In recent years, two-dimensional van der Waals (2D vdWs) heterostructures have attracted great research interest due to their great potential in fundamental physics research and spintronic devices (such as MTJs). Due to its excellent scalability, controllable magnetism and out-of-plane anisotropy, the compact nonvolatile memory controller (NV-MC) based on spintronics is expected to solve the memory bottle-neck problem. At present, a series of in-depth studies have been conducted on advanced 2D vdWs materials, such as MoS2, WSe2, and Fe3GeTe2 (FGT). The results show that the 2D vdWs materials have great TMR value and high SOT switching efficiency, both theoretically reported and experimentally verified. In the paper, a novel MTJ device based on the FGT/WTe2 heterostructure is proposed. In the absence of an external magnetic field, the magnetization direction of the MTJ free layer can still be reversed with certainty when the unipolar write current reaches about 5 mA. Moreover, the DMI effect generated between 2-D material/FM interfaces is also considered, which can promote the performance of SOT-MTJ without the external field. The reading reliability of SOT-MRAM is improved in comparison with the traditional CoFeB-based MTJ device.

Spintronic devices play an important role in data processing applications such as deep learning, machine learning, and neuromorphic computing with its non-volatile property, enabling high-speed data transfer between on-chip memory and CPU. In recent years, several non-volatile memory (NVM) architectures have been invented, including resistive random access memory (RRAM), phase change memory, racetrack memory, magneto-resistance random access memory (MRAM).1 Spin-transfer torque magnetic tunnel junction (STT-MTJ) is a flexible non-volatile device with the merits of high memory density, low power consumption and high durability.2 Despite these benefits, STT-MTJ still experiences latency issues due to input delays.3 In addition, since STT-MTJ devices use the same current path for write and read operations, barrier oxide may get breakdown. Leading to the memory degradation.4 Spin–orbit torque MTJ (SOT-MTJ) device is proposed with better performance.

SOT-MTJ device consists of a ferromagnetic (FM) layer deposited on nonmagnetic (NM) layers. The NM layer is mainly composed of heavy metals (HM) with significant spin orbit coupling (SOC), such as W, Ta, or Pt, which act as the SOC layer in SOT-MTJ device, generating the SOT required for magnetization switching between adjacent FM layers.5 Charge-to-spin conversion in the NM layer is due to the spin Hall effect (SHE) in bulk material and the Rashba effect at interfaces.6 By applying spin torque on adjacent FM layers, the transverse spin current in the NM layer can cause the magnetization switching.7 Perpendicular magnetic anisotropy (PMA) SOT-MTJ provides excellent scalability, read/write route isolation, and thermal stability.

There are some issues that need to be addressed for SOT-MTJ device. The SOT efficiency of HMs is still not optimistic, ranging from 0.1 to 0.3.8 In addition, the deterministic switching of FM layer magnetization in PMASOT-MTJ requires the assistance of an external magnetic field.9 

In the literature, various alternative technologies for achieving field-free magnetization switching in SOT-MTJ device have been proposed. One approach is to explore the utility of emergent quantum systems as a spin-source material wherein SOTs can be controlled by crystal symmetries. Recently, transition-metal dichalcogenides with low-symmetry crystal structures,10 such as WTe2, exhibit an out-of-plane anti-damping torque when a charge current is applied along the low-symmetry axis of the WTe2/FM bi-layer system.11 

In the paper, a novel SOT-MTJ structure based on WTe2/Fe3GeTe2 is constructed, which can be determined switched without external field assistance. We demonstrate that field-free deterministic switching of vertically polarized magnets can be achieved due to the strong charge spin conversion efficiency in WTe2. In addition, the Dzyaloshinskii–Moriya interaction (DMI) between FM and 2D materials is also considered, It is discovered that DMI can help achieve field-free switching magnetization.

In SOT-induced magnetic switching, a current charge density JC (in the x direction) flows on the double layered structural plane of the spin-source material and a ferromagnetic layers, which results in a spin current flowing in the out-of-plane direction (z direction) via spin galvanic effects.12 This spin current applies torque to a magnetization of a nearby magnetic layer. This torque has an anti-damping component τADm×p×m and a field-like component τFLp×m, where m is the magnetization direction and p is the spin polarization direction. Due to the symmetry of the heavy metal (HM)/FM double heterostructure, spin polarization occurs in y direction. The in-plane anti-damping torque (τIPAD) has the form of m×y×m. Recently, WTe2/FM bilayer system13 has been shown to exhibit an out-of-plane anti-damping torque (τIPAD) of the form m×z×m when a charge current is applied along the low-symmetry axis.14 

The presence of a strong τIPAD in a WTe2/Permalloy heterostructure was previously probed by spin–torque ferromagnetic resonance. The parameter τIPAD is independent of the reversal of magnetization, and is reversed with the current direction. It can be used efficiently to switch a perpendicular magnetization.10 Unlike traditional spin-source systems, two-fold rotational symmetry is broken in WTe2 structure. It means that WTe2 has no mirror symmetry in the AC plane. As shown in Fig. 1(a), it was found experimentally that a non-zero τIPAD exists with a charge current applied along the A-axis. On the other hand, as shown in Fig. 1(b), when a charge current is applied along the B axis of WTe2, the preserved mirror symmetry in the BC plane leads to τIPAD=0. Essentially, when a current is applied along the B axis and a BC plane mirror operation is performed, the current isn’t switched. However, the sign of the out-of-plane anti-damping torque could be changed (from τIPAD to τIPAD). The sign of the current-induced torque is also changed with the current reversal, requiring that τIPAD = 0 when the current is along the b axis.15 

Chiral magnetic structures, such as chiral domain walls,16 helical structures,17 and magnetic skyrmions,18 hold promise for potential applications for future spintronic device. Microscopically, the Dzyaloshinskii–Moriya interaction (DMI), which favors canted spin configurations, plays an essential role in the formation of such non-collinear magnetic nanostructures. Two-dimensional (2-D) materials exhibit an extremely small thickness with novel physical properties related to their 2D characteristics.19  Figure 2 shows the crystal structure of AX2 monolayer. Each A atom is tetrahedrally surrounded by four X ligands.

Topological insulators (TIs) and transition-metal dichalcogenides (TMDs) show extraordinary tunable electrical and optoelectronic properties.20 Both TI/FM and TMD/FM heterostructures show large DMI. The effect of DMI on SOT switching is studied by experimental simulation. The DMI interaction considerably impacts the magnetization reversal process, and can produce nonuniform magnetization textures with a distinct chirality.21 In this way, the DMI effect can be used to promote the performance of SOT-MTJ device with field-free switching.

A modified three-terminal SOT-MTJ structure with WTe2/FGT is shown in Fig. 3. The tunnel oxide layer is constructed by MgO, which is used to separate the free layer (FL) from the pinned layer (PL). To achieve a high efficiency of charge-to-spin conversion, TIs/TMDs with high SOC are adopted as the NM layers rather than conventional HMs. To investigate the impact of DMI on magnetization switching, a built-in DMI package is employed. This improved SOT-MTJ might be included in the existing MTJ structure for SOT-based memory and logic applications.

The time development of magnetization switching can be expressed by a modified Landau–Lifshitz–Gilbert (LLG) equation as follows:
dm̂dt=γ×Ĥ+α×dm̂dtτDLm̂×m̂×σ̂τFLm̂×σ̂
(1)

In the equation above, γ is gyromagnetic ratio, α is damping constant. The third and the fourth terms denote the damping-like and field-like terms, respectively. The effective magnetic field is denoted by H, which comprises the anisotropy field, the external field, the DMI effective field, and the demagnetization field.

The damping-like term is expressed as
τDL=γh̄2eMStFMJS
(2)
where h is the reduced Planck’s constant, MS is the magnetization saturation, tFM is the FM layer thickness, and JS is the spin current density.

The simulation parameters used in the micromagnetic simulation are presented in Table I.

For the conventional SOT MTJ device, a critical current density of 2 × 1011 A/m2 is needed for the switching of FL magnetization when the SHA efficiency is 0.3. However, in the proposed TI/FM SOT-MTJ structure, the critical charge current density JC is varied intensely with the value of the SHA. The TI/FM SOT-MTJ structure with SHA of 8.5 requires a critical current density of 1.5 × 109 A/m2. It can be seen that the critical current density of 2-D SOT-MTJ device is greatly reduced compared with that of the conventional SOT-MTJ device.

Experimental research and micromagnetic simulation are used to examine the impact of DMI on SOT switching. Both TI/FM and TMD/FM heterostructures are reported with large DMI interaction.9–11 The DMI considerably impacts the magnetization reversal process and can produce nonuniform magnetization textures with a distinct chirality.

The performance parameters, including the critical current density and the switching time of the proposed SOT-MTJ structure, are evaluated based on the micromagnetic simulation results. Firstly, a current pulse is applied to generate out-of-plane anti-damping SOT at the interface of FGT and WTe2. Secondly, the DMI generated by the HM/FM interface leads the switching of the SOT device by the magnetization reversal. The macro spin approximate micromagnetic simulation of single magnetic domain is carried out to simulate the precession of mz in the free layer. The applied current pulse is J = 8 × 1010 A/m2, with its direction along the x axis and 1-ns pulse width.

As shown in Fig. 4, the temporal micro-magnetic simulation results are obtained. The switching process of the designed field-free SOT-MTJ device is divided into two steps. In the first step, the orientation of the magnetization is mainly from (0, 0, 1) to (−1, 0, 0), mainly due to the out-of-plane anti-damping SOT at the interface of FGT and WTe2. In the second step, the magnetization is from (−1, 0, 0) to (0, 0, −1), in which the deterministic switching is taken place due to the DMI occurred by TI/FM heterostructure after the current is removed.

In addition, when field-free switching of the perpendicular magnetization of FGT employs the charge-current-induced SOTs in WTe2, the final deterministic state is determined by the polarity of the current. A positive current favors up-magnetization, while a negative current favors down-magnetization. τOPAD is generated by the out-of-plane spin polarization (pz), which is injected from WTe2.

Next, tilt one-dimensional model (Tilt-1DM) is considered to increase the simulation efficiency. The DW is modeled as a thin line. There are three time-dependent variables about the DW motion, as shown in the following:
dqdt=Δcosχ1+α2ΩA+αΩB
(3)
dΦdt=11+α2αΩA+ΩB
(4)
dχdt=6γ0/αμ0MSΔπ2tan2χ+Ly/πΔcosχ2ΩC
(5)
Δ=A/KUμ0MS2/2
(6)
where q denotes the position of the center of the DW, Φ the magnetization angle at the center of the DW, χ the geometric tilt angle of the DW, Δ the width of the DW.
The terms ΩA, ΩB, ΩC are expressed as follows:
ΩA=12γ0HKsin2Φχπ2γ0HycosΦ+π2γ0HxsinΦ+π2γ0HDsinΦχ
(7)
ΩB=γ0QHZ+π2γ0ϱHSHEcosΦ
(8)
ΩC=σsinχ+πDQsinΦχμ0HKMSΔsin2Φχ
(9)
where HK = (Nx − Ny) MS, and the shape-dependent demagnetization factors Nx = Lz/(Lz + Δ), Ny = Lz/(Lz + Ly), Nz = 1 − Nx − Ny.22 Q is +1 or −1 corresponding to up-down or down-up DW configurations. Hx, Hy, Hz are the components of the applied filed in three directions.

The DW position, the DW angle and the tilting angle are shown in Fig. 5.

The domain wall adopts the up-down configuration, in which the initial values of q, Φ and χ are all 0. Figure 5(b) shows the enlarged view near t = 0 ns. The DW position switches from zero to negative value at the beginning, then it turns to positive value and begins increasing. DW angle rises rapidly from 0 to 200, and tends to stabilize at 2 × 10−6 ns. There is an oscillation at the beginning of the solution in timing angle, but it quickly returns to zero and remains stable. With the q moving, the DMI helps to achieve field-free switching.

The relationship of Ja and the velocity are shown as Fig. 6. The DW velocity increases linearly with Ja in low-current regime. However, the velocity tends to the saturation when Ja increases to high-current regime. Therefore, to avoid the saturation, the current should not exceed 1 × 1012 A/m2.

The deterministic switching probability is closely related to the size of the MTJ device. For the MTJ device with large-size, nucleation in the sub-volume domain takes precedence over nucleation in the edge domain.23 However, with the size of the MTJ device decreased, the impact of DMI becomes significant, as the chiral domain nucleation potential barrier overcomes the sub volume domain nucleation.24 With the MTJ device continuously decreasing, once the DMI magnitude is strong enough, the magnetization switching could occur at the edge domain.25 

In the paper, the field-free switching based on 2-D material and DMI is proposed. Micromagnetic simulations are performed to evaluate the performance characteristics in the 2-D SOT-MTJ device. Due to the adoption of the 2-D materials with high SOC and the DMI effect, low-power memory and logic applications are now possible. The DMI effect greatly improves the switching speed and critical current density required for the magnetic switching. The temporal evolution of the DW position, the DW angle and the tilting angle can be determined by solving the one-dimensional q-Φ-χ model.

Based on this study, it is demonstrated that the transition metal dichalcogenides compounds can generate strong out-of-plane anti-damping moments due to their unique crystal symmetry and lower symmetry crystal structure.

This work was supported by Open Project Funding of State Key Laboratory of Processor Chip (Grant No. CLQ202303) and NSFC under Grant No. 61774078.

The authors have no conflicts to disclose.

Yuhai Yuan: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Writing – original draft (lead). Yanfeng Jiang: Methodology (lead); Supervision (lead); Writing – review & editing (lead).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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