_2{\mathrm{N}}_4$ barrier layers. Our findings demonstrate that asymmetric $\mathrm{Ag}\left(111\right)/\text{bilayer}\text{−}{\mathrm{VSi}}_2{\mathrm{N}}_4/\mathrm{Au}\left(111\right)$ MFTJs can exhibit multiple nonvolatile resistance states by manipulating the ferroelectric polarization and magnetization alignment of the bilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$, achieving maximum tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) ratios of up to $1.01\times {10}^5\%$ and 37.3%, respectively. More intriguingly, the TER ratio can be further increased to 448.3% by employing left and right symmetric Au(111) electrodes and trilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$ barrier layers. Additionally, we reveal that layered ${\mathrm{VSi}}_2{\mathrm{N}}_4$ possesses intrinsic multiferroicity with the coexistence of the out of plane ferroelectricity and interlayer A-type antiferromagnetism. Through an analysis of electronic structure and Berry curvature, we elucidate the coupling between ferroelectricity and antiferromagnetism via a ferrovalley, enabling electrically controlled magnetism in the bilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$ by interlayer sliding. Our results demonstrate that giant TMR, large TER, and multiferroic coupling can coexist in layered ${\mathrm{VSi}}_2{\mathrm{N}}_4$, with potential applications in other vdW layered multiferroics. The controllable interlayer sliding of vdW MFTJs offers promising opportunities for the design of next-generation logic and memory devices.

Figure 1

(a), (b) Side views of bilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$ crystal structures in AB and BA stacking states, where red, blue, and silver spheres indicate the V, Si, and N atoms, respectively. The red arrows indicate the directions of ferroelectric polarization. Charge density differences and plane-averaged charge density differences along the $z$ direction of (c) state I and (d) state II, respectively. Yellow and blue regions represent electron accumulation and depletion, respectively. The isosurface value is set to $0.0001e/{\AA }^3$.

Figure 2

The plane-averaged electrostatic potential of bilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$ under (a) AB stacking, (b) BA stacking, and (c) AA stacking along the $z$ direction. The insets describe the corresponding stacking structures. (d) Ferroelectric switching pathways and energy barriers through interlayer sliding in bilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$. The inset describes the geometry structure of the paraelectric (PE) state.

Figure 3

(a) Three different magnetic configurations of bilayer ${\mathrm{VSi}}_2{\mathrm{N}}_4$ in ferromagnetic (FM), interlayer antiferromagnetic-1 (AFM1), and interlayer antiferromagnetic-2 (AFM2). Green arrows represent the nearest intralayer ($d_{\mathrm{AA}}$) and interlayer ($d_{\mathrm{AB}}$) distance of magnetic atoms. Red and black arrows represent the different spin directions of magnetic atoms. The spin-polarized band structures without spin-orbit coupling (SOC) and layer-resolved density of states are displayed for (b) AFM1 configuration and (c) AFM2 configuration.

Figure 4

(a) The band structure and (c) the enlarged low-energy band structure and (e) Berry curvatures with SOC of ferroelectric antiferromagnetic configurations P$\uparrow$M$\uparrow \downarrow$ and P$\downarrow$M$\downarrow \uparrow$. (b) The band structure and (d) the enlarged low-energy band structure and (f) Berry curvatures with SOC of configurations P$\uparrow$M$\downarrow \uparrow$ and P$\downarrow$M$\uparrow \downarrow$. The Fermi level is set to 0 eV. The blue (red) colors represent the bands of the spin projection in the positive (negative) directions of the $z$ axis (spin-up or spin-down).

Figure 5

(a) Three different stacking orders of metal/BLVSN interfaces, i.e., metal-N, metal-Si, and metal-V interfaces. (b) The relative total energy of three different interfaces of Ag(111) and Au(111). Schematic diagrams of Ag(111)/BLVSN/Au(111) MFTJs devices with (c) AB stacking BLVSN and (d) BA stacking BLVSN using Ag(111) as the left electrode and Au(111) as the right electrode. The left and right electrodes extend to semi-infinity. These devices are periodic in the $xy$ plane and the current flows in the $z$ direction.

Figure 6

(a) The crystal structures of TLVSN with four different ferroelectric configurations (ferroelectric polarizations are indicated by red arrows), i.e., down-FE (ABC stacking), up-FE (CBA stacking), t-AFE (BAB stacking), and h-AFE (ABA stacking). (b) The switching pathways and energy barriers of the four different ferroelectric configurations. The left and right electrodes extend to semi-infinity. The $xy$ plane is periodic and in a hexagonal lattice and the current flows in the $z$ direction for these MFTJs.

Figure 7

Schematic diagrams of two-probe Au(111)/TLVSN/Au(111) MFTJ devices with three different out of plane FE polarization directions of (a) down-FE, (b) t-AFE, and (c) h-AFE with Au(111) as the left and right electrodes.

Figure 8

(a) Schematic diagrams of MFTJs with sliding multiferroics. Blue regions are the left and right electrodes using suitable metallic materials. Red regions are the scattering region using sliding multiferroics. The mechanism of MFTJs can be extended to other sliding multiferroics, (b) bilayer ${\mathrm{VS}}_2$ and (c) bilayer ${\mathrm{VSi}}_2{\mathrm{P}}_4$.