Large and tunable magnetoresistance in van der Waals ferromagnet/semiconductor junctions

Two-Dimensional Cr3Te4/WS2/Fe3GeTe2/WTe2 Magnetic Memory with Field-Free Switching and Low Power Consumption

Spin-orbit torque (SOT) magnetic memory technology has garnered significant attention due to its ability to enable field-free switching of magnets with strong perpendicular magnetic anisotropy (PMA). However, concerns regarding power consumption of SOT-memory are persisting. Here, this work proposes a method to construct magnetic tunnel junction (MTJ) by transferring chemically vapor-deposited two-dimensional (2D) Cr3Te4/WS2 van der Waals (vdW) heterostructures onto 2D Fe3GeTe2 (FGT) magnet. The robustness and tunability of 2D magnets allow MTJs to exhibit non-volatility, multiple output states, and impressive cycling durability. MTJs with thin WS2 barriers (fewer than six layers) exhibit a linear tunneling effect, achieving a low resistance-area product (RA) of 15.5 kΩ·µm2 using bilayer WS2, which facilitats low-power operation. Furthermore, the different 2D magnets display a significant anti-parallel window of up to 8 kOe. SOT-memory based on the typical MTJ demonstrates a low write consumption of 0.3 mJ and read consumption of 9.7 nJ, marking a significant advancement in 2D vdW SOT-memory. This research has pointed out a new direction for constructing low power consumption SOT-memory with PMA field-free switching.

Spintronic Devices upon 2D Magnetic Materials and Heterojunctions

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.

Spin-Valve Effect in Junctions with a Single Ferromagnet

Spin valves are essential components in spintronic memory devices whose conductance is modulated by controlling spin-polarized electron tunnelling through the alignment of the magnetization in ferromagnetic elements. Whereas conventional spin valves unavoidably require at least two ferromagnetic elements, here we demonstrate a van der Waals spin valve based on a tunnel junction that comprises only one such ferromagnetic layer. Our devices combine an Fe3GeTe2 electrode acting as a spin injector together with a paramagnetic tunnel barrier, formed by a CrBr3 multilayer operated above its Curie temperature. We show that these devices exhibit a conductance modulation with values comparable to those of conventional spin valves. A quantitative analysis of the magnetoconductance that accounts for the field-induced magnetization of CrBr3, including the effect of exchange interaction, confirms that the spin valve effect originates from the paramagnetic response of the barrier, in the absence of spontaneous magnetization in CrBr3.

Artificially Creating Emergent Interfacial Antiferromagnetism and Its Manipulation in a Magnetic van der Waals Heterostructure

van der Waals (vdW) magnets, with their two-dimensional (2D) atomic structures, provide a unique platform for exploring magnetism on the nanoscale. Although there have been numerous reports on their diverse quantum properties, the emergent interfacial magnetism─artificially created at the interface between two layered magnets─remains largely unexplored. This work presents observations of such emergent interfacial magnetism at the ferromagnet-antiferromagnet interface in a vdW heterostructure. We report the discovery of an intermediate Hall resistance plateau in the anomalous Hall loop indicative of emergent interfacial antiferromagnetism fostered by the heterointerface. This plateau can be stabilized and further manipulated under varying pressures but collapses at high pressures over 10 GPa. Our theoretical calculations reveal that charge transfer at the interface is pivotal in establishing the interlayer antiferromagnetic spin-exchange interaction. This work illuminates the previously unexplored emergent interfacial magnetism at a vdW interface comprising a ferromagnetic metal and an antiferromagnetic insulator and highlights its gradual evolution under increasing pressure. These findings enrich the portfolio of emergent interfacial magnetism and support further investigations of vdW magnetic interfaces and the development of next-generation spintronic devices.

Thermally-Stable Temperature-Independent Tunneling Magnetoresistance in all van der Waals Fe3GaTe2/GaSe/Fe3GaTe2 Magnetic Tunnel Junctions

Thermal stability is of great significance for the next-generation two-dimensional (2D) non-volatile spintronic devices. Typically, as the temperature increases, the spin polarization of materials decreases rapidly following the Bloch 𝑇3/2 law in low-temperature regions, resulting in a rapid decrease in the tunneling magnetoresistance (TMR) of the magnetic tunnel junction (MTJ). Owing to the thermal effects induced by current during the writing processes, even small temperature fluctuations can result in significant variations in the TMR of MTJs, hindering their practical applications. In this paper, all-van der Waals Fe3GaTe2/GaSe/Fe3GaTe2 (FGaT/GaSe/FGaT) MTJ devices are constructed, achieving a TMR ratio of 47% at low temperatures and 17% at room temperature. Importantly, the TMR ratio remains stable within a temperature range from 2 to 160 K, breaking the Bloch 𝑇3/2 law. The temperature-independent TMR is highly related to the enhanced perpendicular magnetic anisotropy (PMA) with reduced dimensionality is demonstrated. This work paves a promising path to achieve high-performance, thermally stable 2D spintronic memory chips.

Large Tunneling Magnetoresistance in Nonvolatile 2D Hybrid Spin Filters

Ferromagnetic semiconductors offer an efficient way to achieve high spin polarization via spin filtering effect. Large tunneling magnetoresistance (TMR) can then be realized when multiple spin filters are put in series, as recently demonstrated in van der Waals 2D A-type antiferromagnets such as CrI_{3} and CrSBr. However, the interlayer antiferromagnetic ground state of these magnets inherently results in a high resistance state at zero field, and this volatile behavior limits potential applications. Here we fabricate hybrid spin filters using 2D ferromagnetic metal Fe_{3}GeTe_{2} and semiconductor CrBr_{3}, which are nonvolatile as two magnets are magnetically decoupled. We achieve large TMR of around 100%, with its temperature dependence well fitted by the extended Jullière model. Additionally, the devices allow spin injection tuned through bias voltage, and TMR polarity reversals are observed. Our work opens a new route to develop 2D magnetic semiconductor based spintronics.

Antisymmetric Magnetoresistance in a CrTe2/Bi2Te3/CrTe2 van der Waals Heterostructure Grown by MBE

The magnetoresistance (MR) of spin valves usually displays a symmetric dependence on the magnetic field. An antisymmetric MR phenomenon has been discovered recently that breaks field symmetry and has the potential to realize polymorphic memory. In this work, centimeter-size and high-quality CrTe2/Bi2Te3/CrTe2 van der Waals (vdWs) heterostructure devices have been prepared using molecular beam epitaxy (MBE). By changing the magnetization direction of the top and bottom layers of CrTe2, an antisymmetric MR effect with high, intermediate, and low resistance states has been found and persists up to 75K. The emergence of this antisymmetric MR phenomenon is attributed to the spin Hall effect, which generates spin currents with both spin-up and spin-down orientations on the upper and lower surfaces of Bi2Te3. The spin currents diffuse or reflect at the Bi2Te3/CrTe2 interfaces alongside the additional charge currents induced by the inverse spin Hall effect (ISHE). Through theoretical calculations, the existence of the antisymmetric MR effect has also been confirmed. Our work emphasizes the use of the MBE technology to grow vdWs heterostructures to explore new physical phenomena and potential applications of spin electronic devices in polymorphic solid-state storage.

Pseudotunnel Magnetoresistance in Twisted van der Waals Fe3GeTe2 Homojunctions

Twistronics, a novel engineering approach involving the alignment of van der Waals (vdW) integrated two-dimensional materials at specific angles, has recently attracted significant attention. Novel nontrivial phenomena have been demonstrated in twisted vdW junctions (the so-called magic angle), such as unconventional superconductivity, topological phases, and magnetism. However, there have been only few reports on integrated vdW layers with large twist angles θt, such as twisted interfacial Josephson junctions using high-temperature superconductors. Herein, vdW homojunctions of the thin-magnetic flakes, Fe3GeTe2 (FGT), with large θt ranging from 0° to 90°, without inserting any tunnel barriers are assembled. Nevertheless, these vdW homojunctions exhibit tunnel-magnetoresistance (TMR) like behavior (pseudo-TMR (PTMR) effect) with the ratios highly sensitive to the θt values, revealing that the vdW gap at the junction interface between the twisted FGT layers behaves like a tunnel barrier and the θt serves a control parameter for PTMR by drastically varying magnitudes of the lattice-mismatch and the subsequent appearance of antiferromagnetic (AFM) spin alignment. First-principles calculations considering vacuum gaps indicate strong dependence of TMR on the θt driven by the sixfold screw rotational symmetry of bulk FGT. The present homojunctions hold promise as a platform for novel AFM spin-dependent phenomena and spintronic applications.

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.

Room-temperature spin-valve devices without spacer layers based on Fe3GaTe2 van der Waals homojunctions

In the advancement of spintronic devices, spin valves play a critical role, especially in the sensor and information industries. The emergence of two-dimensional (2D) van der Waals (vdW) magnetic materials has opened up new possibilities for the development of high-performance spin-valve devices. However, the Curie temperature (TC) of most 2D vdW ferromagnets falls below room temperature, resulting in a scarcity of room-temperature spin-valve devices. In this study, we have prepared spin-valve devices without spacer layers based on Fe3GaTe2 vdW homojunctions and observed notable two-state magnetoresistance (MR) from 2 K to room temperature. A maximum MR of 50% surpasses some heterojunctions with spacer-layer structures and it remains 0.6% at room temperature. Furthermore, spin-valve devices exhibit favorable ohmic contact and low operating current as low as 10 nA. These findings demonstrate the enormous potential of Fe3GaTe2-based room-temperature devices and the simplified two-layer structure shows significant prospect in the context of the ongoing trend towards miniaturization of contemporary devices.

Half-Metallic Transport and Spin-Polarized Tunneling through the van der Waals Ferromagnet Fe4GeTe2

We examine the coherent spin-dependent transport properties of the van der Waals (vdW) ferromagnet Fe4GeTe2 using density functional theory combined with the nonequilibrium Green's function method. Our findings reveal that the conductance perpendicular to the layers is half-metallic, meaning that it is almost entirely spin-polarized. This property persists from the bulk to a single layer, even under significant bias voltages and with spin-orbit coupling. Additionally, using dynamical mean field theory for quantum transport, we demonstrate that electron correlations are important for magnetic properties but minimally impact the conductance, preserving almost perfect spin-polarization. Motivated by these results, we then study the tunnel magnetoresistance (TMR) in a magnetic tunnel junction consisting of two Fe4GeTe2 layers with the vdW gap acting as an insulating barrier. We predict a TMR ratio of ∼500%, which can be further enhanced by increasing the number of Fe4GeTe2 layers in the junction.

Growth of Quasi-Two-Dimensional CrTe Nanoflakes and CrTe/Transition Metal Dichalcogenide Heterostructures

Two-dimensional (2D) van der Waals layered materials have been explored in depth. They can be vertically stacked into a 2D heterostructure and represent a fundamental way to explore new physical properties and fabricate high-performance nanodevices. However, the controllable and scaled growth of non-layered quasi-2D materials and their heterostructures is still a great challenge. Here, we report a selective two-step growth method for high-quality single crystalline CrTe/WSe2 and CrTe/MoS2 heterostructures by adopting a universal CVD strategy with the assistance of molten salt and mass control. Quasi-2D metallic CrTe was grown on pre-deposited 2D transition metal dichalcogenides (TMDC) under relatively low temperatures. A 2D CrTe/TMDC heterostructure was established to explore the interface's structure using scanning transmission electron microscopy (STEM), and also demonstrate ferromagnetism in a metal-semiconductor CrTe/TMDC heterostructure.

Sign-flipping intrinsic anomalous Hall conductivity with Berry curvature tunability in a half-metallic ferromagnet NbSe2-VSe2 lateral heterostructure

Single-layer half-metal magnets offer exciting scope in spin electronic quantum applications owing to improved spin transport, reduced interfacial resistance and streamlined device fabrication. Herein, we report the emergence of sign-flipping intrinsic anomalous Hall conductivity (AHC) as a result of changes in Berry curvature under an external electric field and half metallicity in a lateral heterostructure composed of centrosymmetric metallic monolayers 1T-NbSe2 and 1T-VSe2. The metallic monolayers 1T-NbSe2 and 1T-VSe2 laterally interfaced along the zigzag orientation break inversion symmetry at the interface and result in distinctive Berry curvature features. Furthermore, the half-metallic character was prominent with gapped states in the spin-up channel, while the spin-down state remained conductive; we observed the unique manifestation of sign-flipping intrinsic AHC at the Fermi level in addition to the electron- and hole-doped regions. This sign-flipping aspect of AHC at the Fermi level is of fundamental importance from the prospect of real-time device applications as it eliminates the necessity of supplementary actions, such as doping and strain engineering, which are traditionally employed to achieve AHC sign reversal. Additionally, a phase transition from half metal to metal occurs at a field of 0.5 V Å-1 and beyond. Half metallicity with sign switching AHC via external electric field makes the lateral NbSe2-VSe2 heterostructure a potential candidate for real-time energy-efficient low-power spintronic devices.

Recent advances in two-dimensional intrinsic ferromagnetic materials Fe3X(X=Ge and Ga)Te2 and their heterostructures for spintronics

Owing to their atomic thicknesses, atomically flat surfaces, long-range spin textures and captivating physical properties, two-dimensional (2D) magnetic materials, along with their van der Waals heterostructures (vdWHs), have attracted much interest for the development of next-generation spin-based materials and devices. As an emergent family of intrinsic ferromagnetic materials, Fe3X(X=Ge and Ga)Te2 has become a rising star in the fields of condensed matter physics and materials science owing to their high Curie temperature and large perpendicular magnetic anisotropy. Herein, we aim to comprehensively summarize the recent progress on 2D Fe3X(X=Ge and Ga)Te2 and their vdWHs and provide a panorama of their physical properties and underlying mechanisms. First, an overview of Fe3X(X=Ge and Ga)Te2 is presented in terms of crystalline and electronic structures, distinctive physical properties and preparation methods. Subsequently, the engineering of electronic and spintronic properties of Fe3X(X=Ge and Ga)Te2 by diverse means, including strain, gate voltage, substrate and patterning, is surveyed. Then, the latest advances in spintronic devices based on 2D Fe3X(X=Ge and Ga)Te2 vdWHs are discussed and elucidated in detail, including vdWH devices that exploit the exchange bias effect, magnetoresistance effect, spin-orbit torque effect, magnetic proximity effect and Dzyaloshinskii-Moriya interaction. Finally, the future outlook is given in terms of efficient large-scale fabrication, intriguing physics and important technological applications of 2D Fe3X(X=Ge and Ga)Te2 and their vdWHs. Overall, this study provides an overview to support further studies of emergent 2D Fe3X(X=Ge and Ga)Te2 materials and related vdWH devices for basic science and practical applications.

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.

Tunable asymmetric magnetoresistance in an Fe3GeTe2/graphite/Fe3GeTe2 lateral spin valve

van der Waals (vdW) ferromagnetic heterojunctions, characterized by an ultraclean device interface and the absence of lattice matching, have emerged as indispensable and efficient building blocks for future spintronic devices. In this study, we present a seldom observed antisymmetric magnetoresistance (MR) behavior with three distinctive resistance states in a lateral van der Waals (vdW) structure comprising Fe3GeTe2 (FGT)/graphite/FGT. In contrast to traditional spin valves governed by the magnetization configurations of ferromagnetic electrodes (FEs), this distinct feature can be attributed to the interaction between FGT and the FGT/graphite interface, which is primarily influenced by the internal spin-momentum locking effect. Furthermore, modulation of the MR behavior is accomplished by employing the coupling between antiferromagnetic and ferromagnetic materials to adjust the coercive fields of two FEs subsequent to the in situ growth of an FGT oxide layer on FGT. This study elucidates the device physics and mechanism of property modulation in lateral spin valves and holds the potential for advancing the development of gate-tunable spintronic devices and next-generation integrated circuits.