Recent Advances in Two-Dimensional Spintronics

Room‐Temperature Antisymmetric Magnetoresistance in van der Waals Ferromagnet Fe3GaTe2 Nanosheets

Van der Waals (vdW) ferromagnetic materials have emerged as a promising platform for the development of 2D spintronic devices. However, studies to date are restricted to vdW ferromagnetic materials with low Curie temperature (Tc) and small magnetic anisotropy. Here, a chemical vapor transport method is developed to synthesize a high-quality room-temperature ferromagnet, Fe3GaTe2 (c-Fe3GaTe2), which boasts a high Tc = 356 K and large perpendicular magnetic anisotropy. Due to the planar symmetry breaking, an unconventional room-temperature antisymmetric magnetoresistance (MR) is first observed in c-Fe3GaTe2 devices with step features, manifesting as three distinctive states of high, intermediate, and low resistance with the sweeping magnetic field. Moreover, the modulation of the antisymmetric MR is demonstrated by controlling the height of the surface steps. This work provides new routes to achieve magnetic random storage and logic devices by utilizing the room-temperature thickness-controlled antisymmetric MR and further design room-temperature 2D spintronic devices based on the vdW ferromagnet c-Fe3GaTe2.

Transition Metal Dichalcogenides: Making Atomic-Level Magnetism Tunable with Light at Room Temperature

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D-DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M-doped TX2 , where M = V, Fe, and Cr; T = W, Mo; X = S, Se, and Te), may lead to innovative applications in spintronics, spin-caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D-TMD DMSs and heterostructures. By combining magneto-LC resonance (MLCR) experiments with density functional theory (DFT) calculations, we show that the magnetization can be enhanced using light in V-doped TMD monolayers (e.g., V-WS2 , V-WSe2 ). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D-TMD heterostructures (VSe2 /WS2 , VSe2 /MoS2 ), the significance of proximity, charge-transfer, and confinement effects in amplifying light-mediated magnetism is demonstrated. We attributed this to photon absorption at the TMD layer that generates electron-hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D-TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next-generation magneto-optic nanodevices.

Spin-Orbit Torques and Magnetization Switching in (Bi,Sb)2Te3/Fe3GeTe2 Heterostructures Grown by Molecular Beam Epitaxy

Topological insulators (TIs) hold promise for manipulating the magnetization of a ferromagnet (FM) through the spin-orbit torque (SOT) mechanism. However, integrating TIs with conventional FMs often leads to significant device-to-device variations and a broad distribution of SOT magnitudes. In this work, we present a scalable approach to grow a full van der Waals FM/TI heterostructure by molecular beam epitaxy, combining the charge-compensated TI (Bi,Sb)2Te3 with 2D FM Fe3GeTe2 (FGT). Harmonic magnetotransport measurements reveal that the SOT efficiency exhibits a non-monotonic temperature dependence and experiences a substantial enhancement with a reduction of the FGT thickness to 2 monolayers. Our study further demonstrates that the magnetization of ultrathin FGT films can be switched with a current density of Jc ∼ 1010 A/m2, with minimal device-to-device variations compared to previous investigations involving traditional FMs.

Regulating the electronic properties of the WGe2N4 monolayer by adsorption of 4d transition metal atoms towards spintronic devices

We study the regulation of the electronic and spin transport properties of the WGe2N4 monolayer by adsorbing 4d transition metal atoms (Y-Cd) using density functional theory combined with non-equilibrium Green's function. It is found that the adsorption of transition metal atoms (except Pd, Ag and Cd atoms) can introduce a magnetic moment into the WGe2N4 monolayer. Among the transition metal atoms, the adsorption of Nb and Rh atoms transforms WGe2N4 from a semiconductor to a half-metal and a highly spin-polarized semiconductor, respectively. The half-metallic Nb-adsorbed WGe2N4 system is selected to investigate the spin transport properties, and a high magnetoresistance ratio of 107% is achieved. In both parallel and antiparallel magnetization configurations, the spin filtering efficiency reaches close to 100% in the whole bias range, and the antiparallel magnetization configuration exhibits a dual spin filtering effect with a rectification ratio of up to 104. Our study predicts that the adsorption of 4d transition metal heteroatoms is an effective method to regulate the electronic and magnetic properties of WGe2N4 towards high-performance spintronic devices.

Strain-controlled spin transport in a two-dimensional (2D) nanomagnet

Semiconductors with controllable electronic transport coupled with magnetic behaviour, offering programmable spin arrangements present enticing potential for next generation intelligent technologies. Integrating and linking these two properties has been a long standing challenge for material researchers. Recent discoveries in two-dimensional (2D) magnet shows an ability to tune and control the electronic and magnetic phases at ambient temperature. Here, we illustrate controlled spin transport within the magnetic phase of the 2D semiconductor CrOBr and reveal a substantial connection between its magnetic order and charge carriers. First, we systematically analyse the strain-induced electronic behaviour of 2D CrOBr using density functional theory calculations. Our study demonstrates the phase transition from a magnetic semiconductor → half metal → magnetic metal in the material under strain application, creating intriguing spin-resolved conductance with 100% spin polarisation and spin-injection efficiency. Additionally, the spin-polarised current-voltage (I-V) trend displayed conductance variations with high strain-assisted tunability and a peak-to-valley ratio as well as switching efficiency. Our study reveals that CrOBr can exhibit highly anisotropic behaviour with perfect spin filtering, offering new implications for strain engineered magneto-electronic devices.

Graphene Spin Valves for Spin Logic Devices

An alternative to charge-based electronics identifies the spin degree of freedom for information communication and processing. The long spin-diffusion length in graphene at room temperature demonstrates its ability for highly scalable spintronics. The development of the graphene spin valve (SV) has inspired spin devices in graphene including spin field-effect transistors and spin majority logic gates. A comprehensive picture of spin transport in graphene SVs is required for further development of spin logic. This review examines the advances in graphene SVs and their role in the development of spin logic devices. Different transport and scattering mechanisms in charge and spin are discussed. Furthermore, the on/off switching energy between graphene SVs and charge-based FETs is compared to highlight their prospects for low-power devices. The challenges and perspectives that need to be addressed for the future development of spin logic devices are then outlined.

Room-temperature half-metals induced via chemical surface modification: 2D Mn2Se2 monolayer

Compared with various antiferromagnetic (AFM) materials, two-dimensional (2D) room-temperature ferromagnetic (FM) materials are rarely discovered because of the geometrically determined spin interactions. Since 2D FM materials have shown great potential in the next-generational information devices, it is quite important to design new FM materials based on the reported AFM materials. Here, in this study, we found that the Mn₂Se₂ monolayer can be converted to half-metal from AFM semiconductor at room temperature by edge modification of certain chemical groups (such as -Cl, -Br, -I, and -S) based on systematical first-principles calculations. Our results show that the adsorbed chemical groups significantly modify the electronic states of Mn ions and the resulting spin interactions. Moreover, our results indicate that the Curie temperatures (T_c) of some Mn₂Se₂ monolayer derivatives approach or even exceed room temperature, among which Curie temperatures after chemical modification by -Cl, -Br, -I, -S are 290 K, 320 K, 400 K, and 1050 K, respectively. Thus, chemical modifications can be one of the effective methods to construct 2D FM materials in experiments.

High throughput calculations for a dataset of bilayer materials

Bilayer materials made of 2D monolayers are emerging as new systems creating diverse opportunities for basic research and applications in optoelectronics, thermoelectrics, and topological science among others. Herein, we present a computational bilayer materials dataset containing 760 structures with their structural, electronic, and transport properties. Different stacking patterns of each bilayer have been framed by analyzing their monolayer symmetries. Density functional theory calculations including van der Waals interactions are carried out for each stacking pattern to evaluate the corresponding ground states, which are correctly identified for experimentally synthesized transition metal dichalcogenides, graphene, boron nitride, and silicene. Binding energies and interlayer charge transfer are evaluated to analyze the interlayer coupling strength. Our dataset can be used for materials screening and data-assisted modeling for desired thermoelectric or optoelectronic applications.

Two-dimensional half-metallicity and fully spin-polarized topological fermions in monolayer EuOBr

Two-dimensional (2D) half-metal and topological states have been the current research focus in condensed matter physics. Herein, we report a novel 2D material named EuOBr monolayer, which can simultaneously show 2D half-metal and topological fermions. This material shows a metallic state in the spin-up channel but a large insulating gap of 4.38 eV in the spin-down channel. In the conducting spin channel, the EuOBr monolayer shows the coexistence of Weyl points and nodal-lines near the Fermi level. These nodal-lines are classified by type-I, hybrid, closed, and open nodal-lines. The symmetry analysis suggests these nodal-lines are protected by the mirror symmetry, which cannot be broken even spin-orbit coupling is included because the ground magnetization direction in the material is out-of-plane [001]. The topological fermions in the EuOBr monolayer are fully spin-polarized, which can be meaningful for future applications in topological spintronic nano-devices.

Recent Advances in the Spintronic Application of Carbon-Based Nanomaterials

The term "carbon-based spintronics" mostly refers to the spin applications in carbon materials such as graphene, fullerene, carbon nitride, and carbon nanotubes. Carbon-based spintronics and their devices have undergone extraordinary development recently. The causes of spin relaxation and the characteristics of spin transport in carbon materials, namely for graphene and carbon nanotubes, have been the subject of several theoretical and experimental studies. This article gives a summary of the present state of research and technological advancements for spintronic applications in carbon-based materials. We discuss the benefits and challenges of several spin-enabled, carbon-based applications. The advantages include the fact that they are significantly less volatile than charge-based electronics. The challenge is in being able to scale up to mass production.

Quantum magnetic phenomena in engineered heterointerface of low-dimensional van der Waals and non-van der Waals materials

Investigating magnetic phenomena at the microscopic level has emerged as an indispensable research domain in the field of low-dimensional magnetic materials. Understanding quantum phenomena that mediate the magnetic interactions in dimensionally confined materials is crucial from the perspective of designing cheaper, compact, and energy-efficient next-generation spintronic devices. The infrequent occurrence of intrinsic long-range magnetic order in dimensionally confined materials hinders the advancement of this domain. Hence, introducing and controlling the ferromagnetic character in two-dimensional materials is important for further prospective studies. The interface in a heterostructure significantly contributes to modulating its collective magnetic properties. Quantum phenomena occurring at the interface of engineered heterostructures can enhance or suppress magnetization of the system and introduce magnetic character to a native non-magnetic system. Considering most 2D magnetic materials are used as stacks with other materials in nanoscale devices, the methods to control the magnetism in a heterostructure and understanding the corresponding mechanism are crucial for promising spintronic and other functional applications. This review highlights the effect of electric polarization of the adjacent layer, changed structural configuration at the vicinity of the interface, natural strain induced by lattice mismatch, and exchange interaction in the interfacial region in modulating the magnetism of heterostructures of van der Waals and non-van der Waals materials. Further, prospects of interface-engineered magnetism in spin-dependent device applications are also discussed.

Self-Forming p-n Junction Diode Realized with WSe2 Surface and Edge Dual Contacts

Owing to their practical applications, two-dimensional semiconductor p-n diodes have attracted enormous attention. Over the past decade, various methods, such as chemical doping, heterojunction structures, and metallization using metals with different work functions, have been reported for fabrication of such devices. In this study, a lateral p-n junction diode is formed in tungsten diselenide (WSe₂ ) using a combination of edge and surface contacts. The appearance of amorphous tungsten oxide at etched WSe₂ , and the formation of a junction near the edge contact, are verified by high-resolution transmission electron microscopy. The device demonstrates high on/off ratio for both the edge and surface contacts, with respective values of 10⁷ and 10⁸ . The diode can achieve extremely high mobility of up to 168 cm² V^-1 s^-1 and a rectification ratio of 10³ . The ideality factor is 1.11 at a back gate voltage V_G   = 60 V at 300 K. The devices with encapsulation of hexagonal boron nitride exhibit good stability to atmospheric exposure, over a measured period of 2 months. In addition, the architecture of the contacts, which is based on a single-channel device, should be advantageous for the implementation of more complicated applications such as inverters, photodetectors, and light-emitting diodes.

Thermally-driven large current-perpendicular-to-plane magnetoresistance in ultrathin flakes of vanadium diselenide

Current-perpendicular-to-plane magnetoresistance (CPP MR) in layered heterojunctions is at the heart of modern magnetic field sensing and data storage technologies. van der waals heterostructures and two-dimensional (2D) magnets opened a new playground for exploring this effect, although most 2D magnets exhibit large CPP MR only at very low temperatures due to their very low Curie temperatures. vanadium diselenide (VSe₂) is a promising material since its monolayers can potentially act as room temperature ferromagnets. VSe₂multilayers have been predicted to exhibit CPP MR effects, although experimental work in this area remains scarce. In this work we investigate CPP MR in 1T-VSe₂ultrathin flakes, revealing alarge (∼60%-70%), positive, linear, and non saturating CPP MR, which persists close to room temperature (∼250 K), in a relatively small magnetic field range of ±12 kG. The CPP MR has been found to increase with decreasing flake thickness. The CPP MR originates due to the intrinsic inhomogeneity in the CPP transport path, andexhibits unprecedented immunity against thermal fluctuations, leading to increasingly enhanced MR as temperature is increased, even significantly beyond the charge density wave transition temperature. The observed 'thermally-driven' MR features are remarkably robust and reproducible, and can offer a viable route for developing practical room temperature 2D based magnetic sensor technologies. Our results also suggest that harnessing similar effects in other 2D systems could result in large MR as well, thereby motivating further research on CPP transport in these systems, which has been relatively unexplored so far.

Room temperature giant magnetoresistance in half-metallic Cr2C based two-dimensional tunnel junctions

Two-dimensional (2D) magnetic materials inherit enormous potential to revolutionize next-generation spintronic technology. The majority of prior investigations using 2D ferromagnet-based tunnel junctions have shown encouraging tunnel magnetoresistance (TMR) at low temperatures. Using first-principles-based calculations, here we investigate the magnetic properties of commercially available Cr₂C crystals at their monolayer limit and reveal their half metallicity properties far beyond room temperature. We then design hetero-multilayer structures combining Cr₂C with graphene and hexagonal boron nitride (h-BN) and report their magnetoresistance using spin-polarized quantum transport calculations. While graphene based devices, adsorbed on the metal contact, reveal a very high TMR (1200%), it can be further increased to 1500% by changing the barrier layer to h-BN. The dependence of TMR on the number of barrier layers and different metallic electrode materials (Ti, Ag, and Au) are also studied. Our investigation suggests that Cr₂C based spin valves can serve as the perfect building blocks for room temperature all-2D spintronic devices.

Visualizing Atomically Layered Magnetism in CrSBr

2D materials can host long-range magnetic order in the presence of underlying magnetic anisotropy. The ability to realize the full potential of 2D magnets necessitates systematic investigation of the role of individual atomic layers and nanoscale inhomogeneity (i.e., strain) on the emergence of stable magnetic phases. Here, spatially dependent magnetism in few-layer CrSBr is revealed using magnetic force microscopy (MFM) and Monte Carlo-based simulations. Nanoscale visualization of the magnetic sheet susceptibility is extracted from MFM data and force-distance curves, revealing a characteristic onset of both intra- and interlayer magnetic correlations as a function of temperature and layer-thickness. These results demonstrate that the presence of a single uncompensated layer in odd-layer terraces significantly reduces the stability of the low-temperature antiferromagnetic (AFM) phase and gives rise to multiple coexisting magnetic ground states at temperatures close to the bulk Néel temperature (T_N ). Furthermore, the AFM phase can be reliably suppressed using modest fields (≈16 mT) from the MFM probe, behaving as a nanoscale magnetic switch. This prototypical study of few-layer CrSBr demonstrates the critical role of layer parity on field-tunable 2D magnetism and validates MFM for use in nanomagnetometry of 2D materials (despite the ubiquitous absence of bulk zero-field magnetism in magnetized sheets).

Recent advances in two-dimensional ferromagnetism: strain-, doping-, structural- and electric field-engineering toward spintronic applications

Since the first report on truly two-dimensional (2D) magnetic materials in 2017, a wide variety of merging 2D magnetic materials with unusual physical characteristics have been discovered and thus provide an effective platform for exploring the associated novel 2D spintronic devices, which have been made significant progress in both theoretical and experimental studies. Herein, we make a comprehensive review on the recent scientific endeavors and advances on the various engineering strategies on 2D ferromagnets, such as strain-, doping-, structural- and electric field-engineering, toward practical spintronic applications, including spin tunneling junctions, spin field-effect transistors and spin logic gate, etc. In the last, we discuss on current challenges and future opportunities in this field, which may provide useful guidelines for scientists who are exploring the fundamental physical properties and practical spintronic devices of low-dimensional magnets.

Recent Developments in van der Waals Antiferromagnetic 2D Materials: Synthesis, Characterization, and Device Implementation

Magnetism in two dimensions is one of the most intriguing and alluring phenomena in condensed matter physics. Atomically thin 2D materials have emerged as a promising platform for exploring magnetic properties, leading to the development of essential technologies such as supercomputing and data storage. Arising from spin and charge dynamics in elementary particles, magnetism has also unraveled promising advances in spintronic devices and spin-dependent optoelectronics and photonics. Recently, antiferromagnetism in 2D materials has received extensive attention, leading to significant advances in their understanding and emerging applications; such materials have zero net magnetic moment yet are internally magnetic. Several theoretical and experimental approaches have been proposed to probe, characterize, and modulate the magnetic states efficiently in such systems. This Review presents the latest developments and current status for tuning the magnetic properties in distinct 2D van der Waals antiferromagnets. Various state-of-the-art optical techniques deployed to investigate magnetic textures and dynamics are discussed. Furthermore, device concepts based on antiferromagnetic spintronics are scrutinized. We conclude with remarks on related challenges and technological outlook in this rapidly expanding field.

Gate Control of Spin-Layer-Locking FETs and Application to Monolayer LuIO

A recent 2D spinFET concept proposes to switch electrostatically between two separate sublayers with strong and opposite intrinsic Rashba effects, exploiting the spin-layer-locking mechanism in centrosymmetric materials with local dipole fields. Here, we propose a novel monolayer material within this family, lutetium oxide iodide (LuIO). It displays one of the largest Rashba effects among 2D materials (up to k_R = 0.08 Å^-1), leading to a π/2 rotation of the spins over just 1 nm. The monolayer was predicted to be exfoliable from its experimentally known 3D bulk counterpart, with a binding energy lower than graphene. We characterize and simulate the interplay of the two gate-controlled parameters for such devices: doping and spin channel selection. We show that the ability to split the spin channels in energy diminishes with doping, leading to specific gate-operation guidelines that can apply to all devices based on spin-layer locking.

CrI3/Y2CH2 Heterointerface-Induced Stable Half-Metallicity of Two-Dimensional CrI3 Monolayer Ferromagnets

Two-dimensional (2D) CrI₃ monolayer ferromagnets are key to the development of future miniature spintronic devices and modulating them into a half-metal will greatly expand the application scenarios of CrI₃ in nanospintronics. Nevertheless, existing strategies to induce half-metallicity of a CrI₃ monolayer remain experimentally challenging and have unstable issues. In this work, the introduction of a 2D electride [Y₂C]²⁺·2e^- as an auxiliary layer is shown to be an effective way to achieve the generation of stable half-metallicity in the CrI₃ monolayer. When the fully hydrogenated Y₂CH₂ and ferromagnetic CrI₃ monolayer combine to form a heterostructure, surprisingly the appropriate amount of charge injection (0.72 e) turns CrI₃ into a half-metal. Hetero-interfacial half-metallicity in CrI₃ is an intrinsic one and does not require any chemical functionalization or external physical modification. Therefore, it is advantageous for practical applications of CrI₃ in miniature spintronic devices, such as magnetic tunnel junctions, spin valves or spin field-effect transistors. A new strategy of the stable CrI₃/Y₂CH₂ heterostructure was successfully developed to induce the half-metallicity of 2D CrI₃ ferromagnets, which is experimentally feasible and half-metallic stable enough. This work paves the way for the application of the CrI₃ monolayer in half-metallic-based spintronics.