Proximity-Coupling-Induced Significant Enhancement of Coercive Field and Curie Temperature in 2D van der Waals Heterostructures

Magnetism in 2D has long been the focus of condensed matter physics due to its important applications in spintronic devices. A particularly promising aspect of 2D magnetism is the ability to fabricate 2D heterostructures with engineered optical, electrical, and quantum properties. Recently, the discovery of intrinsic ferromagnetisms in atomic thick materials has provided a new platform for investigations of fundamental magnetic physics. In contrast to 2D CrI₃ and Cr₂Ge₂Te₆ insulators, itinerant ferromagnetic Fe₃GeTe₂ (FGT), which has a larger intrinsic perpendicular anisotropy, higher Curie temperature (T_C), and relatively better stability, is a promising candidate for achieving permanent room-temperature ferromagnetism through interface or component engineering. Here, it is shown that the ferromagnetic properties of FGT thin flakes can be modulated through coupling with a FePS₃. The magneto-optical Kerr effect results show that the T_C of FGT is improved by more than 30 K and that the coercive field is increased by ≈100% due to the proximity coupling effect, which changes the spin textures of FGT at the interface. This work reveals that antiferromagnet/ferromagnet coupling is a promising way to engineer the magnetic properties of itinerant 2D ferromagnets, which paves the way for applications in advanced magnetic spintronic and memory devices.     

Controllable Magnetic Proximity Effect and Charge Transfer in 2D Semiconductor and Double-Layered Perovskite Manganese Oxide van der Waals Heterostructure

Optically generated excitonic states (excitons and trions) in transition metal dichalcogenides are highly sensitive to the electronic and magnetic properties of the materials underneath. Modulation and control of the excitonic states in a novel van der Waals (vdW) heterostructure of monolayer MoSe₂ on double-layered perovskite Mn oxide ((La_0.8Nd_0.2)_1.2Sr_1.8Mn₂O₇) is demonstrated, wherein the Mn oxide transforms from a paramagnetic insulator to a ferromagnetic metal. A discontinuous change in the exciton photoluminescence intensity via dielectric screening is observed. Further, a relatively high trion intensity is discovered due to the charge transfer from metallic Mn oxide under the Curie temperature. Moreover, the vdW heterostructures with an ultrathin h-BN spacer layer demonstrate enhanced valley splitting and polarization of excitonic states due to the proximity effect of the ferromagnetic spins of Mn oxide. The controllable h-BN thickness in vdW heterostructures reveals a several-nanometer-long scale of charge transfer as well as a magnetic proximity effect. The vdW heterostructure allows modulation and control of the excitonic states via dielectric screening, charge carriers, and magnetic spins.     

Strain-Sensitive Magnetization Reversal of a van der Waals Magnet

By virtue of the layered structure, van der Waals (vdW) magnets are sensitive to the lattice deformation controlled by the external strain, providing an ideal platform to explore the one-step magnetization reversal that is still conceptual in conventional magnets due to the limited strain-tuning range of the coercive field. In this study, a uniaxial tensile strain is applied to thin flakes of the vdW magnet Fe₃GeTe₂ (FGT), and a dramatic increase of the coercive field (H_c) by more than 150% with an applied strain of 0.32% is observed. Moreover, the change of the transition temperatures between the different magnetic phases under strain is investigated, and the phase diagram of FGT in the strain–temperature plane is obtained. Comparing the phase diagram with theoretical results, the strain-tunable magnetism is attributed to the sensitive change of magnetic anisotropy energy. Remarkably, strain allows an ultrasensitive magnetization reversal to be achieved, which may promote the development of novel straintronic device applications.     

Layered Antiferromagnetism Induces Large Negative Magnetoresistance in the van der Waals Semiconductor CrSBr

The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compounds has attracted considerable interest in these materials for both fundamental research and technological applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temperatures, and poor charge transport properties. Here the magnetic and electronic properties of CrSBr are reported, an air-stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Néel temperature, T_N = 132 ± 1 K, CrSBr adopts an A-type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is Δ_E = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large negative magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin-based electronics.     

Magnetic Structure and Metamagnetic Transitions in the van der Waals Antiferromagnet CrPS4

In 2D magnets, interlayer exchange coupling is generally weak due to the van der Waals layered structure but it still plays a vital role in stabilizing the long-range magnetic ordering and determining the magnetic properties. Using complementary neutron diffraction, magnetic, and torque measurements, the complete magnetic phase diagram of CrPS₄ crystals is determined. CrPS₄ shows an antiferromagnetic ground state (A-type) formed by out-of-plane ferromagnetic monolayers with interlayer antiferromagnetic coupling along the c axis below T_N = 38 K. Due to small magnetic anisotropy energy and weak interlayer coupling, the low-field metamagnetic transitions in CrPS_4, that is, a spin-flop transition at ≈0.7 T and a spin-flip transition from antiferromagnetic to ferromagnetic under a relatively low field of 8 T, can be realized for H∥c. Intriguingly, with an inherent in-plane lattice anisotropy, spin-flop-induced moment realignment in CrPS₄ for H∥c is parallel to the quasi-1D chains of CrS₆ octahedra. The peculiar metamagnetic transitions and in-plane anisotropy make few-layer CrPS₄ flakes a fascinating platform for studying 2D magnetism and for exploring prototype device applications in spintronics and optoelectronics.     

Room-Temperature Magnetic Skyrmions and Large Topological Hall Effect in Chromium Telluride Engineered by Self-Intercalation

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr_1+xTe₂ with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333 K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr_1.53Te₂ with a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106 nΩ cm is observed at 290 K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.     

Air-stable ultrathin Cr3Te4 nanosheets with thickness-dependent magnetic biskyrmions

Two-dimensional (2D) materials with intrinsic magnetism have drawn intense interest for fundamental research and potential application in spintronics and valleytronics. Here we synthesized 2D Cr₃Te₄ nanosheets with controllable thickness by chemical vapor deposition approach. Reflection magnetic circular dichroism and magneto-transport measurements demonstrated that the Cr₃Te₄ nanosheets feature excellent environmental stability and have a thickness-dependent ferromagnetic behavior with a high Curie temperature of 165–235 K. Lorentz transmission electron microscopy studies revealed topologically nontrivial magnetic-spin states in the ultrathin Cr₃Te₄ nanosheets. The magnetic stripe domains in Cr₃Te₄ can transform into biskyrmion bubbles when an external magnetic field is applied perpendicularly to the nanosheet. The size and density of the biskyrmions can be tuned by the external applied magnetic field and thickness of the nanosheets, respectively. Our findings of topologically nontrivial magnetic-spin states in air-stable 2D magnets could promise new designs of magnetic memory storage, sensors, and spintronics.     

Bridging the van der Waals Interface for Advanced Optoelectronic Devices

Van der Waals (vdW) heterostructures exhibit excellent optoelectronic properties and novel functionalities. However, their applicability is impeded due to the common issue of the tunneling barrier, which arises from the vdW gap; this significantly increases the injection resistance of the photoexcited carriers. Herein, a generic strategy is demonstrated to eliminate the vdW gap in a broad class of heterostructures. It is observed that the vdW gap in the interface is bridged via strong orbital hybridization between the interface dangling bonds of nonlayered chalcogenide semiconductors and the artificially induced vacancies of transition metal chalcogenides (TMDCs). The photoresponse times of bridged PbS/ReS₂, PbS/MoSe₂, and PbS/MoS₂ are ≈30, 51, and 43 µs, respectively. The photon-triggered on/off ratio of the bridged PbS/MoS₂, ZnSe/MoS₂, and ZnTe/MoS₂ heterostructures exceed 10⁶, 10⁵, and 10⁵, respectively. These are several orders of magnitude higher than common vdW heterostructures. The findings obtained in this study present a versatile strategy for overcoming the performance limitations of vdW heterostructures.     

Synthesis and Broadband Photodetection of a P-Type 1D Van der Waals Semiconductor HfSnS3

1D van der Waals (vdW) materials have attracted significant interest in recent years due to their giant anisotropic and weak interlayer-coupled characters. More 1D vdW materials are urgently to be exploited for satisfying the practice requirement. Herein, the study of 1D vdW ternary HfSnS₃ high-quality single crystals grown via the chemical vapor transport technique is reported. The Raman vibration modes and band structure of HfSnS₃ are analyzed via DFT calculations. Its strong in-plane anisotropic is verified by the polarized Raman spectroscopy. The field-effect transistors (FETs) based on the HfSnS₃ nanowires demonstrate p-type semiconducting behavior as well as outstanding photoresponse in a broadband range from UV to near-infrared (NIR) with short response times of ≈0.355 ms, high responsivity of ≈11.5 A W⁻¹, detectivity of ≈8.2 × 10¹¹, external quantum efficiency of 2739%, excellent environmental stability, and repeatability. Furthermore, a typical photoconductivity effect of the photodetector is illustrated. These comprehensive characteristics can promote the application of the p-type 1D vdW material HfSnS₃ in optoelectronics.     

Efficient Spin-Orbit Torque Switching of the Semiconducting Van Der Waals Ferromagnet Cr2Ge2Te6

Being able to electrically manipulate the magnetic properties in recently discovered van der Waals ferromagnets is essential for their integration in future spintronics devices. Here, the magnetization of a semiconducting 2D ferromagnet, i.e., Cr₂Ge₂Te₆, is studied using the anomalous Hall effect in Cr₂Ge₂Te₆/tantalum heterostructures. The thinner the flakes, hysteresis and remanence in the magnetization loop with out-of-plane magnetic fields become more prominent. In order to manipulate the magnetization in such thin flakes, a combination of an in-plane magnetic field and a charge current flowing through Ta—a heavy metal exhibiting giant spin Hall effect—is used. In the presence of in-plane fields of 20 mT, charge current densities as low as 5 × 10⁵ A cm^–2 are sufficient to switch the out-of-plane magnetization of Cr₂Ge₂Te₆. This finding highlights that current densities required for spin-orbit torque switching of Cr₂Ge₂Te₆ are about two orders of magnitude lower than those required for switching nonlayered metallic ferromagnets such as CoFeB. The results presented here show the potential of 2D ferromagnets for low-power memory and logic applications.     

Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

Although cubic rock salt‐CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here, a detailed magnetic and neutron diffraction study on zinc blende and wurtzite CoO nanoparticles is presented. The zinc blende‐CoO phase is antiferromagnetic with a 3rd type structure in a face‐centered cubic lattice and a Néel temperature of T_N (zinc‐blende) ≈225 K. Wurtzite‐CoO also presents an antiferromagnetic order, T_N (wurtzite) ≈109 K, although much more complex, with a 2nd type order along the c‐axis but an incommensurate order along the y‐axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic‐like behavior even at room temperature.     

Modulation of Metal and Insulator States in 2D Ferromagnetic VS2 by van der Waals Interaction Engineering

2D transition‐metal dichalcogenides (TMDCs) are currently the key to the development of nanoelectronics. However, TMDCs are predominantly nonmagnetic, greatly hindering the advancement of their spintronic applications. Here, an experimental realization of intrinsic magnetic ordering in a pristine TMDC lattice is reported, bringing a new class of ferromagnetic semiconductors among TMDCs. Through van der Waals (vdW) interaction engineering of 2D vanadium disulfide (VS₂), dual regulation of spin properties and bandgap brings about intrinsic ferromagnetism along with a small bandgap, unravelling the decisive role of vdW gaps in determining the electronic states in 2D VS₂. An overall control of the electronic states of VS₂ is also demonstrated: bond‐enlarging triggering a metal‐to‐semiconductor electronic transition and bond‐compression inducing metallization in 2D VS₂. The pristine VS₂ lattice thus provides a new platform for precise manipulation of both charge and spin degrees of freedom in 2D TMDCs availing spintronic applications.     

Large-Area Synthesis of Ferromagnetic Fe5−xGeTe2/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 Fe_5−xGeTe₂/graphene heterostructures is achieved by vdW epitaxy of Fe_5−xGeTe₂ on epitaxial graphene. Structural characterization confirms the realization of a continuous vdW heterostructure film with a sharp interface between Fe_5−xGeTe₂ 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.     

Crystal chemistry and magnetic properties of CrIIBIIIF5 compounds

Three new compounds isostructural with Cr₂F₅ have been prepared: CrAlF₅, CrTiF₅, CrVF₅. Their magnetic properties have been investigated: CrAlF₅ is paramagnetic above 4.2 K with a Weiss constant θ = ?3K, CrTiF₅ and CrVF₅ are ferrimagnetic with T_C = 26 K and 40 K, Cr₂F₅ is antiferromagnetic with T_N = 40 K. From the structure and the rules of super-exchange, the magnetic order and spin orientations below the magnetic-ordering temperature are predicted to exhibit weak, antiferromagnetic spin canting in Cr₂F₅ and strong ferromagnetic canting in CrTiF₅ and CrVF₅ where interatomic-exchange interactions are competitive.     

Low-temperature properties of Ga0.5Mo2S4

Specific heat measurements from 2 to 27 K and crystal structure and magnetic properties for 4.2–300 K are reported for the spinel Ga_0.5Mo₂S₄. AtT _p =42 K this ternary chalcogenide undergoes a phase transition. At liquid helium temperatures it has a rhombohedral crystal structure with lattice parametera=9.275 Å and rhombohedral angle =90.45°. It follows from the magnetization measurements that ferromagnetic ordering takes place at about 20 K, and close to the Curie point an anomaly in the temperature dependence ofC _p has been found. The magnetic behavior of Ga_0.5Mo₂S₄ at low temperatures can be explained by a small concentration of magnetic ions and far-distant-neighbor superexchange interactions partly weakened by vacancies located in 4(c) positions. Magnetocaloric measurements indicate the dominance of ferromagnetic interactions.     

Record‐High Superconductivity in Niobium–Titanium Alloy

The extraordinary superconductivity has been observed in a pressurized commercial niobium–titanium alloy. Its zero‐resistance superconductivity persists from ambient pressure to the pressure as high as 261.7 GPa, a record‐high pressure up to which a known superconducting state can continuously survive. Remarkably, at such an ultra‐high pressure, although the ambient pressure volume is shrunk by 45% without structural phase transition, the superconducting transition temperature (T_C) increases to ≈19.1 K from ≈9.6 K, and the critical magnetic field (H_C2) at 1.8 K has been enhanced to 19 T from 15.4 T. These results set new records for both the T_C and the H_C2 among all the known alloy superconductors composed of only transition metal elements. The remarkable high‐pressure superconducting properties observed in the niobium–titanium alloy not only expand the knowledge on this important commercial superconductor but also are helpful for a better understanding on the superconducting mechanism.     

Magnetic Behaviors of Ferromagnetic Semiconductor Zn1?xCrxTe Grown by MBE

The magnetic and structural properties of MBE-grown films of Zn_1–xCr_xTe were investigated. The magnetization versus magnetic field (M–H) measurement of Zn_1–xCr_xTe (x = 0.01–0.17) showed clear hysteresis loop at low temperatures. The ferromagnetic transition temperature (T_C) estimated from the Arrott-plot analysis increased almost linearly with the Cr composition (x) up to 275 K at x = 0.17. However, in the magnetization versus temperature (M–T) measurement, the irreversibility between the zero-field-cooled (ZFC) and field-cooled (FC) processes was observed. This is typically observed in the magnetic random system such as spin-glass or superparamagnetic phase. In the high resolution transmission microscopy (HRTEM) observations, structural defects such as stacking faults and polycrystalline-like structure were observed at high Cr compositions, whereas any apparent precipitates of different phases were not seen in all the range of Cr compositions examined. The correlation of the observed magnetic randomness with the local structural defects was discussed.     

Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling

Ultrafast interlayer charge transfer is one of the most distinct features of van der Waals (vdW) heterostructures. Its dynamics competes with carrier thermalization such that the energy of nonthermalized photocarriers may be harnessed by band engineering. In this study, nonthermalized photocarrier energy is harnessed to achieve near-infrared (NIR) to visible light upconversion in a metal–insulator–semiconductor (MIS) vdW heterostructure tunnel diode consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS₂). Photoexcitation of the electrically biased heterostructure with 1.58 eV NIR laser in the linear absorption regime generates emission from the ground exciton state of WS₂, which corresponds to upconversion by ≈370 meV. The upconversion is realized by electrically assisted interlayer transfer of nonthermalized photoexcited holes from FLG to WS₂, followed by formation and radiative recombination of excitons in WS₂. The photocarrier transfer rate can be described by Fowler–Nordheim tunneling mechanism and is electrically tunable by two orders of magnitude by tuning voltage bias applied to the device. This study highlights the prospects for realizing novel electro-optic upconversion devices by exploiting electrically tunable nonthermalized photocarrier relaxation dynamics in vdW heterostructures.     

Imaging Domain Reversal in an Ultrathin Van der Waals Ferromagnet

The recent isolation of 2D van der Waals magnetic materials has uncovered rich physics that often differs from the magnetic behavior of their bulk counterparts. However, the microscopic details of fundamental processes such as the initial magnetization or domain reversal, which govern the magnetic hysteresis, remain largely unknown in the ultrathin limit. Here a widefield nitrogen-vacancy (NV) microscope is employed to directly image these processes in few-layer flakes of the magnetic semiconductor vanadium triiodide (VI₃). Complete and abrupt switching of most flakes is observed at fields H_c ≈ 0.5–1 T (at 5 K) independent of thickness. The coercive field decreases as the temperature approaches the Curie temperature (T_c ≈ 50 K); however, the switching remains abrupt. The initial magnetization process is then imaged, which reveals thickness-dependent domain wall depinning fields well below H_c. These results point to ultrathin VI₃ being a nucleation-type hard ferromagnet, where the coercive field is set by the anisotropy-limited domain wall nucleation field. This work illustrates the power of widefield NV microscopy to investigate magnetization processes in van der Waals ferromagnets, which can be used to elucidate the origin of the hard ferromagnetic properties of other materials and explore field- and current-driven domain wall dynamics.     

Tunable Modal Birefringence in a Low‐Loss Van Der Waals Waveguide

van der Waals (vdW) crystals are promising candidates for integrated phase retardation applications due to their large optical birefringence. Among the two major types of vdW materials, the hyperbolic vdW crystals are inherently inadequate for optical retardation applications since the supported polaritonic modes are exclusively transverse‐magnetic (TM) polarized and relatively lossy. Elliptic vdW crystals, on the other hand, represent a superior choice. For example, molybdenum disulfide (MoS₂) is a natural uniaxial vdW crystal with extreme elliptic anisotropy in the frequency range of optical communication. Both transverse‐electric (TE) polarized ordinary and TM polarized extraordinary waveguide modes can be supported in MoS₂ microcrystals with suitable thicknesses. In this work, low‐loss transmission of these guided modes is demonstrated with nano‐optical imaging at the near‐infrared (NIR) wavelength (1530 nm). More importantly, by combining theoretical calculations and NIR nanoimaging, the modal birefringence between the orthogonally polarized TE and TM modes is shown to be tunable in both sign and magnitude via varying the thickness of the MoS₂ microcrystal. This tunability represents a unique new opportunity to control the polarization behavior of photons with vdW materials.