A Room-Temperature Spin-Valve with van der Waals Ferromagnet Fe5 GeTe2 /Graphene Heterostructure

Recent Advances in Two-Dimensional Ferromagnetic Materials-Based van der Waals Heterostructures

Two-dimensional (2D) ferromagnetic materials are subjects of intense research owing to their intriguing physicochemical properties, which hold great potential for fundamental research and spintronic applications. Specifically, 2D van der Waals (vdW) ferromagnetic materials retain both structural integrity and chemical stability even at the monolayer level. Moreover, due to their atomic thickness, these materials can be easily manipulated by stacking them with other 2D vdW ferroic and nonferroic materials, enabling precise control over their physical properties and expanding their functional applications. Consequently, 2D vdW ferromagnetic materials-based heterostructures offer a platform to tailor magnetic properties and explore advanced spintronic devices. This review aims to provide an overview of recent developments in emerging 2D vdW ferromagnetic materials-based heterostructures and devices. The fabrication approaches for 2D ferromagnetic vdW heterostructures are primarily summarized, followed by a review of two categories of heterostructures: ferromagnetic/ferroic and ferromagnetic/nonferroic vdW heterostructures. Subsequently, the progress made in modulating magnetic properties and emergence of various phenomena in these heterostructures is highlighted. Furthermore, the applications of such heterostructures in spintronic devices are discussed along with their future perspectives and potential directions in this exciting field.

Efficient spin filtering through Fe4GeTe2-based van der Waals heterostructures

Utilizing ab initio simulations, we study the spin-dependent electronic transport characteristics within Fe4GeTe2-based van der Waals heterostructures. The electronic density of states for both free-standing and device-configured Fe4GeTe2 (F4GT) confirms its ferromagnetic metallic nature and reveals a weak interface interaction between F4GT and PtTe2 electrodes, enabling efficient spin filtering. The ballistic transport through a double-layer F4GT with a ferromagnetic configuration sandwiched between two PtTe2 electrodes is predicted to exhibit an impressive spin polarization of 97% with spin-up electrons exhibiting higher transmission probability than spin-down electrons. Moreover, we investigate the spin transport properties of Fe4GeTe2/GaTe/Fe4GeTe2 van der Waals heterostructures sandwiched between PtTe2 electrodes to explore their potential as magnetic tunnel junctions in spintronic devices. The inclusion of monolayer GaTe as a 2D semiconducting spacer between F4GT layers results in a tunnel magnetoresistance of 487% at a low bias and decreases with increasing bias voltage. Overall, our findings underscore the potential of F4GT/GaTe/F4GT heterostructures in advancing spintronic devices based on van der Waals materials.

Photoresponsive Two-Dimensional Magnetic Junctions for Reconfigurable In-Memory Sensing

Magnetic tunneling junctions (MTJs) lie in the core of magnetic random access memory, holding promise in integrating memory and computing to reduce hardware complexity, transition latency, and power consumption. However, traditional MTJs are insensitive to light, limiting their functionality in in-memory sensing─a crucial component for machine vision systems in artificial intelligence applications. Herein, the convergence of magnetic memory with optical sensing capabilities is achieved in the all-two-dimensional (2D) magnetic junction Fe3GaTe2/WSe2/Fe3GaTe2, which combines 2D magnetism and optoelectronic properties. The clean intrinsic band gap and prominent photoresponse of interlayer WSe2 endow the tunneling barrier with optical tunability. The on-off states of junctions and the magnetoresistance can be flexibly controlled by the intensity of the optical signal at room temperature. Based on the optical-tunable magnetoresistance in all-2D magnetic junctions, a machine vision system with the architecture of in-memory sensing and computing is constructed, which possesses high performance in image recognition. Our work exhibits the advantages of 2D magneto-electronic devices and extends the application scenarios of magnetic memory devices in artificial intelligence.

Twist-assisted all-antiferromagnetic tunnel junction in the atomic limit

Antiferromagnetic spintronics1,2 shows great potential for high-density and ultrafast information devices. Magnetic tunnel junctions (MTJs), a key spintronic memory component that are typically formed from ferromagnetic materials, have seen rapid developments very recently using antiferromagnetic materials3,4. Here we demonstrate a twisting strategy for constructing all-antiferromagnetic tunnel junctions down to the atomic limit. By twisting two bilayers of CrSBr, a 2D antiferromagnet (AFM), a more than 700% nonvolatile tunnelling magnetoresistance (TMR) ratio is shown at zero field (ZF) with the entire twisted stack acting as the tunnel barrier. This is determined by twisting two CrSBr monolayers for which the TMR is shown to be derived from accumulative coherent tunnelling across the individual CrSBr monolayers. The dependence of the TMR on the twist angle is calculated from the electron-parallel momentum-dependent decay across the twisted monolayers. This is in excellent agreement with our experiments that consider twist angles that vary from 0° to 90°. Moreover, we also find that the temperature dependence of the TMR is, surprisingly, much weaker for the twisted as compared with the untwisted junctions, making the twisted junctions even more attractive for applications. Our work shows that it is possible to push nonvolatile magnetic information storage to the atomically thin limit.

Van der Waals Magnetic Electrode Transfer for Two-Dimensional Spintronic Devices

Two-dimensional (2D) materials are promising candidates for spintronic applications. Maintaining their atomically smooth interfaces during integration of ferromagnetic (FM) electrodes is crucial since conventional metal deposition tends to induce defects at the interfaces. Meanwhile, the difficulties in picking up FM metals with strong adhesion and in achieving conductance match between FM electrodes and spin transport channels make it challenging to fabricate high-quality 2D spintronic devices using metal transfer techniques. Here, we report a solvent-free magnetic electrode transfer technique that employs a graphene layer to assist in the transfer of FM metals. It also serves as part of the FM electrode after transfer for optimizing spin injection, which enables the realization of spin valves with excellent performance based on various 2D materials. In addition to two-terminal devices, we demonstrate that the technique is applicable for four-terminal spin valves with nonlocal geometry. Our results provide a promising future of realizing 2D spintronic applications using the developed magnetic electrode transfer technique.

Bulk ferromagnetism in cleavable van der Waals telluride NbFeTe2

A van der Waals telluride, NbFeTe2, has been synthesized using chemical vapor transport reactions. The optimized synthetic conditions yield high-quality single crystals with a novel monoclinic crystal structure. Monoclinic NbFeTe2 demonstrates a (100) cleavage plane, bulk ferromagnetism below 87 K, and a metallic ground state-the necessary prerequisites for needed spintronics technologies.

Enhancing the electronic and optical properties of the metal/semiconductor NbS2/BSe nanoheterostructure towards advanced electronics

Metal-semiconductor (M-S) contacts play a vital role in advanced applications, serving as crucial components in ultracompact devices and exerting a significant impact on overall device performance. Here, in this work, we design a M-S nanoheterostructure between a metallic NbS2 monolayer and a semiconducting BSe monolayer using first-principles prediction. The stability of such an M-S nanoheterostructure is verified and its electronic and optical properties are also considered. Our results indicate that the NbS2/BSe nanoheterostructure is structurally, mechanically and thermally stable. The formation of the NbS2/BSe heterostructure leads to the generation of a Schottky contact with the Schottky barrier ranging from 0.36 to 0.51 eV, depending on the stacking configurations. In addition, the optical absorption coefficient of the NbS2/BSe heterostructure can reach up to 5 × 105 cm-1 at a photon energy of about 5 eV, which is still greater than that in the constituent NbS2 and BSe monolayers. This finding suggests that the formation of the M-S NbS2/BSe heterostructure gives rise to an enhancement in the optical absorption of both NbS2 and BSe monolayers. Notably, the tunneling probability and the contact tunneling-specific resistivity at the interface of the NbS2/BSe heterostructure are low, indicating its applicability in emerging nanoelectronic devices, such as Schottky diodes and field-effect transistors. Our findings offer valuable insights for the practical utilization of electronic devices based on the NbS2/BSe heterostructure.

Strong In-Plane Magnetization and Spin Polarization in (Co0.15Fe0.85)5GeTe2/Graphene van der Waals Heterostructure Spin-Valve at Room Temperature

Van der Waals (vdW) magnets are promising, because of their tunable magnetic properties with doping or alloy composition, where the strength of magnetic interactions, their symmetry, and magnetic anisotropy can be tuned according to the desired application. However, so far, most of the vdW magnet-based spintronic devices have been limited to cryogenic temperatures with magnetic anisotropies favoring out-of-plane or canted orientation of the magnetization. Here, we report beyond room-temperature lateral spin-valve devices with strong in-plane magnetization and spin polarization of the vdW ferromagnet (Co0.15Fe0.85)5GeTe2 (CFGT) in heterostructures with graphene. Density functional theory (DFT) calculations show that the magnitude of the anisotropy depends on the Co concentration and is caused by the substitution of Co in the outermost Fe layer. Magnetization measurements reveal the above room-temperature ferromagnetism in CFGT and clear remanence at room temperature. Heterostructures consisting of CFGT nanolayers and graphene were used to experimentally realize basic building blocks for spin valve devices, such as efficient spin injection and detection. Further analysis of spin transport and Hanle spin precession measurements reveals a strong in-plane magnetization with negative spin polarization at the interface with graphene, which is supported by the calculated spin-polarized density of states of CFGT. The in-plane magnetization of CFGT at room temperature proves its usefulness in graphene lateral spin-valve devices, thus revealing its potential application in spintronic technologies.

Room-Temperature van der Waals Ferromagnet Switching by Spin-Orbit Torques

The emerging wide varieties of the van der Waals (vdW) magnets with atomically thin and smooth interfaces hold great promise for next-generation spintronic devices. However, due to the lower Curie temperature of the vdW ferromagnets than room temperature, electrically manipulating its magnetization at room temperature has not been realized. In this work, it is demonstrated that the perpendicular magnetization of the vdW ferromagnet Fe3 GaTe2 can be effectively switched at room temperature in the Fe3 GaTe2 /Pt bilayer by spin-orbit torques (SOTs) with a relatively low current density of 1.3 × 107 A cm-2 . Moreover, the high SOT efficiency of ξDL ≈ 0.28 is quantitatively determined by harmonic measurements, which is higher than those in Pt-based heavy metal/conventional ferromagnet devices. The findings of room-temperature vdW ferromagnet switching by SOTs provide a significant basis for the development of vdW-ferromagnet-based spintronic applications.

Surface Stability and Exfoliability of Non-van der Waals Magnetic Chromium Tellurides

Exfoliation of two-dimensional (2D) magnetic materials from non-van der Waals (non-vdW) materials has attracted increasing attention because it provides a great platform for the construction of 2D magnetic materials. For non-vdW magnetic chromium tellurides with high Curie temperatures, their few-layer samples show promising applications in the field of spintronics. However, there is still no consensus on whether the surface structures of few-layer chromium tellurides should be terminated by Cr or Te atoms. By calculating the surface and exfoliation energy, we find that which structure is more stable depends greatly on the value of the chemical potential of Te atoms, and the few-layer sample with a Cr-terminated surface is easier to exfoliate than that with both Te-terminated surfaces. Finally, we propose that different exfoliated structures can be identified by using the atomic number ratio of Cr to Te and the average magnetic moment of Cr atoms in few-layer samples.

Progress and Prospects in Metallic FexGeTe2 (3 ≤ x ≤ 7) Ferromagnets

Thermal fluctuations in two-dimensional (2D) isotropy systems at non-zero finite temperatures can destroy the long-range (LR) magnetic order due to the mechanisms addressed in the Mermin-Wanger theory. However, the magnetic anisotropy related to spin-orbit coupling (SOC) may stabilize magnetic order in 2D systems. Very recently, 2D FexGeTe2 (3 ≤ x ≤ 7) with a high Curie temperature (TC) has not only undergone significant developments in terms of synthetic methods and the control of ferromagnetism (FM), but is also being actively explored for applications in various devices. In this review, we introduce six experimental methods, ten ferromagnetic modulation strategies, and four spintronic devices for 2D FexGeTe2 materials. In summary, we outline the challenges and potential research directions in this field.

Large-Area Synthesis of Ferromagnetic Fe5- x GeTe2 /Graphene van der Waals Heterostructures with Curie Temperature above Room Temperature

Van der Waals (vdW) heterostructures combining layered ferromagnets and other 2D crystals are promising building blocks for the realization of ultracompact devices with integrated magnetic, electronic, and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing for realizing highly uniform heterostructures with well-defined interfaces between different 2D-layered materials. It is also required that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, it is demonstrated that the large-area growth of Fe5- x GeTe2 /graphene heterostructures is achieved by vdW epitaxy of Fe5- x GeTe2 on epitaxial graphene. Structural characterization confirms the realization of a continuous vdW heterostructure film with a sharp interface between Fe5- x GeTe2 and graphene. Magnetic and transport studies reveal that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond nonscalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.

2D Ferroic Materials for Nonvolatile Memory Applications

The emerging nonvolatile memory technologies based on ferroic materials are promising for producing high-speed, low-power, and high-density memory in the field of integrated circuits. Long-range ferroic orders observed in 2D materials have triggered extensive research interest in 2D magnets, 2D ferroelectrics, 2D multiferroics, and their device applications. Devices based on 2D ferroic materials and heterostructures with an atomically smooth interface and ultrathin thickness have exhibited impressive properties and significant potential for developing advanced nonvolatile memory. In this context, a systematic review of emergent 2D ferroic materials is conducted here, emphasizing their recent research on nonvolatile memory applications, with a view to proposing brighter prospects for 2D magnetic materials, 2D ferroelectric materials, 2D multiferroic materials, and their relevant devices.