Spintronic Devices upon 2D Magnetic Materials and Heterojunctions

Zhiyan Jia; Mengfan Zhao; Qian Chen; Yuxin Tian; Lixuan Liu; Fang Zhang; Delin Zhang; Yue Ji; Bruno Camargo; Kun Ye; Rong Sun; Zhongchang Wang; Yong Jiang

doi:10.1021/acsnano.4c14168

. 2025 Mar 7;19(10):9452–9483. doi: 10.1021/acsnano.4c14168

Spintronic Devices upon 2D Magnetic Materials and Heterojunctions

Zhiyan Jia ^†, Mengfan Zhao ^†, Qian Chen ^‡, Yuxin Tian ^†, Lixuan Liu ^†, Fang Zhang ^†, Delin Zhang ^†, Yue Ji ^†, Bruno Camargo ^∥, Kun Ye ^†,^*, Rong Sun ^¶,^*, Zhongchang Wang ^§, Yong Jiang ^†,^*

PMCID: PMC11924334 PMID: 40053908

Abstract

graphic file with name nn4c14168_0013.jpg

In spintronics, there has been increasing interest in two-dimensional (2D) magnetic materials. The well-defined layered crystalline structure, interface conditions, and van der Waals stacking of these materials offer advantages for the development of high-performance spintronic devices. Spin–orbit torque (SOT) devices and the tunneling magnetoresistance (TMR) effect based on these materials have emerged as prominent research areas. SOT devices utilizing 2D magnetic materials can efficiently achieve SOT-driven magnetization switching by modulating the interaction between spin and orbital degrees of freedom. Notably, crystal structure symmetry breaking in 2D magnetic heterojunctions leads to field-free perpendicular magnetization switching and an extremely low SOT-driven magnetization switching current density of down to 10⁶ A/cm². This review provides a comprehensive overview of the construction, measurement, and mechanisms of 2D SOT heterojunctions. The TMR effect observed in 2D materials also exhibits significant potential for various applications. Specifically, the spin-filter effect in layered A-type antiferromagnets has led to giant TMR ratios approaching 19,000%. Here, we review the physical mechanisms underlying the TMR effect, along with the design of high-performance devices such as magnetic tunnel junctions (MTJ) and spin valves. This review summarizes different structural types of 2D heterojunctions and key factors that enhance TMR values. These advanced devices show promising prospects in fields such as magnetic storage. We highlight significant advancements in the integration of 2D materials in SOT, MTJ, and spin valve devices, which offer advantages such as high-density storage capability, low-power computing, and fast data transmission rates for Magnetic Random Access Memory and logic integrated circuits. These advancements are expected to revolutionize future developments in information technology.

Keywords: heterojunctions, logic circuits, magnetic random access memory, magnetic tunnel junction, spin−orbit torque, spin valve, spintronic, two-dimensional

Introduction

With the rapid advancements in information storage and processing technology, the size of integrated chips and transistor density per unit area have approached the limits set by Moore’s law.¹⁻⁴ Spintronic devices, which utilize electronic spin as the primary information carrier, possess several advantages such as compact size, high speed, and low power consumption. Consequently, they are strong contenders for information storage devices in the post-Moore era.⁵⁻⁹ Notably, the discovery of two-dimensional (2D) magnetic materials provides a material foundation and a research platform for novel spintronic device construction.¹⁰⁻¹⁵ Differing from traditional three-dimensional (3D) magnetic materials, these 2D counterparts exhibit physical and chemical properties including enhanced controllability, reduced dimensionality effects, and larger specific surface areas.¹⁶⁻¹⁸ In general, spatial inversion symmetry breaking resulting from symmetry reduction in 2D magnetic materials leads to pronounced magnetic anisotropy that enables long-range atomic layer thickness maintenance of the magnetic order.¹⁹⁻²⁴ Moreover, the geometric potential of 2D materials, influenced by their inherent curvature, has emerged as a significant factor in shaping their electronic and spintronic characteristics.²⁵ Given the numerous advantages and potentials, extensive research efforts have been focused on the development of two-dimensional magnetic materials, including the creation of room-temperature organic magnets based on sp³-functionalized graphene, representing a breakthrough in graphene-based spintronics and materials science.²⁶

Recent research focuses on spin–orbit torque (SOT) control based on heterojunctions comprising 2D magnetic metals,²⁷⁻³⁴ as well as magnetoresistance detection through magnetic tunnel junctions (MTJs),^16,35−37 representing key areas of investigation concerning 2D magnetic materials. Given their inherent strong magnetic anisotropy, layer correlation, and switchable magnetism characteristics, and abundant spin physical properties,³⁸⁻⁴⁰ providing an effective platform for electronic spin manipulation.^41,42 Compared with conventional materials, devices based on heterojunctions involving 2D magnetic metals have potential for achieving lower energy consumption, smaller dimensions, faster response times, and higher information storage densities.⁴³ Field-free magnetization switching has been achieved in 2D Fe_2.78GeTe₂ based SOT devices, and the critical current density is as low as 9.8 × 10⁶ A/cm².⁴⁴ In the heterojunction of 2D Fe₃GaTe₂, an impressive tunneling magnetoresistance (TMR) of 85% at room–temperature is achieved, which closely approaches the parameter value available in the industry.⁴⁵ Therefore, further scientific and practical investigations on the construction and spin manipulation of 2D magnetic material heterojunctions are crucial to develop simple preparation methods for these heterojunctions, to achieve low critical current density for SOT switching, and high TMR values for MTJ applications.^46,47 Moreover, recent predictions by researchers suggest that logic functional devices implementing Boolean logic gate operations can utilize 2D magnetic metal heterojunctions.^43,48,49 The investigation on SOT control and MTJ based on metal heterojunctions of 2D magnetic materials provides essential support for the advancement of Magnetic Random Access Memory (MRAM) and logic integrated circuits employing 2D materials, thereby holding tremendous potential in achieving fully electronically controlled memory and computing.⁵⁰⁻⁵²

As depicted in Figure 1, this paper provides a comprehensive review of the preparation techniques for 2D magnetic materials and their heterojunctions, as well as the potential mechanisms, measurements, and applications in spintronic devices. The physical basis of spintronics relies on the quantum property of electron spin, allowing for information storage, processing, and transmission through manipulation and transmission of electron spin information. One important application is information storage technology, with the current magnetization switching strategy in SOT-driven MTJ representing cutting-edge information storage based on spintronics. Therefore, we discuss recent progress in SOT, MTJ, and spin valve devices within 2D heterojunctions while also presenting future prospects for constructing and validating all electrically controlled storage systems and Boolean logic functional devices based on 2D magnetic materials.

2D Magnetic Materials

According to the Mermin–Wagner theorem, a long-range magnetic ordering of nature cannot be achieved at finite temperatures in the 2D Heisenberg model. This theorem can also be applied to other systems with continuous symmetry and is an extension of the conclusion that there is no Bose–Einstein condensation in 2D systems due to their uniform density of states.⁶⁵⁻⁶⁷ However, experimental work on intrinsic magnetic materials in 2D van der Waals (vdWs) has shown that magnetic anisotropy effectively eliminates thermal fluctuations, thereby enabling research on 2D intrinsic magnetic materials such as Cr₂Ge₂Te₆ and CrI₃ with Curie temperatures (T_C) of 45 (monolayer) and 40 K (bilayer), respectively (Figure 2a).^19,20 These works point out that magnetic anisotropy effectively eliminates thermal fluctuations in 2D systems, such as Ising ferromagnetism, kicking off the research on 2D intrinsic magnetic materials, which can provide a novel material basis and research platform for developing spintronic materials and devices.^10,68 Nevertheless, most 2D magnetic materials currently have T_C lower than room temperature, which greatly limits their applications in spintronic devices.⁶⁹

2D Layered Magnetic Materials

Since the discovery of 2D intrinsic magnetic materials, significant advancements have been made in layered magnetic materials.^19,20 Numerous 2D layered ferromagnetic materials (FMs) have been theoretically and experimentally reported, including Fe₃GeTe₂,⁶³ VSe₂,⁷⁰ and CrSe₂.⁵⁹ As depicted in Figure 2b, the coordination number of atoms decreases with decreasing dimensionality, leading to potential variations in their magnetic properties compared with their bulk counterparts.^63,71 Additionally, the shielding effect of orbital magnetic moments is diminished in the 2D case. Consequently, a wealth of novel and intriguing magnetic phenomena can be observed within 2D magnets.⁷² A prominent characteristic exhibited by layered 2D magnetic materials is their layer-dependent magnetism.^63,73 For instance, Fei et al. discovered that the 2D intrinsic ferromagnetic metal material Fe₃GeTe₂, exhibits superior air stability with T_C of as low as 130 K for thicknesses less than 4 nm (Figure 2c).⁷⁴ Deng et al., conducted anomalous Hall effect (AHE) measurements on Fe₃GeTe₂ films with varying thicknesses from bulk to monolayer, and they revealed that T_C reduces from 180 to 20 K as the number of layers decreases.⁶³ Gong et al. explored the Kerr properties of few-layered and bulk Cr₂Ge₂Te₆ as a function of temperature, and they found that T_C increases from 30 K in bilayer to 68K in bulk.¹⁹ Avsar et al. reported a systematic switching between ferromagnetic and antiferromagnetic ground states in defective PtSe₂ as the number of layers varied between odd and even, demonstrating an ordered behavior (Figure 2d).⁷⁵ The recently discovered MnBi₂Te₄, an intrinsically magnetic topological insulator material, exhibits ferromagnetic couplings within the layers and antiferromagnetic couplings between neighboring layers,⁷⁶⁻⁷⁸ displaying an odd–even effect of spin–flop field dependence with layer number (Figure 2e).⁷⁹ Moreover, the layered 1T–CrTe₂ demonstrated a high T_C close to room temperature (Figure 2f),⁸⁰ which is enhanced monotonically as the thickness decreases from 130 to 7.5 nm (Figure 2g).⁸¹ Given its strong perpendicular anisotropy, the intrinsic ferromagnetic order persists even when reducing the thickness of monolayer 1T–CrTe₂, resulting in a T_C value of up to 200 K.⁸² Additionally, O’Hara et al. observed room-temperature ferromagnetism in MnSe_x film grown by molecular beam epitaxy (MBE), as shown in Figure 2h.⁶⁹

Consequently, the majority of 2D magnetic materials exhibit T_C values lower than room temperature, significantly restricting their potential applications in spintronic devices. Various strategies have been employed to enhance T_C of these materials, including chemical doping, ionic intercalation, pressure manipulation, and interface modulation.^27,73,83 (i) Chemical doping: Fe-rich Fe₄GeTe₂ crystals were prepared by Seo et al., demonstrating ferromagnetic ordering close to room temperature even in dissociated nanosheets as thin as seven layers (Figure 2i).^84,85 Yang et al. utilized Ga ion implantation to elevate T_C of ferromagnetic Fe₃GeTe₂ above room temperature.⁷² Wen et al. achieved elevated T_C values surpassing room temperature through the modulation of transition metal doping concentrations in Mn_0.14Cr_0.86Te and Fe_0.26Cr_0.74Te.⁸⁶ (ii) Ionic intercalation: In 2D layered magnetic materials, changes in carrier concentration can alter the magnetic ion orbital occupancy, exchange interactions, and magnetic anisotropy. Ion intercalation is an effective method for regulating the carrier concentration in these materials.⁸⁷ Deng et al. successfully engineered lithium ion intercalation of Fe₃GeTe₂ using the solid-state electrolyte LiClO₄–PEO. They were able to change the density of states at the Fermi energy level and regulate its T_C to 310 K (Figure 2j).⁶³ Wang et al. utilized an electrochemical method to insert organic cationic tetrabutylammonium root ions into the Cr₂Ge₂Te₆ interlayer, resulting in a significant increase in T_C from 65 to 208 K and a shift in the magnetic anisotropy from out-of-plane (OOP) to in-plane (IP).⁸⁸ (iii) Pressure: Pressure also plays a role in modulating physical parameters such as T_C and coercivity (H_C) in 2D magnetic materials due to their sensitivity to lattice deformation caused by long-range exchange interactions.⁸⁹ Li et al. applied pressure to alter the stacking order of a thin layer of CrI₃ from monoclinic to rhomboid, leading to a transformation of its magnetic properties from interlayer antiferromagnetism to ferromagnetism.⁹⁰ Through in situ uniaxial tensile strain modulation, Wang et al. observed that when applying a strain of 0.32%, T_C linearly increased from 180 to 210 K for Fe₃GeTe₂.⁹¹ Additionally, Li et al. demonstrated that high pressure can modulate the magnetic state of Fe₅GeTe₂, causing it undergo multiple phase transitions and elevating its T_C from near room temperature up beyond 400 K (Figure 2k).⁶² (iv) Interfacial modulation: The interfacial magnetic near-neighbor coupling effect provides a means to manipulate magnetic properties without additional energy consumption.⁴² Wang et al. achieved the growth of wafer-scale 2D Fe₃GeTe₂ integrated with Bi₂Te₃ topological insulator by MBE and utilized Bi₂Te₃ interfacial engineering to enhance its T_C up to 400 K (Figure 2l).⁹² Similar interfacial effects can also occur between Fe₄GeTe₂ and the substrate, resulting in a T_C of approximately 530 K.⁴² Yang et al. fabricated air-stabilized vertical heterojunctions of 2D Cr₅Te₈/CrTe₂ with a T_C exceeding 400 K using chemical vapor deposition (CVD), where biaxial strains and strong interfacial coupling collectively induce high temperature magnetic ordering in the system.⁶¹

Achieving 2D magnetic materials with T_C higher than room temperature can be accomplished through various modulation techniques; however, these methods present challenges for the integration of 2D magnetic materials into devices. Recently, intrinsic ferromagnetic 2D Fe₃GaTe₂,^38,93 MnSiTe₃, and MnGeTe₃⁴¹ have been discovered at room temperature. Notably, Zhang et al. successfully synthesized Fe₃GaTe₂ as a 2D material demonstrating intrinsic ferromagnetism with a T_C surpassing room temperature and excellent perpendicular anisotropy; specifically, T_C of a sample with a thickness of only 9.5 nm reaches ∼350 K.⁹³ The discovery of this material significantly contributes to the potential applications of 2D magnetic materials in research on spintronic devices.

2D Nonlayered Magnetic Materials

Recent scientific investigations demonstrated that ultrathin nonlayered materials can be synthesized using CVD. Upon dimensional reduction, a range of intriguing and unconventional physical phenomena have been observed in these materials, which may pave the way for the development of high T_C and environmentally stable 2D nonlayered magnetic materials.⁹⁴⁻⁹⁸ For instance, the investigation on thickness-dependent T_C of 2D nonlayered CrTe nanoflakes revealed a decrease in T_C from 205 to 140 K as the thickness reduced from 45 to 11 nm (Figure 3a).⁹⁹ However, Wu et al. reported that the nonlayered CrTe nanosheets exhibited a potential T_C up to 367 K.¹⁰⁰ Wen et al. experimentally synthesized single-unit-cell thick nonlayered Cr₂Te₃ and found that its T_C slightly increased with decreasing thickness. When the thickness was reduced to approximately six single-unit-cell layers (∼7.1 nm), they observed a sharp increase in T_C up to 280 K (Figure 3b).¹⁰¹ Additionally, Figure 3c demonstrates the positive modulation of T_C under tensile strain and negative modulation under compressive strain for Cr₂Te₃ nanosheets; moreover, maximum changes in T_C with sample thickness were approximately 40 and 90 K respectively. The H_C of Cr₂Te₃ nanosheets exhibits significant reduction under applied strains indicating either decreased exchange interaction or altered magnetization direction (Figure 3d).⁸⁹ Chen et al. successfully adjusted the stoichiometric ratio of Cr and Te to 5:8, resulting in the synthesis of air-stable 2D nonlayered Cr₅Te₈ nanosheets. The thickness-dependent investigation revealed an increase in their T_C from 100 to 160 K (Figure 3e). These nanosheets exhibited excellent stability in ambient conditions for a period exceeding 3 months.⁵⁶ Furthermore, Meng et al. demonstrated the realization of near room-temperature magnetism (270 K) and ambient stability in 2D nonlayered Cr₇Te₈ nanosheets (Figure 3f). They also achieved electric gate control over the Berry curvature-dependent AHE, showcasing its tunability. The observed gate-tunable sign reversal of AHE can be attributed to the redistribution of Berry curvature within the energy band structure of Cr₇Te₈ due to changes induced by interpolated layers.¹⁰²

Recently, significant advancements have been made in the exploration of 2D nonlayered magnetic materials exhibiting an inherent T_C above room temperature. Examples include 2D nonlayered ε-Fe₂O₃ (Figure 3g)¹⁰³ and ironene (Figure 3h), which demonstrate magnetic order and strong magnetic vortex at room temperature.¹⁰⁴Figure 3i illustrates the magnetic hysteresis (M–H) loops of 2D nonlayered γ-Fe₂O₃ nanoflakes at room temperature, revealing size-dependent magnetic domain states.^105,106 Zhao et al. synthesized Fe₇Se₈ nanosheets via CVD, which exhibit spin reorientation around ∼130 K while maintaining remote magnetically ordered behavior at room temperature. These nanosheets with varying thicknesses all display robust magnetic domain structures at room temperature (Figure 3j).⁵⁸ Wang et al. synthesized centimeter-scale atomically thin Fe₃O₄ arrays with unidirectional orientations, revealing long-range ferrimagnetic order at room temperature and demonstrating tunable magnetic domain evolution in atomically thin Fe₃O₄.¹⁰⁷ Zhao et al. proposed general thermodynamics-triggered competitive growth model that provides a multivariate quantitative criterion for predicting and guiding the growth of 2D nonlayered materials (Figure 3k). On the basis of this model, they designed a universal hydrate-assisted CVD strategy for the controllable synthesis of various nonlayered transition metal oxides. Additionally, four phases of iron oxides with distinct topological structures were selectively grown using this approach.⁹⁸ Notably, 2D Fe₃O₄ exhibited magnetic ordering above room temperature, and shown multilevel resistance under microwave fields due to the structural presence of antiphase boundaries and multispin texture magnetization (Figure 3l).¹⁰⁸ Additionally, the Verwey phase transition in 2D Fe₃O₄ can drive abrupt changes in spin transport, such as an inverse sign change in AHE ratio (Figure 3m).¹⁰⁹ Furthermore, by doping Co elements into Fe₃O₄, 2D nonlayered alloy oxides that displayed high T_C and strong dimensional effects above 390 K were formed.¹¹⁰

The primary challenge faced by most 2D layered magnetic materials is their low T_C, while controlling the size and thickness of 2D nonlayered materials remains a difficult problem. Therefore, future research efforts are directed toward synthesizing large-area and even wafer scale ultrahigh T_C 2D magnetic materials. Table 1 provides a comprehensive overview of various 2D magnetic materials, highlighting their structural properties, T_C(N), and thickness. Among these, 2D layered magnets such as Fe_nGeTe₂ (n = 3, 4, 5), VSe₂, CrTe₂, MnSe_x, Fe_0.26Cr_0.74Te, MnSiTe₃, and MnGeTe₃ exhibit near-room-temperature T_C(N), making them highly promising for practical applications. The ability to maintain high T_C(N) in ultrathin 2D forms of these nonlayered materials, such as Fe₃O₄, CoFe₂O₄, and CrTe, is particularly noteworthy and could open new avenues for the development of advanced magnetic devices. Additionally, the thickness of these materials varies significantly, affecting their electronic and magnetic properties. Thinner materials may offer enhanced spin injection efficiency but could be more challenging to synthesize and integrate into devices. The above demonstrations provide insights into the controllable synthesis of 2D magnetic materials and offer detailed studies on their magnetic anisotropy and T_C (Table 1), providing a material basis for the application of 2D materials in spintronic devices, such as SOT, MTJ, and spin valve circuits.

Table 1. Structural, Electrical, and Magnetic Properties of Some 2D Layered and Nonlayered Magnets^a.

Materials	Structure	Method	Easy axis	T_C(N) (K)	Thickness (nm or L)	ref.
Cr₂Ge₂Te₆	Layered, trigonal system	CVT	//c	65	4.6	(19)
Cr₂Ge₂Te₆	Layered, trigonal system	CVT	//c	49	∼2.3	(19)
CrI₃	Layered, trigonal system	CVT	//c	61	∼2.04	(20)
CrI₃	Layered, trigonal system	CVT	//c	45	∼0.68	(20)
Cr₂Ge₂Te₆	Layered	CVT	⊥c	208	bulk	(88)
Fe₃GeTe₂	Layered, hexgonal, P6₃/mmc	CVT	//c	205	bulk	(63)
Fe₃GeTe₂	Layered, hexgonal, P6₃/mmc	CVT	//c	20	∼0.8	(63)
Fe₃GeTe₂	Layered, hexgonal, P6₃/mmc	CVT	//c	210	120	(74)
				204	3.2
				180	1.6
				130	0.8
Fe₃GeTe₂ (Ga implantation)	Layered, hexgonal, P6₃/mmc	CVT	⊥c	450	bulk	(72)
Fe₄GeTe₂	Layered, rhombohedral, R3̅m	CVT	⊥-//c	270	∼16	(84)
Fe₅GeTe₂	Layered, rhombohedral, R3̅m	CVT	//c	∼280	∼15	(111)
VSe₂	Layered, triclinic	CVT	⊥c	470	∼2.3–4.4	(70)
CrSe₂	Layered, triclinic	CVD	//c	110	10.8	(59)
CrSe₂	Layered, triclinic	CVD	//c	65	∼0.7	(59)
Ta₃FeS₆	Layered, hexagonal, P6₃22	CVT	//c	145	bulk	(71)
Ta₃FeS₆	Layered, hexagonal, P6₃22	CVT	//c	80	6.0	(71)
1T-CrTe₂	Layered, triclinic, P3̅m1	CVT	⊥c	305	8.7	(80)
1T-CrTe₂	Layered, triclinic, P3̅m1	CVD	⊥-//c	212	7.5	(81)
				210	10
				193	15
				187	130
1T-CrTe₂	Layered, triclinic, P3̅m1	MBE	//c	>300	∼4.2	(82)
1T-CrTe₂	Layered, triclinic, P3̅m1	MBE	//c	>200	∼0.6	(82)
MnSe_x	Layered	MBE	//c	>300	∼0.9	(69)
Mn_0.14Cr_0.86Te	Layered, hexagonal	CVD	//c	335	17.9	(86)
Fe_0.26Cr_0.74Te			⊥c	330	83.4
Co_0.4Cr_0.6Te			⊥-//c	175	6.9
Fe₃GaTe₂	Layered, hexagonal, P6₃/mmc	CVT	//c	380	112	(93)
Fe₃GaTe₂	Layered, hexagonal, P6₃/mmc	CVT	//c	350	9.5	(93)
MnSiTe₃	Layered, hexagonal	CVT	//c	378	bulk	(41)
MnGeTe₃	Layered, hexagonal	CVT	//c	349	bulk	(41)
Cr₂Te₃	Nonlayered, trigona, P3̅1c	CVD	//c	180	15.4	(89)
Cr₂Te₃	Nonlayered, trigona, P3̅1c	CVD	//c	140	12.5	(89)
FeTe	Nonlayered, hexagona, P6₃/mmc	CVD	//c	220	30	(95)
				215	12
				170	4
FeTe	Nonlayered, hexagonal, P6₃/mmc	CVD	//c	300	40	(96)
CrTe	Nonlayered, hexagonal, P6₃/mmc	CVD	//c	205	45	(99)
CrTe	Nonlayered, hexagonal, P6₃/mmc	CVD	//c	140	11	(99)
CrTe	Nonlayered, hexagonal, P6₃/mmc	CVD	⊥c	367	20–40	(100)
CrTe	Nonlayered, hexagonal, P6₃/mmc	CVD	⊥c	343	bulk	(100)
Cr₂Te₃	Nonlayered, hexagonal	CVD	//c	280	7.1	(101)
Cr₂Te₃	Nonlayered, hexagonal	CVD	//c	160	40.3	(101)
Cr₅Te₈	Nonlayered, hexagonal	CVD	//c	230	bulk	(56)
				160	30
				100	10
				50	5
Cr₇Te₈	Nonlayered, NiAs-type	CVD	//c	270	20	(102)
γ-Fe₂O₃	Nonlayered, antispinel, P4₁32	CVD	//c	300	∼12.35–48.95	(105)
Fe₇Se₈	Nonlayered, NiAs-type hexagonal	CVD	⊥c	130	45	(58)
Fe₃O₄	Nonlayered, antispinel	CVD	⊥c	400	40	(107)
Fe₃O₄	Nonlayered, antispinel	CVD	⊥c	350	thin	(107)
CoFe₂O₄	Nonlayered, antispinel	CVD	//c	390	2–4	(110)
Fe₃O₄	Nonlayered, antispinel	CVD	⊥c	>800	thin	(108)

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Abbreviations: //c: Parallel to the c axis. ⊥c: Perpendicular to the c axis. T_C(N): Curie (Néel) temperature.

Advantages of 2D Magnetic Materials

Compared to bulk magnetic materials, 2D magnetic materials offer a natural structural advantage, exhibiting a layered structure with layers bound by van der Waals forces and surfaces characterized by atomic-scale flatness, as shown in Figure 4a. In this figure, 1T-CrTe₂ nanosheets display flat surfaces, and the cross-sectional TEM images also highlight their well-layered structure.⁸¹ This structure enables 2D materials to be stacked like building blocks to form flat heterojunctions, as depicted in Figure 4b.¹¹² Similarly, ultrathin nanosheets with atomic-scale flat surfaces, such as Cr₅Te₈ nanosheets in Figure 4c, are also observed in 2D nonlaminated magnets synthesized by CVD methods, making them suitable for heterojunction preparation.¹¹³ This structural advantage over bulk magnets facilitates the fabrication of devices.

Structural and property advantages of 2D materials as well as the numerous and diverse means of modulation. (a) AFM image and the corresponding OM image (upper right inset) of a typical 1T-CrTe₂ hexagonal nanoflake on a 285 nm SiO₂/Si substrate. The height profile in the lower left inset shows that the flake is approximately 1.2 nm thick. The corresponding crystal structures are shown, with Cr in maroon and Te in yellow. Reprinted with permission under a Creative Commons CC BY License from ref (81). Copyright 2021 Springer Nature.⁸¹ (b) A large number of 2D materials are available to create a variety of functional van der Waals heterostructures, typically starting with mechanically peeled and assembled stacks. Reproduced with permission: Copyright 2016, American Association for the Advancement of Science.¹¹² (c) Side view atomic structure and AFM image of the trigonal Cr₅Te₈ flake on a mica substrate. Reproduced with permission: Copyright 2022, American Chemical Society.¹¹³ (d) Antiferromagnetic order (top) in CrSBr (even number of layers) breaks inversion (i) and time-reversal (τ) symmetries, allowing for ED SHG. Ferromagnetic order (bottom) in CrSBr monolayers or antiferromagnetic in odd numbers of layers breaks time-reversal (τ) but not inversion (i) symmetry, preventing ED SHG. Reproduced with permission: Copyright 2021, American Chemical Society.¹¹⁴ (e) Schematics of the spiral configuration of trilayer NiI₂ in top views. The light blue spheres and red arrows represent the Ni atoms in the top layer and their spins. (f) Spin configurations in 2D magnetic CrPS₄ with different thicknesses (bilayer and trilayer). Reprinted with permission under a Creative Commons CC BY License from ref (116). Copyright 2024 Springer Nature.¹¹⁶ (f) Reproduced with permission: Copyright 2021, American Chemical Society.¹²⁰ (g) Schematic of the diamond anvil cell (DAC) and the results of pressurizing 2D Fe₃GaTe₂ with this DAC, where the magnetic susceptibility axis shifts from OOP to IP as the pressure increases. Reprinted with permission under a Creative Commons CC BY License from ref (135). Copyright 2020 Wiley-VCH GmbH. Reprinted under a Creative Commons CC BY License with permission from ref (121). Copyright 2024 Wiley-VCH GmbH.^121,135 (h) Anomalous Hall resistance (R_AH) vs magnetic field (B) curves of the Bi_1.5Sb_0.5Te_1.7Se_1.3/Fe₃GaTe₂ heterojunction at 390 and 400 K. Reproduced with permission: Copyright 2024, American Chemical Society.¹²² (i) Schematic illustration of the crystal structure of Co-doped Fe₃GaTe₂ (left). Normalized Hall resistance R_xy(H) curves at 20 K for samples with different thicknesses. Reproduced with permission: Copyright 2024, American Chemical Society.¹²³ (j) Schematic of the variation of the magnetic moment of bilayer CrI₃ under an applied electric field. Reproduced with permission: Copyright 2020, American Chemical Society.¹²⁴ (k) Measurement schematic of the all-2D Fe₃GaTe₂/WSe₂/Fe₃GaTe₂ magnetic junction. Reproduced with permission: Copyright 2024, American Chemical Society.¹²⁷

2D magnetic materials also exhibit diverse benefits in properties such as layer-dependent T_C, H_C, and the manipulation of electrons and spins. Lee et al. demonstrated via the second harmonic generation (SHG) effect that a monomolecular layer of CrSBr is ferromagnetically ordered up to 146 K, while the ferromagnetic monolayers in multilayers are coupled antiferromagnetically (Figure 4d).¹¹⁴ They also found that CrSBr is an A-type antiferromagnet, and, in contrast to other 2D magnets, its T_N increases as the number of layers decreases. Wilson et al. showed that the magnetic torque of CrSBr shifted from an interlayer antiferromagnetic arrangement to an interlayer ferromagnetic arrangement under a perpendicular magnetic field, altering the spatial distribution of the interlayer electron–hole wave function. Under interlayer antiferromagnetic coupling, the electron and hole wave functions were confined within the same layer, while interlayer ferromagnetic coupling resulted in nonlocalized electron wave functions in both layers.¹¹⁵ Thickness-dependent magnetism exists in 2D NiI₂. Most reports consider NiI₂ to be a type-I helical antiferromagnetism with a propagation vector of [q = (0.138, 0, 1.457)], as shown in Figure 4e. The vector q in multilayered and bulk NiI₂ is mainly determined by the competition of various magnetic exchange interactions between the magnetic atoms. No interlayer interactions are present in monolayer NiI₂. Therefore, the vector q lies in the ab plane and is determined by the intralayer exchange interactions. A helical spin configuration exists in the magnetic ground state of trilayer NiI₂, and the in-plane projection of vector q along the [210] direction is ab plane. For trilayer NiI₂, which has reached the bulk limit, the in-plane projection of vector q is similar to that of the bulk material. However, the magnetic ground state of the few-layer NiI₂ remains elusive.^116,117 Furthermore, Liu et al. predicted intralayer ferromagnetic and interlayer antiferromagnetic ordering in 2D NiI₂, where the even-numbered layers showed zero magnetization, while the odd-numbered layers showed nonzero net magnetization.^118,119 Similarly, in 2D CrPS₄, the ferromagnetic out-of-plane spin orientation is found in the monolayer crystals, in contrast to the antiferromagnetic ordering in the bulk state. The alternating antiferromagnetic and ferromagnetic characteristics observed in even and odd layers suggest a strong antiferromagnetic exchange interaction between the layers.¹²⁰

Another key advantage of 2D magnetic materials is their greater susceptibility to modulation by external fields. Various methods such as pressure, interfacial coupling, doping, electric fields, strain, and square fields can significantly affect the magnetic and electronic properties of 2D magnetic materials. Gao et al. reported the pressure-tuned dome-like ferromagnetic-paramagnetic phase diagram of the 2D ferromagnetic Fe₃GaTe₂, where magnetic anisotropy can be continuously tuned from OOP to IP by applying pressure. This behavior is attributed to the competition between intra- and interlayer exchange interactions and the enhancement of the density of states near the Fermi energy.¹²¹ Similar results were observed in the pressure modulation of Fe₅GeTe₂ properties.⁶² Zhang et al. reported a nondestructive van der Waals interface magnetochemical strategy for above-room-temperature multifaceted modulation of 2D ferromagnetism. By coupling nonmagnetic MoS₂, WSe₂, or Bi_1.5Sb_0.5Te_1.7Se_1.3 with 2D magnetic Fe₃GaTe₂, they increased the T_C to 400 K, tuned the room-temperature perpendicular magnetic anisotropy to 26.8%, and achieved an anomalous Hall effect up to 340 K. These phenomena arise from magnetic exchange interactions induced by interfacial charge transfer, spin–orbit coupling (SOC), and changes in magnetic anisotropy energy.¹²² This approach paves the way for designing multifunctional 2D heterostructures and modulating the properties of 2D magnetic materials through van der Waals interfacial coupling. Wang et al. reported the doping of Co into 2D ferromagnetic Fe₃GaTe₂, producing an antiferromagnetic material with distinct parity layer effects, observed through layer-dependent Hall resistance and magnetoresistance measurements, suggesting an A-type antiferromagnetic structure. This doping introduces layer-dependent magnetism in Fe₃GaTe₂, enriching materials for spintronic applications.¹²³ Xu et al. observed an electric field-induced phase transition from bilayer interlayer antiferromagnetic to ferromagnetic in CrI₃, with a critical field as low as 0.12 V/Å. The antiferromagnetic-ferromagnetic energy difference (ΔE) increases with the electric field and correlates with the field-induced on-site energy difference, which is the splitting between the electronic states of the two vdW layers. Their tight-binding model agrees with ΔE, providing consistent estimates for orbital hopping, exchange splitting, and crystal field splitting. Furthermore, a CrI₃-based spin field-effect device was proposed, where the spin current is controlled by the electric field switch.¹²⁴ The theoretical study was also confirmed experimentally by Huang et al., who realized electric field-controlled switching between antiferromagnetic and ferromagnetic states by applying a fixed magnetic field near the metamagnetic transition.¹²⁵ Jiang et al. applied electrostatic doping in bilayered CrI₃ thereby significantly altering the magnetic order of the interlayers, which then led to the transition from an antiferromagnetic ground state in the pristine form to a ferromagnetic ground state above a critical electron density.¹²⁶ The modulation of 2D magnetic materials by electric fields is fully demonstrated. Zhu et al. achieved the fusion of spin information storage and optical sensing capabilities in a fully 2D magnetic tunnel junction, Fe₃GaTe₂/WSe₂/Fe₃GaTe₂, which integrates 2D magnetic and optoelectronic properties. The intrinsic bandgap and photoresponse of the interlayer WSe₂ impart optical tunability to the tunneling barrier. The junction’s on–off state and magnetoresistance can be regulated by the intensity of the optical signal at room temperature, with the material and spintronic device tuning achieved using the optical field.¹²⁷ A prominent advantage of 2D magnetic materials is the thickness-associated dielectric screening. For example, properly functioning field-effect transistors (FETs) have been realized using CrPS₄, a 2D magnetic semiconductor with a large bandwidth. As a results of the electrostatic shielding effect of CrPS₄, the charge in the transistor is forced to accumulate in the vicinity of CrPS₄, producing gate modulation.¹²⁸ A similar phenomenon is observed in 2D NiI₂, and gate-tunable semiconductor properties are observed in FETs that utilize NiI₂ in addition to gate-tunable magnetoelectric effects.¹²⁹

Moreover, 2D magnetic materials are expected to enable devices with lower energy consumption compared to conventional magnetic materials. For instance, considering only the spin Hall effect (SHE)-driven magnetization flip-write current density threshold J_C0-SHE, it can be approximated from a macroscopic spin model derived from

where e represents the elementary charge, μ₀ is the vacuum permeability, ℏ is the reduced Planck constant, θ_SH is the spin Hall angle, M_S is the saturation magnetization, H_K,eff is the effective perpendicular anisotropic field, and t_F represents the free layer thickness.¹³⁰ The overall power consumption of the device is proportional to the square of the current density, which in turn is proportional to the M_S strength of the material, as shown in eq 1.¹³¹ The M_S of 2D Fe₃GaTe₂ is reported to be 2 orders of magnitude smaller than that of conventional magnetic materials, suggesting that the power consumption of SOT devices based on 2D magnetic materials could be reduced by 4 orders of magnitude.^132−134 This highlights the significant potential advantages of 2D magnetic materials.

In conclusion, there are significant structural differences between 2D layered and nonlayered magnetic materials. Most 2D layered magnetic materials exhibit T_C below room temperature, while most nonlayered magnetic materials have higher T_C.^22,136 The study of ultrathin nonlayered magnetic materials is limited by synthesis methods, but ultrathin and layered magnetic nanosheets can be easily obtained using various methods, which facilitates the fabrication of heterojunctions.¹³⁷ Moreover, layered magnetic materials have the advantage of dangling-bond-free surfaces, and nonlayered nanosheets have also been reported to have atomically flat surfaces, leading to 2D magnetic materials that offer significant structural advantages over bulk magnetic materials. The magnetic properties of 2D magnetic materials are highly dependent on the number or thickness of layers and can be adjusted by various means, such as pressure, electric fields, doping, interfacial coupling, and optical fields. This tunable magnetic property enables broader applications in the development of low-power spintronic devices.^138−140

SOT Devices Based on 2D Magnetic Materials

The traditional SOT heterojunction, which consists of a non-magnetic layer (heavy metal) with strong SOC and magnetic films, faces challenges related to complex preparation processes, uncontrollable interfaces, relies on external assistance for magnetic field, and high power consumption.^141,142 By contrast, the utilization of 2D materials with layered structures, low crystal symmetry, strong SOC, and controllable perpendicular magnetic anisotropy (PMA), has emerged as a promising approach for fabricating heterojunctions with sharp and clean interfaces in SOT switching applications.^55,143,144 Compared to traditional SOT devices, the primary advantage of 2D SOT devices lies in their potential for achieving smaller size and field-free magnetization switching, making them more practical for future applications.¹⁴⁵

Mechanisms and Testing Techniques of SOT Heterojunctions

Microscopic Origin of SOT

The SOT phenomenon refers to the influence of spin-polarized electrons on magnetic moments.^{142,146−149} The mechanism of SOT origins from two components. First, spin-polarized electrons are generated by electric current due to the SOC effect. As depicted in Figure 5a and 5b, the primary SOC effects that result in spin accumulation are the spin Hall effect (SHE) and the interfacial Rashba–Edelsteion effect (REE).^150−152 Second, neighboring magnetospheres experience torque from these spin-polarized electrons, leading to dissipation, progression, or switching of magnetic moments.^31,153−156 In conventional SOT heterostructures, SHE occurs when an unpolarized charge current J_C is injected into a source material with strong SOC. As a result of asymmetric scattering correlated with spins, up- and down-spin electrons are deflected in opposite directions resulting in a transverse spin current J_S perpendicular to the direction of charge current and cumulative spin polarization σ.³¹ The SHE can be mathematically represented as follows¹⁵⁷

where ℏ is the reduced Planck constant divided by 2π, e is the elementary charge, and θ_SH is the spin Hall angle. The value of θ_SH determines the charge–spin conversion efficiency of the source material, and the sign of θ_SH indicates the polarization direction of spin accumulation at the interface of the heterostructure.^28,31,148

Schematic illustrations of the SOT effect and measurement technology. (a–c) Schematics depict the current induced spin Hall effect (a), Rashba–Edelstein effect (b), and Rashba-like vector and Dresselhaus-like vector in the k-plane (c). (d) Components of IP (τ_DL) and OOP (τ_OOP) damping-like torques, as well as field-like torque (τ_FL). (a–d) Reproduced with permission: Copyright 2020, American Institute of Physics.³³ (e) A schematic diagram illustrates the setup for spin-transfer ferromagnetic resonance (ST–FMR) measurements. The upper panel shows a ST–FMR device connected to a measurement circuit, where an RF current from a signal generator is injected via a bias tee. The lower panel depicts spin momentum locking and SOT-induced magnetization dynamics during ST–FMR measurements. The large blue arrow represents electron movement direction opposite to I_RF direction, while green and red arrows with balls denote generated spin angular momentum at top and bottom surfaces of Bi₂Se₃, respectively. (f) A typical ST–FMR signal from a Bi₂Se₃ 20 quintuple layers/CoFeB 7 nm device at 6 GHz is shown with fits, where blue and green lines represent symmetric Lorentzian (VSFS) and antisymmetric Lorentzian components (VAFA), respectively. Panel (e) and (f) were reprinted with permission under a Creative Commons CC BY License from ref (175). Copyright 2017 Springer Nature.¹⁷⁵ (g) The Hall resistance versus pulsed DC current I curves for different assisting external magnetic fields. (h) Magneto-optical Kerr images recorded for different DC current stages 1–5. (g,h) Reproduced with permission: Copyright 2020, Wiley-VCH GmbH.¹⁷³ (i) A schematic illustration demonstrates the 2ω Hall measurement in Fe₃GeTe₂/Pt device. I_AC represents the injected AC current amplitude, H₀ and M are the applied IP field and magnetic moment, respectively, and φ is the azimuthal angle. Reproduced with permission: Copyright 2019, American Chemical Society.¹⁷⁰ (j,k) Second harmonic voltages for the longitudinal effective field (j) and transverse effective field (k), respectively. H_L and H_T are the applied longitudinal magnetic field along the current direction (x axis) and transverse magnetic field transverse to the current direction (y axis). (j,k) Reproduced with permission: Copyright 2019, American Association for the Advancement of Science.³⁴ (l) Plots of the damping-like field as a function of variable I_AC for Fe₃GaTe₂/Pt are shown for -M_z (black circles) and + M_z (red circles) states, respectively, after subtracting the thermoelectric contribution. The red solid lines represent the linear regression results with zero intercept. Reproduced with permission: Copyright 2023, Wiley-VCH GmbH.¹⁷²

Another effective approach for generating spin-polarized electrons is interfacial current-induced spin accumulation, known as the REE.¹⁵¹ The REE occurs at interfaces where the inversion symmetry is broken, leading to the generation of an internal electric field E along the direction of symmetry breaking (z), as depicted in Figure 5c. This electric field arises from a potential drop and lifts the band degeneracy, resulting in spin polarization at the interface. Additionally, within the crystal itself, an inversion asymmetry gives rise to a Dresselhaus-like interaction.³³ When conduction electrons with momentum p move near the interface, an effective magnetic field parallel to E × p is generated. Therefore, we can describe the interface Rashba effect using this Hamiltonian quantity^31,158

where α_R represents the Rashba parameter influenced by the interfacial potential drop. The Rashba effect induces spin splitting and spin-momentum locking on 2D dispersive surfaces, which has been extensively investigated on various surfaces and interfaces.¹⁵⁹ Interfacial alloying of heavy elements with medium-heavy metals can amplify the in-surface potential gradient through hybridization, thereby resulting in a pronounced Rashba effect.¹⁶⁰

Magnetization Dynamics of SOT

As depicted in Figure 5d, SOT can be primarily categorized into two types: damping-like (DL) and field-like (FL) torques. On the one hand, the DL torque can be interpreted as the absorption of spin-polarized angular momentum by the noncollinear magnetization strength.²⁹ Mathematically, it is represented as a cross product between the magnetic moment m and the spin-polarization direction σ, followed by another cross product with m, i.e., m × σ × m. In terms of its physical mechanism, the DL torque mainly originates from the SHE in heavy metals; however, subsequent theories and experiments have demonstrated that other effects such as the Rashba effect can also generate DL torque (τ_DL). On the other hand, FL torque (τ_FL) can be considered an equivalent magnetic field’s influence on the magnetization strength of the magnetic layer. Formally, it is expressed as a cross product between m and σ alone, i.e., m × σ.^151,161 The τ_FL can arise from various physical effects, including the current-induced Oersted field, the Rashba equivalent field at the interface, and the sd exchange field between spin polarization and the magnetization strength.^162−164 Initially considered as the primary cause of τ_FL, later studies have revealed that other effects such as SHE can also generate τ_FL. Both types of torques play a crucial role in SOT-excited magnetic dynamics.²⁹ However, most studies typically focus on the DL term while neglecting the FL term. For practical applications and theoretical investigations, both types of torques must be fully comprehended and utilize. SOT is a complex and evolving research area involving multiple physical effects and mechanisms. Therefore, a comprehensive analysis and exploration of τ_DL and τ_FL in light of recent research progress and experimental data are necessary.^31,148

The spin-polarized electron-induced SOT acts upon the incoming magnetization, and the dynamics of the SOT-induced magnetization are described by the Landau–Lifshitz–Gilbert equation^150,157,165

where γ and α represent the gyromagnetic ratio and Gilbert damping constant, respectively. T denotes the cumulative effect of various SOT,¹⁴⁸ encompassing a field-like torque τ_FL and a damping-like torque τ_DL, which can be mathematically formulated as follows:³¹

SOT Measurement Methods

Currently, the primary techniques employed for measuring the SOT effect include spin-torque ferromagnetic resonance (ST–FMR), magneto-optical Kerr (MOKE), and second-harmonic Hall (SHH) methods.^148,166 As depicted in Figure 5e and 5f, the ST–FMR technique utilizes the influence of spin propagation on the overall magnetization intensity when the FMR signal is driven by an external AC magnetic field generated by a microwave field, thereby quantifying the SOT effect.¹⁶⁷ The MOKE method enables direct observation of domain wall motions, by which magnetization switching can be clearly detected, as shown in Figure 5g and 5h.¹⁶⁸ The predominant approach for measuring SOT is still through second-harmonic methodology which employs low-frequency currents to perturb magnetic moments within a magnetic layer and generate second-harmonic voltage signals using the Hall effect of magnets (Figure 5i). This method accurately quantifies SOT while facilitating separation between DL and FL torques, making it adaptable to various anisotropic systems.¹⁶⁹

Alghamdi et al. found that a DL SOT is exerted on the magnetization of Fe₃GeTe₂ by the spin current generated in Pt, with a critical current density for magnetization switching of approximately 2.5 × 10⁷ A/cm² in the Fe₃GeTe₂/Pt heterostructure. In particular, they investigated the SHH responses while rotating the magnetization of Fe₃GeTe₂ within the plane using an applied magnetic field. The SHH signal (R^2ω_H) can be described by the following equation

where φ represents the azimuthal angle between the magnetic field and current direction, cos φ terms encompass various contributions to DL SOT contribution R^2ω_DL via AHE, thermoelectric contribution R^2ω_TH through the anomalous Nernst effect, Oersted field-induced effect R^2ω_Oe, and the FL SOT via the planar Hall effect R^2ω_FL. The cos (3φ) term combines Oersted-field and FL SOT contributions as R^2ω_Oe + R^2ω_FL.¹⁷⁰ By utilizing SHH measurements, current-induced magnetic moment switching can be quantified in Fe₃GeTe₂/Pt heterojunctions. For second-harmonic measurements, a small alternating current is applied to the device while subjecting it to an IP or OOP magnetic field along the longitudinal or transverse directions respectively, enabling the measurement of longitudinal (H_L) or transverse (H_T) effective fields. Figure 5j and 5k depict the measured second-harmonic voltages under H_L and H_T external magnetic fields, respectively, which are fitted using parabolic and linear functions accordingly. The corresponding ratios for DL and FL torque can be calculated as follows

where V_ω and V_2ω are the first- and second-harmonic voltages, respectively.³⁴ The current-induced effective fields in the longitudinal and transverse directions can then be extracted on the basis of the following equations¹⁶⁶

where R_Planar is the planar Hall resistance and R_AHE is the anomalous Hall resistance. In general, the SOT can be evaluated by the spin torque efficiency

where T_int is the interfacial spin transparency, μ₀ΔH_L(T) is the effective field (μ₀ is the vacuum permeability), M_S is the saturation magnetization, t^eff_FM represents the effective thickness of 2D ferromagnetic, and J is the current density.^34,161,171 Furthermore, Li et al. quantitatively determined high DL SOT efficiency ξ_DL ≈ 0.28 in the Fe₃GaTe₂/Pt heterojunction by using a similar tactic, which is higher than Pt-based heavy metal/conventional ferromagnet devices.¹⁷² The DL effective field versus current density of μ₀H_DL = 2.1 mT per 10⁷ A/cm² is extracted from the second-harmonic signal in Figure 5l, after subtracting the contribution of the thermo-electric effect. Consequently, the DL SOT efficiency ξ_DL is derived from the variational subform of Equation 9

where μ₀H_DL and t_FM are DL effective field and thickness, respectively.¹⁷¹ In general, the presence of a second-harmonic signal is a sign that the magnetic moment can undergo switching. Subsequently, a pulse current (I_Pulse) can be applied to measure R_xy reversal in the heterojunction.^169,173,174

According to the aforementioned statement, SOT refers to the phenomenon wherein a J_C passing through a material with strong SOC can induce a pure spin flow that exerts a torque on the neighboring magnetic layer due to either the SHE or Rashba effect, thereby modulating its magnetization state. SOT can be categorized into two main types: DL and FL torques, both of which play significant roles in the dynamics of magnets driven by SOT. However, as previously pointed out, most studies typically focus solely on the DL term while neglecting the FL term. For practical applications and theoretical investigations, both types of torques should be comprehensively comprehended and harnessed.^29,31

The comprehensive understanding of several crucial parameters is essential for accurate SOT measurements. J denotes the current density, whereas the value of θ_SH determines the charge–spin conversion efficiency of the source material, and the sign of θ_SH indicates the polarization direction of spin accumulation at the heterostructure interface. H_R represents a Hamiltonian quantity that describes the interface Rashba effect. τ_FL and τ_DL correspond to FL and DL torques, respectively. In addition, μ₀ΔH_L(T) refers to an effective field used for evaluating important parameters in SOT analysis: spin torque efficiency ξ (or ξ_DL).

Note that SOT constitutes a multifaceted and dynamic field of study encompassing various physical phenomena and mechanisms. In order to investigate SOT in 2D materials, a deeper understanding is required. To achieve a comprehensive understanding of the DL and FL torques, SOT measurement methods, and applications in 2D systems, thorough analysis and exploration within the context of cutting-edge research advancements and experimental data are necessary.^7,14

Construction of 2D SOT Heterojunctions

In practice, various methods are available for constructing 2D magnetic SOT heterojunctions, including mechanical exfoliation (ME), physical vapor deposition (PVD), CVD, and a combination of mechanical exfoliation with PVD (ME–PVD). Prominent PVD techniques encompass magnetron sputtering (MS), pulsed laser deposition (PLD), electron beam evaporation (EBE), and MBE. (i) ME: Shao et al. employed mechanical stripping to stack 2D materials with strong SOC, namely, WTe₂ and Fe₃GeTe₂, forming a heterojunction. They demonstrated that the AHE of the heterojunction can be effectively regulated by electric current and revealed the significant influence of Joule heating on its switching behavior.¹⁷⁶ (ii) PVD: Mogi et al. fabricated heterojunctions comprising a 2D topological insulator (Bi_1–xSb_x)₂Te₃ and a 2D magnetic Cr₂Ge₂Te₆ film using MBE to achieve current-induced magnetization switching.⁶⁰ Chen et al. employed MBE to synthesize heterojunctions composed of 2D ferromagnetic Fe₃GeTe₂ and 2D topological Bi₂Te₃, enabling ultrafast terahertz spin–current excitation at room temperature. The symmetry of terahertz radiation is effectively controlled by optical pumping incidence and the direction of the external magnetic field, suggesting that the generation mechanism of terahertz radiation involves a spin–charge transition attributed to the inverse Edelstein effect.¹⁷⁷ (iii) CVD: Li et al. employed CVD to fabricate 2D magnetic CrSe₂/WSe₂ heterojunctions, enabling precise control over the thickness of CrSe₂. Reflective magnetic circular dichroism and magneto-transport results revealed a transition from weakly ferromagnetic to strongly ferromagnetic behavior as the thickness increased from monolayer to bilayers.⁵⁹ Similarly, Zeng et al. successfully synthesized 2D magnetic Cr₅Te₈/WSe₂ heterojunctions via CVD,¹⁷⁸ providing insights into the direct preparation of SOT heterojunctions using CVD. (iv) ME–PVD: Wang et al. employed ME to disintegrate a few layers of Fe₃GeTe₂ crystals, followed by MS deposition of 6 nm heavy metal Pt, thereby forming a heterojunction. The SOT flip in the Pt/Fe₃GeTe₂ heterojunction was achieved at 100 K and with an auxiliary magnetic field strength of 50 mT, exhibiting a critical flip current density of approximately 1.2 × 10⁷ A/cm².³⁴ Similarly, mechanically exfoliated Fe₃GaTe₂ combined with MS deposition of Pt films can be utilized to fabricate heterojunctions that demonstrate efficient SOT effects induced by current flow. Moreover, through the design of symmetry-breaking geometries, complete electronically controlled magnetization flipping without assistance from an external magnetic field has been realized.¹⁷⁴ Recently, Zhao et al. successfully prepared (Co_0.5Fe_0.5)₅GeTe₂/Pt heterojunctions using this coupling method (ME–EBE) and observed the phenomenon of all electrically controlled SOT switching without magnetic field at room temperature.¹⁷⁹

The construction of a 2D SOT heterojunction is a crucial aspect and has emerged as one of the prominent research areas in the field of 2D spintronic devices in recent years.^12,33 Among the current methods for their fabrication, ME of crystals exhibits high efficiency; however, it suffers from limitations such as low yield, poor reproducibility, and small size. MBE within PVD is mature but encounters challenges related to low efficiency and high cost. Conversely, CVD offers an inexpensive approach with high reaction efficiency; nevertheless, it faces issues regarding poor reproducibility and a prolonged period required for optimizing conditions. To achieve efficient and cost-effective construction of 2D magnetic metal heterojunctions in future endeavors, researchers are urgently implored to develop or optimize the preparation techniques for 2D SOT heterojunctions.³¹

Investigation of 2D SOT Heterojunctions

The SOT effect refers to the process of altering the spin state of an object through the application of an external torque, which can arise from SOC effects, including the Rashba effect, and the SHE.^152,180,181 In terms of manipulating spins, 2D magnetic heterojunctions offer a means for realizing SOC effects in 2D NM or at their interfaces. The distinctive spin structure and magnetic properties of FMs offer promising platforms for efficient and controllable SOT in 2D magnetic heterojunctions. For instance, by applying external magnetic and electric fields, control over spin-polarized currents and spin accumulation in 2D magnetic heterojunctions can be achieved for information storage, transmission, and processing purposes.^{13,31,182,183}

The 2D heterojunctions fabricated with heavy metals or 2D topological materials with 2D FMs are a prevalent type of SOT devices. Alghamdi et al. discovered in their study of the 2D Fe₃GeTe₂/Pt heterojunction (Figure 6a) that when a current density of 2.5 × 10⁷ A/cm² is applied, the spin flow generated in Pt induces a DL modulation of Fe₃GeTe₂ magnetization (Figure 6b).¹⁷⁰ Shin et al. observed a complete 2D Fe₃GeTe₂/WTe₂ (with low symmetry) heterojunction by applying a current density-driven magnetic moment switch of 3.9 × 10⁶ A/cm² at 150 K assisted by a magnetic field of 30 mT. The current direction of the WTe₂/Fe₃GeTe₂ device was 23° off from the a axis of the WTe₂ crystal.¹⁸⁴ The implementation of field-free SOT magnetization switching is more favorable and practical compared to the aforementioned SOT magnetization switching, which requires magnetic field assistance. Kao et al. utilized the low symmetry of WTe₂ crystals to generate an OOP polarized spin flow and achieve an field-free switching of the 2D magnetic moment in the Fe₃GeTe₂/WTe₂ heterojunction, with a current density as low as 2 × 10⁶ A/cm² (Figures 6c and 6d), and the current direction of the device was parallel to the a axis of WTe₂.⁴⁴ A similar phenomenon of low-current-density-driven (2.56 × 10⁶ A/cm²) field-free switching was also observed in the Fe₃GaTe₂/TaIrTe₄ heterojunctions, with an applied current parallel to the a axis of TaIrTe₄.¹⁸⁵ By incorporating an asymmetric geometric design (wedge-shaped structure) into Fe₃GaTe₂/Pt heterojunctions, Yun et al. successfully achieved field-free magnetization switching by harnessing OOP polarized spin currents.¹⁷⁴ Additionally, achieving room-temperature SOT magnetization switching is a crucial requirement for its practical application. Li et al. successfully achieved SOT-driven switching of the 2D magnetic moment at room temperature using a Hall device based on the 2D Fe₃GaTe₂/Pt structure (Figure 6e), with a relatively low current density requirement of only 1.3 × 10⁷ A/cm² (Figure 6f). Additionally, through second-harmonic measurements, they determined an SOT efficiency value of 0.28 which surpasses those obtained from Pt-based heavy metal/conventional ferromagnetic SOT devices.¹⁷² Recently, Kajale et al. also reported experiments on nonvolatile and deterministic magnetization switching controlled by electric currents for vdW magnetic materials at room temperature. They successfully implemented of SOT switching for the Fe₃GaTe₂/Pt system with a switching temperature up to 320 K and a low switching threshold current density of only 1.69 × 10⁶ A/cm². The inverse damping torque efficiency was found to be 0.093.¹³² Furthermore, they successfully achieved room-temperature SOT field-free switching in a 2D magnetic Fe₃GaTe₂/WTe₂ heterojunction (Figure 6g), demonstrating an unassisted current density of 2.23 × 10⁶ A/cm² and a maximum operating temperature of up to 325 K (Figure 6h), and the current direction of the device was parallel to the a axis of WTe₂.¹⁸⁶

Representative 2D SOT heterojunction devices. (a) The schematic illustration depicts the effective field responsible for switching the magnetic state of Fe₃GeTe₂ in the Fe₃GeTe₂/Pt hybrid devices. J represents the injected current density, H_x denotes the applied IP field, H_DL signifies the effective field from damping-like SOT, and M represents the magnetization of Fe₃GeTe₂. The inset shows case an AFM topographic image of the Fe₃GeTe₂ device. (b) The effective switching current is plotted as a function of applied IP positive bias field. The color scale indicates the switching resistance as a percentage of R_H0 (the absolute value of anomalous Hall resistance at zero current). (a,b) Reproduced with permission: Copyright 2019, American Chemical Society.¹⁷⁰ (c) A schematic illustrating the application of a charge current along the a-axis of WTe₂, demonstrating the OOP component of spin polarization, is a valid form of antidamping SOTs. (d) The magnetization state of Fe₃GeTe₂ is initialized to m_z = −1 (blue and orange data) or m_z = +1 (yellow and purple data). Subsequently, with increasing magnitude, a current pulse is gradually applied in accordance with arrow direction while detecting the magnetization state using AHE. Switching from m_z = −1 to m_z = +1 occurs only when a positive charge current is employed with I⃗//â. Conversely, a negative current induces switch from m_z = +1 to m_z = −1. This observation aligns well with magnetic switching attributed to τ^AD_OP since final state determination relies on current polarity. (c,d) Reproduced with permission: Copyright 2022, Springer Nature.⁴⁴ (e) A typical optical image presents the Fe₃GaTe₂/Pt Hall device. (f) Current-driven perpendicular magnetization switching is demonstrated under ±200 mT IP magnetic field at 300 K. Horizontal dashed lines represent states of saturated magnetization for Fe₃GaTe₂ while arrows indicate sweep direction. (e,f) Reproduced with permission: Copyright 2023, Wiley-VCH GmbH.¹⁷² (g) Response of the Fe₃GaTe₂/WTe₂ device to current pulse sweeps along the a axis was investigated at varying temperatures in the absence of any external field. The curve for each temperature represents an average of four consecutive current pulse sweeps acquired, with error bars indicating the standard deviation across these measurements. To enhance clarity, data has been offset along the y axis. Inset shows an optical image of the Fe₃GaTe₂/WTe₂ device after electrode patterning and encapsulation with hBN. The applied pulse current is parallel to the a-axis direction of WTe₂. Reprinted with permission under a Creative Commons CC BY License from ref (186). Copyright 2024 American Association for the Advancement of Science.¹⁸⁶

Recently, artificial moiré superlattices composed of stacked 2D crystals have been shown to be a potent platform for modulating SOT. Liang et al. demonstrated that SOT can be effectively controlled by varying the torsion angle (θ) and the gate voltage in a SOT device consisting of a twisted WS₂/WS₂ bilayer (t-WS₂) and a CoFe/Pt thin film. Notably, the conductivity of SOT was significantly enhanced by 44.5% at θ = 8.3°. Furthermore, the moiré superlattice in t-WS₂ enables higher gate voltage tunability of the SOT conductivity compared with the WS₂ monolayer and untwisted WS₂/WS₂ bilayer. These results are attributed to the generation of the interfacial moiré magnetic field by the real-space Berry phase in moiré superlattices, which modulates the absorption of the spin-Hall current from Pt via the magnetic proximity effect. This study paves the way for applying moiré patterns in SOT.¹⁸²

Various methods are available for preparing SOT heterojunctions, and different heterojunction configurations can be achieved using 2D magnetic materials with strong PMA.^170,172 The study of 2D SOT heterojunctions is currently focused on three frontier directions. First, the goal is to achieve field-free SOT magnetization switching, which can be realized through the use of 2D topological materials with symmetry-breaking structures, such as WTe₂,^44,186 or by designing special heterojunctions with wedge-shaped structures.¹⁷⁴ Second, efforts are being made to achieve SOT switching at room temperature, which is becoming feasible alongside the development of materials with high T_C.¹³² Finally, there is a push to reduce the critical switching current density, with critical switching current densities on the order of 10⁵ A/cm² already achieved in SOT devices utilizing individual Fe₃GaTe₂ nanosheets.¹⁸⁷ Moiré engineering is a technological tool that may be explored in the future to reduce power consumption. As research progresses, our understanding of 2D SOTs has evolved from mechanisms to applications (Table 2). Based on Table 2, which summarizes recent findings on the magnetization switching conditions of various 2D SOT heterojunctions, including critical switching current density, thickness, and other relevant parameters, several key observations can be made. Notably, all-2D SOT heterojunctions such as Fe₃GeTe₂/WTe₂, Fe₃GaTe₂/WTe₂, and Fe₃GaTe₂/TaIrTe₄ exhibit significant advantages, demonstrating the ability to achieve efficient field-free magnetization switching. Their relatively low current densities are crucial for the development of power-efficient devices. The variation in switching conditions across different heterostructures underscores the importance of material selection and design in optimizing device performance.

Table 2. Comparison of Magnetization Switching Conditions of 2D SOT Heterojunctions.

		Thickness (nm)
SOT heterojunction	Met.	FM layer	SOC layer	J_C (A/cm²)	ξ	H (Oe)	T (K)	I (mA)	∠I and a axis	ref.
PtTe₂/WTe₂/CoFeB	MBE/MS	CoFeB (2.9)	PtTe₂ (2)/WTe₂ (6)	2.6 × 10⁶	N/A	0	300	N/A	N/A	(143)
MoTe₂/Py	ME/MS	Py (6)	MoTe₂ (66.1)	6.71 × 10⁵	0.35	N/A	300	N/A	N/A	(152)
Pt0_.75Ti_0.25/Fe_xTb_1-x	MS	Fe_xTb_1-x (8)	Pt_0.75Ti_0.25 (5.6)	2.2 × 10⁶	0.38 ± 0.02 (x = 1)	N/A	350	N/A	N/A	(161)
(Ga_0.94,Mn_0.06)As/In_0.3Ga_0.7As	MBE	GMA (15)	IGA (500)	4.6 × 10⁴	N/A	500	N/A	N/A	N/A	(162)
PtTe₂/Au/CoTb	MS	CoTb (6)	PtTe₂ (10)	3.1 × 10⁶	N/A	2000	N/A	20	N/A	(167)
Fe₃GeTe₂/Pt	ME/MS	Fe₃GeTe₂ (87)	Pt (6)	1.85 × 10⁶	N/A	500	100	1	N/A	(34)
Fe₃GeTe₂/Pt	ME/MS	Fe₃GeTe₂ (15–23)	Pt (5)	2.5 × 10⁷	0.14 ± 0.01	0	225	2.4	N/A	(170)
Fe₃GeTe₂/WTe₂	ME	Fe₃GeTe₂ (7.3)	WTe₂ (12.6)	3.9 × 10⁶	4.6	300	150	1.5	23°	(184)
Fe_2.78GeTe₂/WTe₂	ME	FGT (4.1)	WTe₂ (25.8)	9.8 × 10⁶	N/A	0	150	9	0°	(44)
Cr₂Ge₂Te₆/(Bi_1-xSb_x)₂Te₃	MBE	Cr₂Ge₂Te₆ (12)	(Bi_1-xSb_x)₂Te₃ (6)	N/A	1.4	1000	N/A	2	N/A	(60)
Fe₃GaTe₂/Pt	ME/MS	Fe₃GaTe₂ (20)	Pt (6)	1.3 × 10⁷	0.28	2000	300	6.3	N/A	(172)
Fe₃GaTe₂/Pt	ME/MS	Fe₃GaTe₂ (15)	Pt (7)	4.8 × 10⁶	N/A	0	300	N/A	N/A	(174)
Fe₃GaTe₂/Pt	ME	Fe₃GaTe₂ (57.9)	Pt (6)	1.69 × 10⁶	0.093	100	320	5.4	N/A	(132)
Fe₃GaTe₂/WTe₂	ME	Fe₃GaTe₂ (17.9)	WTe₂ (23.8)	2.23 × 10⁶	N/A	0	325	8	0°	(186)
Fe₃GaTe₂/TaIrTe₄	ME	Fe₃GaTe₂ (4.8)	TaIrTe₄ (10.3)	2.56 × 10⁶	N/A	0	300	3.4	0°	(185)
(Co_0.5Fe_0.5)₅GeTe₂/Pt	ME/EBE	(Co_0.5Fe_0.5)₅GeTe₂ (30)	Pt (10)	2.4 × 10⁷	N/A	0	300	N/A	N/A	(179)
Fe₃GaTe₂	ME	Fe₃GaTe₂ (12–28)	N/A	1.7 × 10⁵	N/A	80	300	N/A	N/A	(187)

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Magnetoresistance (MR) Effect Based on 2D Magnetic Materials

The 2D MTJ or spin valve devices, as shown in Figure 7a, operate based on the MR and feature a “sandwich structure” comprising two magnetic metal electrodes separated by a spacer layer. The categorization of 2D MTJs and spin valves has been a topic of controversy, also with inconsistent designations of MR.^188,189 Here, we clarify the classification of conventional 3D MTJs and spin valves based on their characteristics and collectively refer to their magnetoresistance as TMR. The MTJ spacer functions as an insulating barrier, whereas the spin valve spacer typically consists of a non-magnetic conductor or semiconductor.^37,190,191 These devices exhibit two resistance states (low and high resistance) corresponding to two different magnetic configurations: parallel and antiparallel orientations, respectively (Figure 7b). In MTJ and spin valve structures, there are two distinct layers: the reference or fixed layer with its magnetization direction aligned along the easy magnetization axis, and the free layer, which can be oriented either parallel or antiparallel to the reference layer (Figure 7c and 7e).^192−194 The magnetization direction of the free layer can be switched using external magnetic field, spin-transfer torque (STT) or SOT.^7,55,73 As depicted in Figure 7d and 7f, when the magnetization directions of the reference layer and the free layer are parallel, the electrons in the “majority-spin” sub-band within the reference layer tunnel through the barrier layer into the free layer and occupy empty states within its corresponding “majority-spin” sub-band. Simultaneously, electrons in the “minority-spin” sub-band within the reference layer also tunnel into corresponding empty states within its counterpart in the free layer. This phenomenon results in a low resistance state for the entire MTJ. Conversely, when antiparallel magnetization directions exist between both layers, electron transport occurs as majority-spin sub-band electrons from the reference layer enter empty states within minority-spin sub-bands of the free layer. Consequently, this phenomenon leads to a reduction in participating electron numbers during transport across tunnel junctions and establishes a high resistance state for device operation.^37,130,195 Similarly, spin valves typically consist of a sandwich core structure comprising two layers of ferromagnetic metal and a non-magnetic intermediate layer. The resistance of the device is modulated by the relative orientation of the two ferromagnetic layers due to the transport of spin-polarized electrons between them.^196−198 Notably, the sharp resistive state flips in 2D MTJ and spin valves are achieved by exploiting the thickness dependence of coercivity in 2D magnetic materials, as illustrated in Figure 7g and 7h. Specifically, an artificial construction involving two layers of 2D electrode materials with different coercivities (corresponding to different thicknesses) enables the realization of antiparallel states for magnetic moments within a certain range of applied magnetic field.^199,200

Working mechanism diagram of MTJ or spin valve device. (a) A schematic diagram depicts a typical 2D MTJ (with test scheme) comprising top and bottom 2D ferromagnetic material (2D FM) layers separated by a 2D spacer. (b) The resistance R vs magnetic field graph demonstrates the switching behavior between low and high voltage levels observed for antiparallel and parallel configurations of the magnets. (c–f) Schematic diagrams (c, d) depict an MTJ in parallel configuration along with its corresponding band diagram, illustrating that both spin-up and spin-down electrons can traverse the barrier from the left electrode to the right one. Schematic diagrams (e, f) illustrate an MTJ in antiparallel configuration along with its corresponding band diagram, where both spin-up and spin-down channels are blocked due to asymmetry between two states. (c–f) Reproduced with permission: Copyright 2021, American Institute of Physics.³⁷ Magneto-transport measurements for Fe₃GeTe₂/h-BN/Fe₃GeTe₂ MTJ device (g) and Fe₃GeTe₂/MoS₂/Fe₃GeTe₂ spin valve device (h). The top panels display magnetoresistance (R₁), while middle and bottom panels show anomalous Hall resistance (R₂ and R₃, respectively). An injection current is 5 μA is applied at a temperature of 15 K. Sharp magnetoresistance jumps are observed in both devices, with perfect two-state behavior exhibited by the Fe₃GeTe₂/h-BN/Fe₃GeTe₂ device, whereas additional minor jumps are seen in the Fe₃GeTe₂/MoS₂/Fe₃GeTe₂ device. Curved arrows indicate scanning directions of the magnetic field, while “Parallel” and “Antiparallel” denote magnetization configurations representing low- and high-resistance states respectively. Vertical dashed lines signify that magnetic field-induced switching occurs at coercive fields (μ₀H_C) specific to Fe₃GeTe₂ layers. Panel (g) and (h) were reprinted with permission under a Creative Commons CC BY License from ref (199). Copyright 2022 Wiley-VCH GmbH.¹⁹⁹

The parameter that characterizes the switching effect in MTJs and spin valves is the attempted penetration magnetoresistance, which can be calculated as follows

where R_P and R_AP represent the resistance when the magnetization directions of the two magnetic layers are parallel and antiparallel, respectively; G_P and G_AP denote the conductance under parallel and antiparallel magnetization configurations respectively. High TMR values at room temperature are imperative to utilize MTJs or spin valves as strong cells in MRAM.^201−203 By replacing conventional magnetic metal films with 2D magnetic materials in heterojunctions, improved interface quality and coupling effects can be achieved, thereby enabling tunneling effects and large TMR values in such MTJ or spin-valve heterojunctions.^{5,6,16,18,37,194}

In 2018, Science published two concurrent studies on spin in heterojunctions of 2D magnetic materials. Song et al. demonstrated the utilization of graphene/CrI₃/graphene heterojunctions as multiple spin-filter MTJs (SF–MTJs), where CrI₃ acted as a potential barrier for spin filters. The TMR exhibited a enhancement with increasing thickness of the CrI₃ layer, reaching a record-breaking value of 19,000% using a four-layer SF–MTJ under low-temperature conditions.³⁶ Klein et al. investigated the temperature-dependent and applied magnetic field-induced tunnelling behavior in graphite/CrI₃/graphite heterojunctions based on 2D materials. The bilayer, trilayer, and tetralayer CrI₃ barrier layers yielded magnetoresistance values of 95%, 300%, and 550%.³⁵ These pioneering works have introduced the application of 2D magnetic materials on MTJs.

Calculation on MTJ Performances

Considering that the MTJ or spin valve devices consist of two magnetic layers and a non-magnetic spacer, their operation is based on the difference in the density of states around the Fermi level of the magnetic electrodes for spin-up and spin-down electrons. The spacer allows independent switching of the two ferromagnetic electrodes under a magnetic field, resulting in low resistance and high resistance states in parallel and antiparallel configurations, respectively.^204,205 The energy state density, Bloch wave vector, transmission, and TMR of MTJ or spin valve should be studied by computational methods.^205−211

Li et al. studied on the spin-dependent electronic transport in vdW MTJs consisting of two layers of Fe₃GeTe₂ and a spacer layer (graphene or h-BN) using density functional theory. Figure 8a illustrates the crystal structure of bulk Fe₃GeTe₂, which exhibits a layered hexagonal crystal structure with Fe₃Ge slabs separated by vdW-bonded Te layers. The unit cell contains Fe atoms located at two distinct Wyckoff sites, denoted as Fe1 and Fe2. The calculated density of states of bulk Fe₃GeTe₂ is shown in Figure 8b. The presence of significant electron density at the Fermi energy and the exchange splitting of spin bands indicate that Fe₃GeTe₂ is a ferromagnetic metal. The resistance across the junction undergoes drastic changes, reaching thousands of percent, when switching the magnetization direction from parallel to antiparallel in the Fe₃GeTe₂ electrodes. This phenomenon is known as giant TMR, which arises due to differences in electronic structures between the two spin-conducting channels within Fe₃GeTe₂. As a result, there is a mismatch between incoming and outgoing Bloch states in the electrodes, leading to suppressed transmission for an antiparallel-aligned MTJ configuration. The spin-dependent ballistic conductance of the MTJs is obtained by the summation of transmission over the 2D Brillouin zone

where T_σ (k_∥) is the spin and k-resolved transmission probability for an electron at the Fermi energy with spin σ and Bloch wave vector k_∥ = (k_x, k_y), and h represents the Plank constant. The calculation of TMR can be achieved by combining Equation 12.^212,213 Additionally, Figure 8c illustrates the calculated k_∥-resolved transmission across Fe₃GeTe₂/h-BN/Fe₃GeTe₂ MTJ. Given its insulating nature, h-BN exhibits a low barrier at the center of the 2D Brillouin zone (2DBZ, Γ̅ point), leading to enhanced transmission in this region. These results suggest that the vacuum barrier is also included for comparison analysis. The total transmission in the parallel magnetization state exceeds that in the antiparallel state by 2 orders of magnitude, thereby contributing to pronounced resistance switching in MTJ.²¹²

Calculated results about the 2D MTJ structure. (a) Side view of the atomic lattice of bulk Fe₃GeTe₂, with dashed rectangular and rhombic shapes representing the crystal unit cell. (b) Spin-resolved density of states for bulk Fe₃GeTe₂. (c) k_∥-resolved electron transmission distribution in the 2DBZ for Fe₃GeTe₂/h-BN(1 L)/Fe₃GeTe₂ MTJ, depicting majority spin transmission for parallel magnetization state (left), minority spin transmission for parallel magnetization state (center), and either spin transmission for antiparallel magnetization state (right). (a–c) Reproduced with permission: Copyright 2019, American Chemical Society.²¹² (d,e) k_∥ dependent transmission spectra of Ru/MnSe₂/h-BN/MnSe₂/Ru MTJs are shown for majority spin channel (left) and minority spin channel (right) in both parallel magnetization configuration (d) and antiparallel magnetization configuration (e). The blue circles indicate the hottest spots in each figure. Panel (d) and (e) were reprinted with permission under a Creative Commons CC BY License from ref (213). Copyright 2019 Institute of Physics.²¹³ (f) Bulk MoS₂ material’s spin Hall conductivity is depicted, where charge current, spin current, and spin directions are mutually perpendicular with vertical direction being that of the spin current. (g) Transmission coefficients versus energy are plotted for 1T VSe₂/1H VSe₂/1T VSe₂ MTJ (VT MTJ), with red color indicating majority spin transmission while blue color line represents minority-spin transmission. Inset shows the transmission coefficients of 1T VSe₂/1H MoS₂/1T VSe₂ MTJ (MT MTJ), where red line overlaps with blue line but remains invisible. (h,i) The upper panel displays transmission eigenchannel wave functions while lower panel presents schematic diagrams of potential profiles corresponding to MT MTJ (h) and VT MTJ (i), respectively. Small arrows depict magnetizations of VSe₂, whereas blue dotted line indicates the minority-spin quantum-well states. The green dashed lines represent E_F location set as zero. (f–i) Reproduced with permission: Copyright 2019, American Chemical Society.²¹⁶

Alternative TMR calculations can also start by calculating the spin-related current, which can be defined using the Landauer–Büttiker equation

in which μ_L(μ_R) and f_L(f_R) represent the electrochemical potential and Fermi distribution function of the left (right) electrode, respectively. V_L/R denotes the bias voltage applied to the electrodes. The transmission coefficient T_σ(E,V_L,V_R), which depends on E, V_L, and V_R, can be utilized to calculate the TMR ratio by employing a method akin to Equation 11

where I_P and I_AP represent the total current across the junction when the magnetization directions of the two magnetic layers are parallel and antiparallel, respectively.^{209,210,212,214}

Similarly, researchers have observed significant TMR values of 1.7 × 10⁵% and 6962% in 2D magnetic MTJs based on VSe₂ and 1T–CrTe₂, respectively.^207,215 Additionally, Feng et al. proposed a novel in-plane MTJ structure consisting of VCl₃/CoBr₃/VCl₃, which exhibited a high TMR of 4.5 × 10¹²% and an excellent spin filtering effect.²¹⁴ This intriguing in-plane heterogeneity presents a new avenue for the advancement of 2D MTJ. Through theoretical investigations, Zhou et al. explored the SOT-driven MTJ (SOT–MTJ) utilizing MoS₂/VSe₂/MoS₂/VSe₂ heterostructures. The intrinsic spin Hall conductivity of the underlying MoS₂ layer was calculated using the QUANTUM ESPRESSO and WANNIER90 packages to demonstrate that SOT can serve as a promising switching method by employing MoS₂ as the SOC layer (Figure 8f). Figure 8g illustrates the transmission-energy curve for 1T VSe₂/1H VSe₂/1T VSe₂ MTJ (VT MTJ) and 1T VSe₂/1H MoS₂/1T VSe₂ MTJ (MT MTJ), where the transmission peaks were observed to deviate from E_F only in VT MTJ but not in MT MTJ. As a result of the sharp attenuation of transmission at E_F for both spins, their transmissions become inconspicuous, resulting in T^↑_APC = 1.19 × 10^–4 (T^↓_APC is the transmission spectra at APC equilibrium state) at E_F. In Figure 8h (upper panel), the eigenchannel wave functions exhibit localization at the left VSe₂ and the middle MoS₂ tunnel layers. Figure 8i (upper panel) illustrates the minority-spin eigenchannel wave functions for VT MTJ, where these wave functions decay along the transport x direction from the incoming MoS₂ electrode. The left 1T VSe₂ and right 1T VSe₂ possess prohibitive magnetization for minority spin, resulting in a significant spin-filtering effect that leads to rapid decay in VT MTJ. The lower panels of Figures 8h and 8i depict schematic representations of potential profile for the model of magnetic quantum wells in VT MTJ and MT MTJ, respectively. The aforementioned investigations demonstrate the sensitivity of 2D heterojunction transport properties to quantum-well resonances, and suggest that the transmission of 2D heterojunctions can be modulated through Fermi energy shifting. Clean interfaces must be maintained in 2D heterojunctions to preserve high TMR and prevent short-circuiting current, as quantum-well states play a critical role in achieving high TMR. By studying the spin-resolved transport properties of SOT–MTJ, researchers have revealed quantum-well resonances in vdW layers and discovered a nonequilibrium large TMR of 846% at room temperature in top sandwich “VSe₂/MoS₂/VSe₂” heterostructures, thereby demonstrating the feasibility of 2D SOT–MTJ.²¹⁶ These findings offer valuable insights into the predictive capabilities of 2D MTJs in terms of heterojunction architectural quality and performance potential, thereby prompting experimental investigations of vdW MTJs and paving the way for their application in spintronics.¹⁸

2D Spin Valve Devices

The diverse range of 2D material families provides a solid foundation for the development of 2D spin valves, encompassing prominent examples such as graphene, MoS₂, WS₂, and WSe₂.^217−225 Pan et al. constructed lateral spin valve heterojunctions (Figure 9a) using Fe₃GaTe₂/graphene/Fe₃GaTe₂, where graphene acts as a spin transport channel and Fe₃GaTe₂ serves as a spin injection and detector. An evident nonlocal spin valve signal was observed when scanning the magnetic field along the OOP direction of Fe₃GaTe₂. This nonlocal spin valve signal persists even at 320 K, achieving room-temperature lateral spin valves in an all-2D heterostructure.²²⁶ However, another widely studied configuration is the vertical structure of spin valve heterojunction. Since 2020, Prof. Wang’s group has conducted a series of experiments on all-2D vertical spin valve devices,^{45,53,188,227−229} observing a TMR value of 3.1% at 10 K in the Fe₃GeTe₂/MoS₂/Fe₃GeTe₂ spin valve heterojunction.⁵³ By replacing the spacer with InSe,²²⁸ WSe₂,²³⁰ GaSe,²²⁹ and MoSe₂¹⁸⁸ (Figure 9b and 9c), they achieved TMR values of 41%, 26%, 192%, and 42%, respectively, at the same temperature. The TMR of these 2D spin valve devices is inversely proportional to the magnitude of the bias current (I_b), bias voltage (V_b), and the thickness (d) of the spacer. Furthermore, they observed a significant TMR value (85%) in the all-2D spin valve device (Fe₃GaTe₂/WSe₂/Fe₃GaTe₂) at room temperature (Figure 9d).⁴⁵ As shown in Figure 9e, Pan et al. also observed a TMR of 50% in the same trilaminar heterojunction at room temperature, and the TMR can be modulated by the I_b.²³¹ During the same period, Prof. Chang’s group observed a weak TMR in the Cr_0.335Sn_0.665Te/Al₂O₃/Fe (TMR = 0.194%)²³² and Fe₃GaTe₂/MoS₂/Fe₃GaTe₂ (TMR = 0.31%)²³³ spin valve devices at room temperature (Figure 9f). However, the TMR significantly increased to 3.7% and 11% in the Fe₃GaTe₂/MoSe₂/Fe₃GaTe₂ (Figure 9f)¹⁸⁹ and Fe₃GaTe₂/WS₂/Fe₃GaTe₂²³⁴ all-2D spin valve devices at room temperature, respectively. They also found that the TMR ratio are strong dependence on the thickness of the spacer layer,^229,230,233 and the polarity of TMR can be reserved by modulating the I_b.^231,234 Additionally, 2D superconductor NbSe₂ can be utilized as a spacer material in spin valve devices such as Fe₃GeTe₂/NbSe₂/Fe₃GeTe₂ to achieve a 17-fold enhancement of magnetoresistance at its superconducting critical temperature of 6.8 K due to band splitting induced by magnetic proximity effect on the materials interface (Figure 9g).²³⁵ Recent studies have demonstrated the successful synthesis of antiferromagnetic materials by introducing Co atoms into Fe₃GaTe₂ [(Fe_0.8Co_0.2)₃GaTe₂].^123,236 By combining this 2D antiferromagnetic material with WSe₂ and Fe₃GaTe₂, researchers were able to form 2D antiferromagnetic/semiconducting/ferromagnetic MTJs which exhibited an impressive TMR value of up to 180% (Figure 9h) at low temperature and the TMR remains near room temperature (280 K).²³⁷ These findings are particularly exciting as they bring us close toward practical applications.^238,239 By continuously updating the 2D ferromagnetic, antiferromagnetic materials, and spacer, a significantly enhanced TMR in the spin valve at room temperature was achieved. Thus, the TMR is intricately linked to various factors such as the type of materials (FM and spacer), I_b, V_b, thickness of spacer, and annealing process (the comparison details are shown in Table 3).^{189,199,229,240} These factors indicate the complexity of optimizing these devices for high-performance applications. A significant advantage of using 2D materials like Fe₃GaTe₂ and Fe₃GeTe₂ in these devices is the potential to achieve high TMR values at room temperature. To maximize the TMR effect in 2D spin valve devices, researchers should focus on selecting appropriate 2D ferromagnetic and spacer materials that can maintain high TMR values at room temperature. Additionally, controlling the thickness of the spacer layer and optimizing the bias conditions will be crucial for achieving the highest possible TMR values. Exploring new 2D materials and their combinations for spin valve devices may lead to significant enhancements in TMR performance.

Schematic diagrams and TMR performances of spin valve devices utilizing 2D magnetic materials and heterojunctions. (a) The schematic diagram illustrates a nonlocal spin valve device constructed using Fe₃GaTe₂ flakes and few-layer graphene (upper panel). Nonlocal spin valve measurement was conducted with a 40 V gate voltage applied to graphene. The black (red) curve represents the up (down) sweep of the OOP magnetic field (lower panel). Reproduced with permission: Copyright 2023, American Chemical Society.²²⁶ (b) Extracted MR ratios of a Fe₃GeTe₂/InSe/Fe₃GeTe₂ device as a function of bias current at 10 K (upper panel). Reproduced with permission: Copyright 2021, Wiley-VCH GmbH.²²⁸ The MR and resistance area as a function of different thicknesses (d) of WSe₂ in Fe₃GeTe₂/WSe₂/Fe₃GeTe₂ device at 10 K (lower panel). Reprinted with permission under a Creative Commons CC BY License from ref (230). Copyright 2022 Springer Nature.²³⁰ (c) Extracted TMR ratios of a Fe₃GeTe₂/GaSe/Fe₃GeTe₂ device as a function of bias voltage measured at temperatures from10 to 190 K, respectively (upper panel). Reprinted with permission under a Creative Commons CC BY License from ref (229). Copyright 2023 Springer Nature.²²⁹ The variation curves of TMR with bias voltage of a Fe₃GeTe₂/MoSe₂/Fe₃GeTe₂ device at different temperatures. Positive (V_p) and negative (V_n) transition voltages obtained at different temperatures remain unchanged (lower panel). Reproduced with permission: Copyright 2024, American Institute of Physics.¹⁸⁸ (d) The schematic illustrates the Fe₃GaTe₂/WSe₂/Fe₃GaTe₂ spin valve device (upper panel). Room-temperature resistance and TMR are measured against magnetic field for this device under constant bias voltage V_b = 50 mV. Reprinted with permission under a Creative Commons CC BY License from ref (45). Copyright 2022 Institute of Physics.⁴⁵ (e) TMR for Fe₃GaTe₂/WSe₂/Fe₃GaTe₂ spin valve device is measured at the various positive DC current bias at 300 K. Reprinted with permission under a Creative Commons CC BY License from ref (231). Copyright 2023 Wiley-VCH GmbH.²³¹ (f) Resistance and TMR of a Fe₃GaTe₂/MoS₂/Fe₃GaTe₂ spin valve device are plotted against OOP magnetic fields at 300 K with a bias current set to be 1 μA. Arrows represent magnetization alignment directions in Fe₃GaTe₂ (upper panel). Reprinted with permission under a Creative Commons CC BY License from ref (233). Copyright 2023 Royal Society of Chemistry.²³³ The resistance of a Fe₃GaTe₂/MoSe₂/Fe₃GaTe₂ spin valve device as a function of OOP magnetic field is measured at a fixed bias current of 1 μA and temperature of 300 K. The red and blue horizontal arrows indicate the sweeping directions of the magnetic field, while the vertical arrows represent the OOP magnetization of the two Fe₃GaTe₂ electrodes, as depicted in the lower panel. Reprinted with permission under a Creative Commons CC BY License from ref (189). Copyright 2023 Royal Society of Chemistry.¹⁸⁹ (g) The schematic diagram of the Fe₃GeTe₂/NbSe₂/Fe₃GeTe₂ spin valve device and magneto-transport test setup is shown in the upper panel. The MR ratios of the spin valve device as a function of temperatures (lower panel). Reproduced with permission: Copyright 2023, American Chemical Society.²³⁵ (h) Schematic diagram of a (Fe_0.8Co_0.2)₃GaTe₂/WSe₂/Fe₃GaTe₂ device (upper panel). Resistance and TMR versus OOP magnetic field at 2 K with a bias voltage of 1 mV (lower panel). Reproduced with permission: Copyright 2024, Wiley-VCH GmbH.²³⁷

Table 3. Comparison of the TMR in MTJ and Spin Valve Devices.

		d (nm or L)
Heterogenous	Spacer	Bottom	Spacer	Top	TMR (%)	T_w (K)	I_b (nA)	V_b (mV)	T_a (K)	Ref.
VSi₂N₄/MoSi₂N₄/VSi₂N₄ (calculation)	MoSi₂N₄	1 L	1.1	1 L	10¹⁰	N/A	N/A	0	N/A	(205)
VSi₂N₄/MoSi₂N₄/VSi₂N₄ (calculation)	MoSi₂N₄	1 L	1.1	1 L	1000	N/A	N/A	100	N/A	(205)
1T–CrTe₂/Graphene–B/1T–CrTe₂(calculation)	Graphene doping with B	N/A	7 L	N/A	6962	300	N/A	N/A	N/A	(207)
VCl₃/CoBr₃/VCl₃ (calculation)	CoBr₃	N/A	N/A	N/A	4.5 × 10¹²	N/A	N/A	N/A	N/A	(214)
2H -MoSe₂/1T-VSe₂/2H-WSe₂/1T-VSe₂/2H -MoSe₂(calculation)	2H-WSe₂	N/A	N/A	N/A	1.7 × 10⁵	300	100	0.5	N/A	(215)
VSe₂/MoS₂/VSe₂ (calculation)	MoS₂	1 L	1 L	1 L	846	300	N/A	500	N/A	(216)
VSe₂/MoS₂/VSe₂ (calculation)	MoS₂	1 L	1 L	1 L	1277	100	N/A	500	N/A	(216)
Graphene/CrI₃/Graphene	CrI₃	N/A	2.7	N/A	19000	2	N/A	300	N/A	(36)
Graphene/CrSBr/Graphene	CrSBr	N/A	4L	N/A	700	2	N/A	N/A	N/A	(244)
Fe_0.25TaS₂/oxide/Fe_0.25TaS₂	Oxide	100	N/A	100	6.0	5	N/A	N/A	N/A	(193)
Fe/Al₂O₃/ Cr_0.335Sn_0.665Te	Al₂O₃	30	1–2	633	0.179	2	N/A	N/A	N/A	(232)
Fe/Al₂O₃/ Cr_0.335Sn_0.665Te	Al₂O₃	30	1–2	633	0.194	300	N/A	N/A	N/A	(232)
Fe₃GeTe₂/MoS₂/Fe₃GeTe₂	MoS₂	10.5	4.2	9.6	3.1	10	1000	N/A	120	(53)
Fe₃GeTe₂/InSe/Fe₃GeTe₂	InSe	6.1	6	12.6	41	10	100	N/A	120	(228)
Fe₃GeTe₂/GaSe/Fe₃GeTe₂	GaSe	8.0	8.2	12	192	10	N/A	10	120	(229)
Fe₃GeTe₂/MoSe₂/Fe₃GeTe₂	MoSe₂	10–20	5	10–20	42	10	N/A	10	N/A	(188)
Fe₃GeTe₂/h-BN/Fe₃GeTe₂	h-BN	20	0.9	7	160	4.2	500	N/A	N/A	(54)
Fe₃GaTe₂/WSe₂/Fe₃GaTe₂	WSe₂	11	7	17	85	300	N/A	50	N/A	(45)
Fe₃GaTe₂/WSe₂/Fe₃GaTe₂	WSe₂	11	7	17	164	10	N/A	50	N/A	(45)
Fe₃GaTe₂/WSe₂/Fe₃GaTe₂	WSe₂	18	7	40	340	2	1	N/A	N/A	(231)
Fe₃GaTe₂/WSe₂/Fe₃GaTe₂	WSe₂	18	7	40	50	300	1	N/A	N/A	(231)
Fe₃GaTe₂/MoS₂/Fe₃GaTe₂	MoS₂	9.5	4.5	16.8	0.31	300	10	N/A	N/A	(233)
					15.89	2.3
					11.97	10
Fe₃GaTe₂/MoSe₂/Fe₃GaTe₂	MoSe₂	9	6.1	9.4	3.28	300	10	N/A	N/A	(189)
Fe₃GaTe₂/MoSe₂/Fe₃GaTe₂	MoSe₂	9	6.1	9.4	37.7	2	1000	N/A	N/A	(189)
Fe₃GaTe₂/WS₂/Fe₃GaTe₂	WS₂	11	4.2	12	213	10	10	N/A	N/A	(234)
Fe₃GaTe₂/WS₂/Fe₃GaTe₂	WS₂	11	4.2	12	11	300	10	N/A	N/A	(234)

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2D MTJ Devices

Since 2015 the 2D TaS₂ crystal intercalated by Fe atoms (Fe_0.25TaS₂) has been utilized to fabricate bilayer MTJ (Figure 10a), exhibiting a TMR of 6.5% at 5 K (Figure 10b). This result can be attributed to the ferromagnetic ordering and strong PMA of Fe_0.25TaS₂, along with the presence of a Ta₂O₅ oxide barrier spacer in the interface between top and bottom exfoliated Fe_0.5TaS₂.¹⁹³ Additionally, 2D insulating h-BN is commonly employed as a favorable barrier layer for MTJ spacers.^16,204,241 Wang et al. employed h-BN as the spacer in the Fe₃GeTe₂/h-BN/Fe₃GeTe₂ MTJ device (Figure 10c), which exhibited a TMR of 160% at 4.2 K and spin polarizability of 0.66 (Figure 10d).⁵⁴ Zhou et al. also demonstrated the experimental realization of vertical Fe₃GeTe₂/h-BN/Fe₃GeTe₂ MTJ using an h-BN spacer (Figure 10e). By applying electrolyte gating, they were able to elevate the TMR ratio from 26% to 65%, providing promising control over magnetization configuration through electrical gating (Figure 10f). Notably, the magnetic fields at which the TMR switches for the MTJ can be modulated by electrical gating, providing a promising method to control the magnetization configuration of the MTJ.¹⁹⁹

Schematic of the 2D magnet-based MTJ devices. (a) Optical micrograph of a Fe_0.25TaS₂/Fe_0.25TaS₂ heterojunction, which contains a native oxide layer. (b) External magnetic field dependence of the four-terminal resistance measured from the heterojunction at 5 K (upper). The two-terminal resistances (middle and lower) measured on the right and left Fe_0.25TaS₂ flakes, respectively. The measurement is performed in two-terminal resistance geometry. The current used for the measurement is I = 2 μA in all the cases. (a,b) Reproduced with permission: Copyright 2024, American Institute of Physics.¹⁹³ (c) Schematic representation (left) and optical microscope image (right) of the Fe₃GeTe₂/h-BN/Fe₃GeTe₂ heterostructures. The Fe₃GeTe₂ crystals shows different shape and thickness (L1 and L2, ∼ 7 and 20 nm thick), respectively. (d) Tunneling resistance of a Fe₃GeTe₂/h-BN/Fe₃GeTe₂ heterostructure measured at 4.2 K with an OOP magnetic field (B). Very sharp resistance jumps are observed for B ≈ ± 0.7 T. The lower left panel zooms in on the magnetoresistance (upper), where sharp switches originate from magnetization reversal around ∼0.7 T, while the lower right panel zooms in on the magnetoresistance (upper), where sharp switches originate from magnetization reversal around +0.7 T. The blue and green arrows indicate the magnetization orientation at different magnetic fields in L1 and L2 layers determined from anomalous Hall resistance measurements. The “jumps” in tunneling magnetoresistance result from reversible changes in electrode magnetizations; minimum resistance occurs when their orientations are parallel, and maximum resistance occurs when they are antiparallel as expected for a MTJ. (c,d) Reproduced with permission: Copyright 2018, American Chemical Society.⁵⁴ (e) Optical image (upper) and schematic diagram (lower) of an ion-gating MTJ (Fe₃GeTe₂/h-BN/Fe₃GeTe₂). The dotted red and blue lines outline bilateral edges of Fe₃GeTe₂ flakes, while dotted black line of the h-BN spacing layer, and the scale bar represents 20 μm. The bottom electrodes are patterned to apply an injection current (I+ and I-) and measure the four-terminal magnetoresistance of the vertical junction (V₁) and anomalous Hall resistance of the Fe₃GeTe₂ flakes (V₂ and V₃). The orange rectangles represent Au electrodes in a schematic diagram, while the light blue shape represents the LiClO₄/PEO electrolyte. (f) TMR of the Fe₃GeTe₂/h-BN/Fe₃GeTe₂ MTJ under 0 V (open points) and 2.8 V (solid points) gate bias at temperatures ranging from 2 to 20 K. The scale bar indicates a range of 100%, which corresponds to the offset. Panel (e) and (f) were reprinted with permission under a Creative Commons CC BY License from ref (199). Copyright 2023 Wiley-VCH GmbH.¹⁹⁹

Antiferromagnetic materials have experienced rapid development in recent years for applications in spintronic devices. From magnetic tunnel junctions based on 2D antiferromagnetic CrI₃, demonstrating 19,000% TMR at low temperatures, to the application of novel 2D antiferromagnetic CrSBr.^35,36,68 Chen et al. reported a twisting strategy for constructing fully antiferromagnetic tunnel junctions at the atomic limit. By twisting a CrSBr bilayer (t-CrSBr), a TMR of over 700% was observed at zero magnetic field, with the entire t-CrSBr layer serving as the tunneling barrier. This effect arises from twisting two CrSBr monolayers, where the TMR results from coherent tunneling accumulated on the individual CrSBr monolayers. The dependence of the TMR on the twist angle is calculated based on the parallel momentum-dependent decay of electrons on the twisted monolayers. Interestingly, the temperature dependence of TMR was found to be much weaker for twisted junctions compared to nontwisted junctions, making twisted junctions more promising for applications.²³⁶ These findings demonstrate the feasibility of applying 2D antiferromagnetic materials in nonvolatile magnetic information storage and modulating magnetic tunneling junctions using moiré engineering of torsion angles.

The results of a series of studies on structure, thickness, work temperature (T_w), anneal temperature (T_a), I_b, and V_b associated TMR are summarized in Table 3. These findings offer insights into achieving high TMR values in 2D spin valve and MTJ devices, facilitating the modulation of spin transport within these systems and highlighting potential applications such as electrically controlled nonvolatile memories.^5,13,37,55 Although the exploration of 2D MTJ is still in its early stages, it is evident that it offers advantages over traditional MTJs due to its inherent vdWs structure and abundant varieties of 2D magnetic materials.^242,243 However, challenges such as the low T_C and TMR of most materials and MTJs still need to be addressed in a serious manner in the future.^13,139

Application Scenarios of 2D SOT and MTJ (or Spin Valve) Devices

A promising future direction for SOT and MTJ (or spin valve) applications involves the use of high and low resistance state switching driven by a field-free and low current density SOT in MTJ (or spin valve) devices.^{5,55,245−247} The integration of SOT–MTJ can find practical implementation in two specific circuits, namely MRAM and logic circuitry, which are the primary applications of spintronics in the field of information. The high and low resistance configurations of the SOT-MTJ devices can serve as both binary data “0” and “1” signals for MRAM, as well as logic levels.^248,249 The advantages offered by 2D FMs can be fully harnessed in the fabrication of these integrated circuits, enabling the realization of all-2D MRAMs and logic circuits.^250−254

2D MRAMs

MRAM is a type of solid-state storage circuit that utilizes magnetoresistive device to store data as stable magnetic states. Data are read by measuring the resistance of these devices to determine their magnetic states, making it a nonvolatile memory device.^7,255,256 The MTJ serves as one core memory cell in MRAM technology. Therefore, the development of 2D materials and their vdWs MTJ heterojunctions provides a pathway for advancing the development of all-2D MRAM.^192,257 Liao et al. developed a room-temperature, all-2D magnetic memory based on the WTe₂/Fe₃GaTe₂/h-BN/Fe₃GaTe₂ heterostructure (as depicted in Figure 11a), enabling all-electric data reading and writing. The data reading process relies on the TMR of Fe₃GaTe₂/h-BN/Fe₃GaTe₂, and data writing utilizes the orbit transfer torque (OTT) effect induced by the orbital Hall effect-induced polarization current in WTe₂ to exert a torque on Fe₃GaTe₂.²⁵⁸ This study seamlessly integrates the construction of 2D OTT–MTJ heterojunctions with spin–flow manipulation for efficient magnetization switching of resistance states using 2D magnetic moments alone, offering promising prospects for ultra–low–power and all–electric operation memory and logic functional devices.³⁷

Application scenario of 2D MTJ. (a) Schematic diagram illustrating the OTT–MTJ structure composed of all-2D WTe₂/Fe₃GaTe₂/BN/Fe₃GaTe₂ heterostructures. Reprinted with permission under a Creative Commons CC BY License from ref (258). Copyright 2023 Elsevier Besloten Vennootschap (B. V.).²⁵⁸ (b) Stack architecture of SOT–MTJs consisting of Au/Capping layer/IrMn/Co/W/CoFeB/MgO/CoFeB/W layers. Reproduced with permission: Copyright 2024, American Chemical Society.²⁵⁰ (c) A 2D SOT–MTJ model incorporating a 2D SOT layer to enable energy-efficient magnetization switching. Reproduced with permission: Copyright 2019, Springer Nature.⁵⁵ (d) The procedure for generating random numbers following a specified probability distribution function. Reproduced with permission. (e) A proposal circuitry implementation for cascading sampling of the Bernoulli bits A, B, C, and D. Panel (d) and (e) were reprinted with permission under a Creative Commons CC BY License from ref (252). Copyright 2024 Wiley-VCH GmbH.²⁵² (f) Diagram depicting the structure of a 3T2M memory cell. (g) Read/write circuit architecture designed specifically for accessing data in a 3T2M Fe₃GeTe₂–MTJ memory cell. The schematic diagram shows cases the sub-circuit structure responsible for logic operations using an asymmetric pre-charge sense amplifier configuration. (h) The Monte Carlo simulation results in the distribution of sensing voltage for storing diverse data in the memory cell. Panel (f)-(h) were reprinted with permission under a Creative Commons CC BY License from ref (269). Copyright 2022 Multidisciplinary Digital Publishing Institute.²⁶⁹

Regarding current spintronic research, STT and SOT are two promising methods for electrically switching MTJs using charge current in the industry.²⁴⁵ The STT switching has not been reported in 2D magnets, Yang et al. suggest that insufficient studies on exchange bias and the requirement of relatively large currents for achieving magnetization switching may be two obstacles.⁷³ The SOT switching has been successfully demonstrated in Fe₃GeTe₂, Cr₂Ge₂Te₆, and Fe₃GaTe₂ under field-free or a small IP in heterostructures with heavy metal or topological materials.^{33,44,60,172,174} This technique can be further utilized to fabricate memory devices with MTJ or spin valve devices known as SOT-driven MRAM (SOT–MRAM; was shown in Figure 11b).^250,259 Additionally, an all-2D SOT–MRAM can be achieved by replacing heavy metals with 2D materials with large SOC,^55,73 as depicted in Figure 11c. We believe that further solid work is required to improve the efficiency and stability of SOT modulation. Future studies must reduce the current density required for SOT switching while optimizing the structure energy bands and properties of 2D magnetic materials to enhance tunneling efficiency and TMR of 2D MTJs. Furthermore, utilizing these advancements will allow for an extensive investigation into the controllability of heterojunction devices consisting of 2D magnetic materials within SOT–MRAM systems to ensure reliable information storage devices.^7,14,37

2D Logic Circuits

Existing structural logic circuits primarily consist of complementary metal oxide semiconductor (CMOS) devices. However, as the characteristic size of CMOS devices approaches the quantum limit, the quantum effect and short-channel effect become increasingly prominent.^260-262 The role of electron spin becomes significant and cannot be disregarded, which can result in high power consumption and slow performance enhancement for existing logic devices. Additionally, device stability deteriorates over time.^263,264 The current CMOS logic circuits lack data non–volatility and require constant power supply to maintain data integrity, leading to additional power consumption. To tackle these challenges, researchers have proposed a range of novel devices and circuit structures, among which MTJ (or spin valve) devices stand out as one of the most prominent options. These devices can potentially replace the voltage representation employed in traditional CMOS circuits by leveraging two distinct resistive states to signify “0” and “1” logic states.^7,9,242,265 The resistive state of MTJ remains unaffected during power down events, rendering it inherently non-volatile. Furthermore, MTJs offer several advantages including low read/write currents, fast read/write speeds, unlimited erase/write cycles capability, and compact size due to their compatibility with CMOS processes. By leveraging this compatibility with CMOS technology, effective in-store computation can be achieved to address the memory wall issue associated with Von Neumann architecture.^37,266,267

Zhang et al. employed SOT–MTJ devices with high barriers as potential candidates for true random number generators (TRNGs) to generate Bernoulli bits and various probability distribution functions, such as Gaussian, uniform exponential, chi square, and even user-defined distributions. They proposed a schematic design of a 4-bit TRNG comprising four SOT–MTJs (A, B, C, and D) (Figure 11d). This TRNG can produce a total of 16 quasi-continuous outcomes ranging from 0 to 15. The resolution can be further enhanced by increasing the number of bits. The aforementioned TRNG can be implemented using these four SOT–MTJs (A, B, C, and D) along with corresponding variable resistors (Figure 11e), which store the resistance states of the preceding SOT–MTJ nodes. The blue labels on the transistors indicate their conduction sequence. In a continuous five-step operation, the transistors are sequentially turned on while remaining off at all other times. For the second to fourth MTJs, each write voltage is influenced by the state of all previous MTJs. This circuit design facilitates the transfer of conditional probabilities described in Figure 11e. The utilization of SOT–MTJ outputs presented in this technique offers valuable insights for Boolean logic circuits; however, how 2D materials can be utilized for such SOT–MTJ structures must be explored.²⁶⁸

Lin et al.²⁶⁹ proposed an all-electrical control technique for compact SOT–MTJ Boolean logic circuits, which utilizes a structure consisting of three transistors (T1, T2, and T3) and two Fe₃GeTe₂–MTJs (MTJ0 and MTJ1; Fe₃GeTe₂/MgO/Fe₃GeTe₂). As depicted in Figure 11f, the word line (WL) is connected to the gates of transistor T1 and T3 to regulate the memory cell’s read/write process and the on/off state of transistors during operation. Transistor T2 is linked to the reading and writing control line (R/W Ctrl), enabling separate paths for writing and reading operations on MTJ0 and MTJ1, respectively. Both top Fe₃GeTe₂ layers are connected to the source line (SL), serving as pathways for reading data and storing calculation results. The key operations involved in the memory cell along with their corresponding voltage bias states are illustrated in Figure 11g. An initialization step is required when the memory cells are first utilized. During this step, a current passes through two MTJs in opposite directions, initializing them to complementary states using the STT effect. In this context, the low- and high-resistance states of the MTJ are denoted as “0” and “1”, respectively. Consequently, if the initialization current aligns the magnetization of the free layer (FL) of MTJ0 parallel to the reference layer (RL), MTJ0 represents “0” while MTJ1 represents “1”. To fully leverage the advantages offered by 2D Fe₃GeTe₂, they propose a storage cell based on 2D MTJ heterojunctions consisting of three transistors and two MTJs (3T2M). The sensing margin for reading “1” and “0” in 3T2M is increased by 201.4% and 276%, respectively, compared with traditional 1T1M²⁷⁰ and 2T1M²⁷¹ cells. Moreover, by combining a basic cell structure with an asymmetric pre-charge sense amplifier, they achieve high-speed AND/NAND and OR/NAND Boolean operations with low power consumption. They have also developed core cells for 3T2M MTJs combined with symmetric pre-charge sense amplifiers to enable basic AND/NAND and OR/NAND Boolean operations (Figure 11h), as well as non-volatile in-memory computing circuits that offer high speed and low power consumption. The logic operation circuit depicted in Figure 11h comprises a memory array, voltage reading circuitry, and an asymmetric sense amplifier.

With extended and generalized 2D materials including 2D antiferromagnets with the joint effect of Dzyaloshinskii–Moriya interaction and spin current for deterministic and fast switching, the proposed 3T2M MTJs logic cell structure is promising for applications in the fields of compact spintronic devices and non-volatile in-memory computing.^46,272,273

Conclusions and Outlook

The development of 2D magnetic materials and their applications in spintronic devices face several challenges, including low T_C, the complexity of heterostructure construction, high power consumption, unclear interfacial coupling mechanisms, and an incomplete understanding of multifield modulation. These challenges have hindered the progress in the development and optimization of room-temperature 2D magnetic heterojunction devices. For the research and application of 2D magnetic materials and their heterojunctions (Figure 12), we propose further investigations in the following areas:

(1)
Exploring 2D magnetic materials with high T_C: Currently, only a few 2D intrinsic magnetic materials have T_C above room temperature. Although some groups have established databases of 2D magnetic materials, the synthesis and environmental stability of related materials still present significant challenges.^274−276 Therefore, it is crucial to continue exploring 2D magnetic materials with high T_C. High-throughput data analysis, including machine learning methods, mean-field approximation, Monte Carlo methods, and phonon spectrum calculations, combined with artificial intelligence techniques, can help enhance the efficiency of material screening and optimize experimental conditions. This will aid in the development of air-stabilized 2D magnetic materials with T_C significantly higher than room temperature and strong PMA, ultimately achieving 2D materials with T_C above 450 K and wafer-scale sizes.
(2)
Addressing oxidation of 2D materials: A significant issue with known 2D magnetic materials is that their surfaces are prone to oxidation, thereby complicating the construction of high-quality heterojunction interfaces. For example, Fe₃GeTe₂ nanosheets are oxidized and exhibit antiferromagnetism, which is beneficial for studying exchange bias but hinders spin transport, limiting their applicability in spintronic devices.^277−279 Surface oxidation can be avoided by preparing 2D magnetic materials and heterojunctions in a full-vacuum or inert-gas-protected environment.^{127,185,244,280} In addition, encapsulation of 2D magnetic materials by atomic layer deposition of perylenetetracarboxylic dianhydride has been experimentally demonstrated to both maintain magnetism and prevent degradation.²⁸¹ These measures will help maintain high-quality heterojunction interfaces and promote significant spin transport.
(3)
Reducing power consumption: Theoretical studies indicate that due to the smaller M_S of 2D magnetic materials, power consumption in 2D SOT devices can be lower.^93,130,132 However, there is still potential to experimentally reduce the critical current density further. It is essential to reduce power consumption in 2D magnetic spintronic devices and understand the key factors influencing T_C, magnetic anisotropy, and the underlying physical mechanisms governing magnetic ordering.
(4)
Exploiting novel physical effects: Previous studies have shown that novel physical effects, such as moiré engineering and ion implantation engineering, can positively impact SOT and MTJ devices.^182,199 However, these applications are still limited, and further detailed investigations are needed to explore interface and bulk effects, such as strong SOC, electron correlation, and field-effect phenomena, in future heterojunctions constructed from 2D magnetic materials and heavy metals (or 2D topological materials and semiconductors). The goal is to establish a theoretical model for interfacial coupling in 2D magnetic heterostructures to achieve ultrahigh spin injection efficiency.
(5)
Enhancing external field modulation: 2D magnetic materials are highly susceptible to external field modulation, such as interfacial coupling and pressure-field modulation of T_C.^121,122 However, these modulation methods have been underexplored in 2D spintronic devices. Comprehensive studies on the external field control of magnetic properties are necessary to improve the efficiency and stability of modulation. Additionally, structure optimization, energy band engineering, and property manipulation of 2D magnetic materials can enhance the switching efficiency of SOT devices and the TMR of MTJs.
(6)
Exploratory research on SOT-MTJs for information storage and logic devices: Research on SOT-MTJ devices for information storage and logic operations based on 2D magnetic heterojunctions is promising. Achieving TMR through 2D magnetization switching driven by SOT technology in an all-2D magnetic heterojunction (SOT-MTJ) and using the output of the SOT-driven 2D magnetization switching as a signal source for Boolean logic operations is an exciting frontier. Future research on novel SOT-MTJ heterojunction devices employing 2D materials is crucial for advancing high-performance, low-energy information memory and logic devices that can operate reliably at room temperature.

RoadMap of 2D magnets, spintronic devices, and application circuits.

Acknowledgments

This work was supported by the National Key Research and Development Program of China (No. 2022YFA1204000), the National Natural Science Foundation of China (No. 52201292, U24A6002, 52302192, 52202155, 52271240, U23A20551), the Key Research and Development Program of Tianjin (No. 24YFXTHZ00150), the Open Project Program of the Hebei Technology Innovation Center of Phase Change Thermal Management of Data Center (No. SKF-2022-06), and the Young Top-Notch Talent program of Science and Technology Project of Hebei Education Department (No. BJK2023082). We acknowledge the European Research Executive Agency (101079184-FUNLAYERS) and Xinjiang Tianchi Talents Project (2023000043). We also thank the financial support of the Young Scientific and Technological Talents in Tianjin (Level Three) and the Tianjin Municipal Science and Technology Bureau.

Glossary

Vocabulary

2D Magnetic Materials: these materials typically denote magnetic substances with a van der Waals structure. Such materials usually have only a few atomic layers in the thickness direction and exhibit a robust magnetic response. In a broad sense, a small number of nonlayered materials can be classified as two-dimensional magnetic materials when they remain ultrathin. The magnetic properties can manifest the types of ferromagnetism, antiferromagnetism, and ferrimagnetism, etc.
Spin–Orbit Torque (SOT): a torque generated through spin–orbit coupling effects, which can manipulate the magnetization direction of magnetic materials, making it a key mechanism for energy-efficient magnetic memory devices.
Magnetic Tunnel Junction (MTJ): a structure consisting of two ferromagnetic layers separated by a thin insulating layer, where the electrical resistance varies with the relative magnetization alignment of the magnetic layers, enabling data storage and readout.
Logic Circuit: a circuit composed of logic gates (e.g., AND, OR, NOT) designed to perform Boolean logic operations and process digital signals, forming the foundation of digital computing systems.
Magnetic Random-Access Memory (MRAM): a nonvolatile memory technology that stores data using the magnetization direction of magnetic materials, offering advantages such as high-speed read/write operations and low power consumption.

Author Contributions

Z.J., M.Z., and Q.C. contributed equally to this work. Z.W. and Y.J. initiated the project. All authors joined in the discussion and commented on the manuscript.

The authors declare no competing financial interest.

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PERMALINK

Spintronic Devices upon 2D Magnetic Materials and Heterojunctions

Zhiyan Jia

Mengfan Zhao

Qian Chen

Yuxin Tian

Lixuan Liu

Fang Zhang

Delin Zhang

Yue Ji

Bruno Camargo

Kun Ye

Rong Sun

Zhongchang Wang

Yong Jiang

Abstract

Introduction

Figure 1.

2D Magnetic Materials

Figure 2.

2D Layered Magnetic Materials

2D Nonlayered Magnetic Materials

Figure 3.

Table 1. Structural, Electrical, and Magnetic Properties of Some 2D Layered and Nonlayered Magnetsa.

Advantages of 2D Magnetic Materials

Figure 4.

SOT Devices Based on 2D Magnetic Materials

Mechanisms and Testing Techniques of SOT Heterojunctions

Microscopic Origin of SOT

Figure 5.

Magnetization Dynamics of SOT

SOT Measurement Methods

Construction of 2D SOT Heterojunctions

Investigation of 2D SOT Heterojunctions

Figure 6.

Table 2. Comparison of Magnetization Switching Conditions of 2D SOT Heterojunctions.

Magnetoresistance (MR) Effect Based on 2D Magnetic Materials

Figure 7.

Calculation on MTJ Performances

Figure 8.

2D Spin Valve Devices

Figure 9.

Table 3. Comparison of the TMR in MTJ and Spin Valve Devices.

2D MTJ Devices

Figure 10.

Application Scenarios of 2D SOT and MTJ (or Spin Valve) Devices

2D MRAMs

Figure 11.

2D Logic Circuits

Conclusions and Outlook

Figure 12.

Acknowledgments

Glossary

Vocabulary

Author Contributions

References

ACTIONS

PERMALINK

RESOURCES

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Links to NCBI Databases

Table 1. Structural, Electrical, and Magnetic Properties of Some 2D Layered and Nonlayered Magnets^a.