In-Plane Magnetic Domains and Néel-like Domain Walls in Thin Flakes of the Room Temperature CrTe2 Van der Waals Ferromagnet

Reactive Molecular Beam Epitaxy Growth of a 1T-FeS2 Single-Layer-Atomic Structure, Moiré, and Decoupling via Intercalation

Two-dimensional (2D) iron disulfide (FeS2), in its 1T polymorph, is a promising candidate for high-Curie-temperature ferromagnetic applications. Unlike typical van der Waals materials, FeS2 lacks a naturally lamellar bulk structure and thus cannot be prepared by exfoliation. Consequently, it exists solely as a synthetic 2D magnet, primarily produced via chemical vapor deposition. Here, we propose an alternative synthesis method: reactive molecular beam epitaxy, where an iron layer predeposited on a Au(111) substrate is sulfurized to form FeS2. Structural and compositional analyses of the resulting 2D layer─employing scanning tunneling microscopy, electron diffraction, Auger electron spectroscopy, and synchrotron surface X-ray diffraction─confirm a nominal Fe ratio of 1:2, essential for achieving a high Curie temperature. Modeling and fitting the three-dimensional X-ray diffraction data further reveals that the layer crystallizes in the desired 1T polymorph. This 1T-FeS2 grown on Au(111) exhibits exceptional crystalline quality, largely surpassing that of other 2D transition metal dichalcogenides epitaxially grown on substrates. In addition, it shows pronounced atomic distortions from an ideal 1T structure, attributed to the strain induced by the substrate to achieve a perfectly commensurate 5 × 5 moiré pattern. The 1T-FeS2 and moiré atomic structures are fully determined with high accuracy on atomic coordinates. Finally, through Cs intercalation, we demonstrate complete decoupling of the FeS2 layer from the substrate and the release of heteroepitaxial strains.

Si-CMOS Compatible Synthesis of Wafer-Scale 1T-CrTe2 with Step-Like Magnetic Transition

2D room-temperature ferromagnet CrTe2 is a promising candidate material for spintronic applications. However, its large-scale and cost-effective synthesis remains a challenge. Here, the fine controllable synthesis of wafer-scale 1T-CrTe2 films is reported on a SiO2/Si substrate using plasma-enhanced chemical vapor deposition at temperatures below 400 °C. Magnetic hysteresis measurements reveal that the synthesized 1T-CrTe2 films exhibit perpendicular magnetic anisotropy along with distinct step-like magnetic transitions. It is found that 1T-CrTe2 is susceptible to oxygen adsorption even in ambient conditions. The theoretical calculations indicate that the oxidation of surface layers is crucial for the absence of out-of-plane easy axis in few-layer CrTe2, while the interlayer antiferromagnetic coupling among the upper surface layers leads to the observed step-like magnetic transitions. The study provides a Si-CMOS compatible approach for the fabrication of magnetic 2D materials and highlights how unintentional adsorbents or dopants can significantly influence the magnetic behaviors of these materials.

Emergent Skyrmions in Cr0.85Te nanoflakes at Room Temperature

Chiral noncollinear magnetic nanostructures, such as skyrmions, are intriguing spin configurations with significant potential for magnetic memory technologies. However, the limited availability of 2D magnetic materials that host skyrmions with Curie temperatures above room temperature presents a major challenge for practical implementation. Chromium tellurides exhibit diverse spin configurations and remarkable stability under ambient conditions, making them a promising platform for fundamental spin physics research and the development of innovative 2D spintronic devices. Here, domain structures of Cr0.85Te nanoflakes synthesized via chemical vapor deposition are investigated, using magnetic force microscopy at room temperature. The results reveal that the domain width of the as-grown nanoflakes scales with the square root of their thicknesses. Notably, the emergence and annihilation of skyrmions are observed, which can be reversibly controlled by external magnetic fields and thermal excitation in ambient air. Micromagnetic simulations suggest that the emergence of skyrmions in Cr0.85Te nanoflakes arises from inversion symmetry breaking due to compositional gradients across the sample thickness, rather than the interfacial Dzyaloshinskii-Moriya interaction. These findings provide new insights into the mechanisms underlying skyrmion formation in 2D ferromagnets and open exciting possibilities for manipulating domain structures at room temperature, offering practical pathways for developing next-generation spintronic devices.

Efficient soft-chemical synthesis of large van-der-Waals crystals of the room-temperature ferromagnet 1T-CrTe2

We herein report on a fast and convenient soft-chemical synthesis approach towards large 1T-CrTe2 van-der-Waals crystals. This compound is formed X-ray diffraction pure, with a complete conversion within just over 2 h from flux-grown LiCrTe2 crystals using diluted acids. Due to the availability of high-quality single crystals, we have confirmed the crystal structure for the first time by single-crystal X-ray diffraction experiments. For the acid deintercalated 1T-CrTe2 crystals, we find long-range ferromagnetic order with a Curie temperature of T C = 318 K. We further revealed the magnetic structure of 1T-CrTe2 using low-temperature neutron powder diffraction experiments and determined the magnetic Hamiltonian using density functional theory. X-ray diffraction experiments of post-annealed crystals suggest a thermal stability of 1T-CrTe2 up to at least 100 °C. Our findings expand the synthesis methods for 1T-CrTe2 crystals, which hold promise for integrated room-temperature spintronics applications.

Harvesting Magneto-Acoustic Waves Using Magnetic 2D Chromium Telluride (CrTe3)

A vast majority of electrical devices have integrated magnetic units, which generate constant magnetic fields with noticeable vibrations. The majority of existing nanogenerators acquire energy through friction/mechanical forces and most of these instances overlook acoustic vibrations and magnetic fields. Magnetic two-dimensional (2D) tellurides present a wide range of possibilities for devising a potential flexible energy harvester. 2D chromium telluride (2D CrTe3) is synthesized, which exhibits ferromagnetic behavior with a higher T c of ≈224 K. The structure exhibits stable high remnant magnetization, making 2D CrTe3 a potential material for harvesting magneto-acoustic waves. A magneto-acoustic nanogenerator (MANG) is fabricated and the basic mechanical stability and sensitivity of the device with change in load conditions are tested. A high surface charge density of 2.919 mC m-2 is obtained for the device. The thermal strain created in the lattice structure is examined using in-situ Raman spectroscopy. The magnetic anisotropy energy (MAE) responsible for long-range FM ordering is calculated by theoretical modelling with insights into opening of electronic bandgap which enhances the flexoelectric effects. The MANG can be a potential NG to synergistically tap into the magneto-acoustic vibrations generated from the frequency changes of a vibrating device such as loudspeakers.

Quasi-2D-Ising-type magnetic critical behavior in trigonal Cr1.27Te2

Single crystal Cr1.27Te2 samples were synthesized by using the chemical vapor transport method. Single crystal x-ray diffraction studies show a trigonal crystal structure with a P3̄m1 symmetry space group. We then systematically investigate magnetic properties and critical behaviors of single crystal Cr1.27Te2 around its paramagnetic-to-ferromagnetic phase transition. The Arrott plot indicates a second-order magnetic phase transition. We estimate critical exponents β = 0.2631 ± 0.002, γ = 1.2314 ± 0.007, and TC = 168.48 ± 0.031 K by using the Kouvel-Fisher method. We also estimate other critical exponents δ = 5.31 ± 0.004 by analyzing the critical isotherm at TC = 168.5 K. We further verify the accuracy of our estimated critical exponents by the scaling analysis. Further analysis suggests that Cr1.27Te2 can be best described as a quasi-2D Ising magnetic system.

Acoustomagnonic Spin Hall Effect in Honeycomb Antiferromagnets

The recently discovered Van der Waals antiferromagnets have suffered from the lack of a comprehensive method to study their magnetic properties. Here, we propose an ac intrinsic magnon spin Hall current driven by surface acoustic waves as a novel probe for such antiferromagnets. Our results pave the way towards mechanical detection and manipulation of the magnetic order in two-dimensional antiferromagnets. Furthermore, they will overcome the difficulties with weak magnetic responses inherent in the use of antiferromagnets and hence provide a building block for future antiferromagnetic spintronics.

Stacking engineering in layered homostructures: transitioning from 2D to 3D architectures

Artificial materials, characterized by their distinctive properties and customized functionalities, occupy a central role in a wide range of applications including electronics, spintronics, optoelectronics, catalysis, and energy storage. The emergence of atomically thin two-dimensional (2D) materials has driven the creation of artificial heterostructures, harnessing the potential of combining various 2D building blocks with complementary properties through the art of stacking engineering. The promising outcomes achieved for heterostructures have spurred an inquisitive exploration of homostructures, where identical 2D layers are precisely stacked. This perspective primarily focuses on the field of stacking engineering within layered homostructures, where precise control over translational or rotational degrees of freedom between vertically stacked planes or layers is paramount. In particular, we provide an overview of recent advancements in the stacking engineering applied to 2D homostructures. Additionally, we will shed light on research endeavors venturing into three-dimensional (3D) structures, which allow us to proactively address the limitations associated with artificial 2D homostructures. We anticipate that the breakthroughs in stacking engineering in 3D materials will provide valuable insights into the mechanisms governing stacking effects. Such advancements have the potential to unlock the full capability of artificial layered homostructures, propelling the future development of materials, physics, and device applications.

Unusual magnetic interaction in CrTe: insights from machine-learning and empirical models

Chromium telluride (CrTe) has received much attention due to its small magnetic anisotropy, which hosts the potential for complex magnetic structures. However, its magnetic properties have been relatively unexplored with numerical simulations, as the magnetic interactions inside are quite unusual. In this study, we employ both a machine-learning model and an empirical model to investigate the magnetic phase transitions of bulk and monolayer CrTe, revealing the existence of unusual magnetic interaction, which can be captured by the machine-learning model but not the simple empirical model. Furthermore, our results also demonstrate that magnetic moments further apart exhibit stronger interactions than those in closer proximity, deviating from typical behavior.

Robust Two-Dimensional Ferromagnetism in Cr5Te8/CrTe2 Heterostructure with Curie Temperature above 400 K

The discovery of ferromagnetism in two-dimensional (2D) van der Waals crystals has generated widespread interest. The seeking of robust 2D ferromagnets with high Curie temperature (Tc) is vitally important for next-generation spintronic devices. However, owing to the enhanced spin fluctuation and weak exchange interaction upon the reduced dimensionalities, the exploring of robust 2D ferromagnets with Tc > 300 K is highly demanded but remains challenging. In this work, we fabricated air-stable 2D Cr5Te8/CrTe2 vertical heterojunctions with Tc above 400 K by the chemical vapor deposition method. Transmission electron microscopy demonstrates a high-quality-crystalline epitaxial structure between tri-Cr5Te8 and 1T-CrTe2 with striped moiré patterns and a superior ambient stability over six months. A built-in dual-axis strain together with strong interfacial coupling cooperatively leads to a record-high Tc for the CrxTey family. A temperature-dependent spin-flip process induces the easy axis of magnetization to rotate from the out-of-plane to the in-plane direction, indicating a phase-dependent proximity coupling effect, rationally interpreted by first-principles calculations of the magnetic anisotropy of a tri-Cr5Te8 and 1T-CrTe2 monolayer. Our results provide a material realization of effectively enhancing the transition temperature of 2D ferromagnetism and manipulating the spin-flip of the easy axis, which will facilitate future spintronic applications.

Recent Advances in 2D Material Theory, Synthesis, Properties, and Applications

Two-dimensional (2D) material research is rapidly evolving to broaden the spectrum of emergent 2D systems. Here, we review recent advances in the theory, synthesis, characterization, device, and quantum physics of 2D materials and their heterostructures. First, we shed insight into modeling of defects and intercalants, focusing on their formation pathways and strategic functionalities. We also review machine learning for synthesis and sensing applications of 2D materials. In addition, we highlight important development in the synthesis, processing, and characterization of various 2D materials (e.g., MXnenes, magnetic compounds, epitaxial layers, low-symmetry crystals, etc.) and discuss oxidation and strain gradient engineering in 2D materials. Next, we discuss the optical and phonon properties of 2D materials controlled by material inhomogeneity and give examples of multidimensional imaging and biosensing equipped with machine learning analysis based on 2D platforms. We then provide updates on mix-dimensional heterostructures using 2D building blocks for next-generation logic/memory devices and the quantum anomalous Hall devices of high-quality magnetic topological insulators, followed by advances in small twist-angle homojunctions and their exciting quantum transport. Finally, we provide the perspectives and future work on several topics mentioned in this review.

Growth, structure, and morphology of van der Waals epitaxy Cr1+δTe2 films

The preparation of two-dimensional magnetic materials is a key process to their applications and the study of their structure and morphology plays an important role in the growth of high-quality thin films. Here, the growth, structure, and morphology of Cr_1+δTe₂ films grown by molecular beam epitaxy on mica with variations of Te/Cr flux ratio, growth temperature, and film thickness have been systematically investigated by scanning tunneling microscopy, reflection high-energy electron diffraction, scanning electron microscope, and X-ray photoelectron spectroscopy. We find that a structural change from multiple phases to a single phase occurs with the increase in growth temperature, irrespective of the Cr/Te flux ratios, which is attributed to the desorption difference of Te atoms at different temperatures, and that the surface morphology of the films grown at relatively high growth temperatures (≥ 300 °C) exhibits a quasi-hexagonal mesh-like structure, which consists of nano-islands with bending surface induced by the screw dislocations, as well as that the films would undergo a growth-mode change from 2D at the initial stage in a small film thickness (2 nm) to 3D at the later stage in thick thicknesses (12 nm and 24 nm). This work provides a general model for the study of pseudo-layered materials grown on flexible layered substrates.

Atomic scale insights into the epitaxial growth mechanism of 2D Cr3Te4 on mica

Two-dimensional (2D) magnetic materials are of wide research interest owing to their promising applications in spintronic devices. Among them, chromium chalcogenide compounds are some of the limited available systems that present both high stability in air and high Curie temperatures. Epitaxial growth techniques based on chemical vapour deposition (CVD) have been demonstrated to be a robust method for growing 2D non-layered chromium chalcogenides. However, the growth mechanism is not well-understood. Here, we demonstrate the epitaxial growth of Cr₃Te₄ nanoplates with high quality on mica. Atomic-resolution scanning transmission electron microscopy (STEM) imaging reveals that the epitaxial growth is based on nanosized chromium oxide seed particles at the interface of Cr₃Te₄ and mica. The chromium oxide nanoparticle exhibits a coherent interface with both mica and Cr₃Te₄ with a lattice mismatch within 3%, suggesting that, as a buffer layer, chromium oxide can release the interfacial strain, and induce the growth of Cr₃Te₄ although there is a distinct oxygen-content difference between mica and Cr₃Te₄. This work provides an experimental understanding behind the epitaxial growth of 2D magnetic materials at the atomic scale and facilitates the improvement of their growth procedures for devices with high crystalline quality.

Anomalous displacement reaction for synthesizing above-room-temperature and air-stable vdW ferromagnet PtTe2Ge1/3

Emerging van der Waals (vdW) magnets provide a paradise for the exploration of magnetism in the ultimate two-dimensional (2D) limit, and the construction of integrated spintronic devices, and have become a research frontier in the field of low-dimensional materials. To date, prototypical vdW magnets based on metals of the first transition series (e.g. V, Cr, Mn and Fe) and chalcogen elements suffer from rapid oxidation restricted by the Hard-Soft-Acid-Base principle, as well as low Curie temperatures (T C), which has become a generally admitted challenge in 2D spintronics. Here, starting from air-unstable Cr2Ge2Te6 vdW thin flakes, we synthesize Ge-embedded PtTe2 (namely PtTe2Ge1/3) with superior air stability, through the displacement reaction in the Cr2Ge2Te6/Pt bilayer. In this process, the anomalous substitution of Cr with Pt in the thermal diffusion is inverse to the metal activity order, which can be attributed to the compatibility between soft-acid (Pt) and soft-base (Te) elements. Meanwhile, the layered uniform insertion of Ge unbalances Pt-Te bonds and introduces long-range ordered ferromagnetism with perpendicular magnetic anisotropy and a Curie temperature above room temperature. Our work demonstrates the anti-metal-activity-order reaction tendency unique in 2D transition-metal magnets and boosts progress towards practical 2D spintronics.

Varied Magnetic Phases in a van der Waals Easy-Plane Antiferromagnet Revealed by Nitrogen-Vacancy Center Microscopy

Interest in van der Waals materials often stems from a desire to miniaturize existing technologies by exploiting their intrinsic layered structures to create near-atomically thin components that do not suffer from surface defects. One appealing property is an easily switchable yet robust magnetic order, which is only sparsely demonstrated in the case of in-plane anisotropy. In this work, we use widefield nitrogen-vacancy (NV) center magnetic imaging to measure the properties of individual flakes of CuCrP₂S₆, a multiferroic van der Waals magnet known to exhibit weak easy-plane anisotropy in the bulk. We chart the crossover between the in-plane ferromagnetism in thin flakes down to the trilayer and the bulk behavior dominated by a low-field spin-flop transition. Further, by exploiting the directional dependence of NV center magnetometry, we are able to observe an instance of a predominantly out-of-plane ferromagetic phase near zero field, in contrast with our expectation and previous experiments on the bulk material. We attribute this to the presence of surface anisotropies caused by the sample preparation process or exposure to the ambient environment, which is expected to have more general implications for a broader class of weakly anisotropic van der Waals magnets.

Observation of Néel-type skyrmions in acentric self-intercalated Cr1+δTe2

Transition-metal dichalcogenides intercalated with 3d-transition metals within the van der Waals (vdW) gaps have been the focus of intense investigations owing to their fascinating structural and magnetic properties. At certain concentrations the intercalated atoms form ordered superstructures that exhibit ferromagnetic or anti-ferromagnetic ordering. Here we show that the self-intercalated compound Cr_1+δTe₂ with δ ≈ 0.3 exhibits a new, so far unseen, three-dimensionally ordered (2×2×2) superstructure. Furthermore, high resolution X-ray diffraction reveals that there is an asymmetric occupation of the two inequivalent vdW gaps in the unit cell. The structure thus lacks inversion symmetry, which, thereby, allows for chiral non-collinear magnetic nanostructures. Indeed, Néel-type skyrmions are directly observed using Lorentz transmission electron microscopy. The skyrmions are stable within the accessible temperature range (100–200 K) as well as in zero magnetic field. The diameter of the Néel skyrmions increases with lamella thickness and varies with applied magnetic field, indicating the role of long-range dipole fields. Our studies show that self-intercalation in vdW materials is a novel route to the formation of synthetic non-collinear spin textures.

Here, Saha et al. show that self-intercalation of e₂Cr atoms in CrTe₂ create an asymmetry in the number of atoms intercalated in the van der Waals gaps between the layers of CrTe₂. This inversion symmetry breaking leads to non-collinear spin-textures and Néel-type magnetic skyrmions over a wide temperature range.

Quasi-1D van der Waals Antiferromagnetic CrZr4 Te14 with Large In-Plane Anisotropic Negative Magnetoresistance

The discovery of 2D van der Waals (vdW) magnetic materials is of great significance to explore intriguing 2D magnetic physics and develop innovative spintronic devices. In this work, a new quasi-1D vdW layered compound CrZr₄ Te₁₄ is successfully synthesized. Owing to the existence of 1D [CrTe₂ ] and [ZrTe₃ ] chains along the b-axis, CrZr₄ Te₁₄ crystals show strong anisotropy of phonon vibrations, electrical transport, and magnetism. Density functional theory calculations reveal the ferromagnetic (FM) coupling within the [CrTe₂ ] chain, while the interchain and interlayer couplings are both weakly antiferromagnetic (AF). Notably, a large intrinsic negative magnetoresistance (nMR) of -56% is achieved at 2 K under 9 T, and the in-plane anisotropic factor of nMR can reach up to 8.2 in the CrZr₄ Te₁₄ device. The 1D FM chains and anisotropic nMR effect make CrZr₄ Te₁₄ an interesting platform for exploring novel polarization-sensitive spintronics.

Ultrathin Van der Waals Antiferromagnet CrTe3 for Fabrication of In-Plane CrTe3 /CrTe2 Monolayer Magnetic Heterostructures

Ultrathin van der Waals (vdW) magnets are heavily pursued for potential applications in developing high-density miniaturized electronic/spintronic devices as well as for topological physics in low-dimensional structures. Despite the rapid advances in ultrathin ferromagnetic vdW magnets, the antiferromagnetic counterparts, as well as the antiferromagnetic junctions, are much less studied owing to the difficulties in both material fabrication and magnetism characterization. Ultrathin CrTe₃ layers have been theoretically proposed to be a vdW antiferromagnetic semiconductor with intrinsic intralayer antiferromagnetism. Herein, the epitaxial growth of monolayer (ML) and bilayer CrTe₃ on graphite surface is demonstrated. The structure, electronic and magnetic properties of the ML CrTe₃ are characterized by combining scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy and confirmed by density functional theory calculations. The CrTe₃ MLs can be further utilized for the fabrication of a lateral heterojunction consisting of ML CrTe₂ and ML CrTe₃ with an atomically sharp and seamless interface. Since ML CrTe₂ is a metallic vdW magnet, such a heterostructure presents the first in-plane magnetic metal-semiconductor heterojunction made of two vdW materials. The successful fabrication of ultrathin antiferromagnetic CrTe₃ , as well as the magnetic heterojunction, will stimulate the development of miniaturized antiferromagnetic spintronic devices based on vdW materials.

ZrTe2/CrTe2: an epitaxial van der Waals platform for spintronics

The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, magnetism, and superconductivity. In this context, the epitaxial synthesis of vdW heterostructures with well-controlled interfaces is an attractive route towards wafer-scale platforms for systematically exploring fundamental properties and fashioning proof-of-concept devices. Here, we use molecular beam epitaxy to synthesize a vdW heterostructure that interfaces two material systems of contemporary interest: a 2D ferromagnet (1T-CrTe2) and a topological semimetal (ZrTe2). We find that one unit-cell (u.c.) thick 1T-CrTe2 grown epitaxially on ZrTe2 is a 2D ferromagnet with a clear anomalous Hall effect. In thicker samples (12 u.c. thick CrTe2), the anomalous Hall effect has characteristics that may arise from real-space Berry curvature. Finally, in ultrathin CrTe2 (3 u.c. thickness), we demonstrate current-driven magnetization switching in a full vdW topological semimetal/2D ferromagnet heterostructure device.

The Magnetic Genome of Two-Dimensional van der Waals Materials

Magnetism in two-dimensional (2D) van der Waals (vdW) materials has recently emerged as one of the most promising areas in condensed matter research, with many exciting emerging properties and significant potential for applications ranging from topological magnonics to low-power spintronics, quantum computing, and optical communications. In the brief time after their discovery, 2D magnets have blossomed into a rich area for investigation, where fundamental concepts in magnetism are challenged by the behavior of spins that can develop at the single layer limit. However, much effort is still needed in multiple fronts before 2D magnets can be routinely used for practical implementations. In this comprehensive review, prominent authors with expertise in complementary fields of 2D magnetism (i.e., synthesis, device engineering, magneto-optics, imaging, transport, mechanics, spin excitations, and theory and simulations) have joined together to provide a genome of current knowledge and a guideline for future developments in 2D magnetic materials research.

Van der Waals Superstructure and Twisting in Self-Intercalated Magnet with Near Room-Temperature Perpendicular Ferromagnetism

The emergence of van der Waals (vdW) magnets has created unprecedented opportunities to manipulate magnetism for advanced spintronics based upon all-vdW heterostructures. Among various vdW magnets, Cr_1+δTe₂ possesses high temperature ferromagnetism along with possible topological spin textures. As this system can support self-intercalation in the vdW gap, it is crucial to precisely pinpoint the exact intercalation to understand the intrinsic magnetism of the system. Here, we developed an iterative method to determine the self-intercalated structures and show evidence of vdW "superstructures" in individual Cr_1+δTe₂ nanoplates exhibiting magnetic behaviors distinct from bulk chromium tellurides. Among 26,332 possible configurations, we unambiguously identified the Cr-intercalated structure as 3-fold symmetry broken Cr_1.5Te₂ segmented by vdW gaps. Moreover, a twisted Cr-intercalated layered structure is observed. The spontaneous formation of twisted vdW "superstructures" not only provides insight into the diverse magnetic properties of intercalated vdW magnets but may also add complementary building blocks to vdW-based spintronics.

Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films

While the discovery of two-dimensional (2D) magnets opens the door for fundamental physics and next-generation spintronics, it is technically challenging to achieve the room-temperature ferromagnetic (FM) order in a way compatible with potential device applications. Here, we report the growth and properties of single- and few-layer CrTe₂, a van der Waals (vdW) material, on bilayer graphene by molecular beam epitaxy (MBE). Intrinsic ferromagnetism with a Curie temperature (T_C) up to 300 K, an atomic magnetic moment of ~0.21 [Formula: see text]/Cr and perpendicular magnetic anisotropy (PMA) constant (K_u) of 4.89 × 10⁵ erg/cm³ at room temperature in these few-monolayer films have been unambiguously evidenced by superconducting quantum interference device and X-ray magnetic circular dichroism. This intrinsic ferromagnetism has also been identified by the splitting of majority and minority band dispersions with ~0.2 eV at Г point using angle-resolved photoemission spectroscopy. The FM order is preserved with the film thickness down to a monolayer (T_C ~ 200 K), benefiting from the strong PMA and weak interlayer coupling. The successful MBE growth of 2D FM CrTe₂ films with room-temperature ferromagnetism opens a new avenue for developing large-scale 2D magnet-based spintronics devices.

Magnetic domains and domain wall pinning in atomically thin CrBr3 revealed by nanoscale imaging

The emergence of atomically thin van der Waals magnets provides a new platform for the studies of two-dimensional magnetism and its applications. However, the widely used measurement methods in recent studies cannot provide quantitative information of the magnetization nor achieve nanoscale spatial resolution. These capabilities are essential to explore the rich properties of magnetic domains and spin textures. Here, we employ cryogenic scanning magnetometry using a single-electron spin of a nitrogen-vacancy center in a diamond probe to unambiguously prove the existence of magnetic domains and study their dynamics in atomically thin CrBr₃. By controlling the magnetic domain evolution as a function of magnetic field, we find that the pinning effect is a dominant coercivity mechanism and determine the magnetization of a CrBr₃ bilayer to be about 26 Bohr magnetons per square nanometer. The high spatial resolution of this technique enables imaging of magnetic domains and allows to locate the sites of defects that pin the domain walls and nucleate the reverse domains. Our work highlights scanning nitrogen-vacancy center magnetometry as a quantitative probe to explore nanoscale features in two-dimensional magnets.

Van der Waals (vdW) magnets have allowed researchers to explore the two dimensional limit of magnetisation; however experimental challenges have hindered analysis of magnetic domains. Here, using an NV centre based probe, the authors analyse the nature of magnetic domains in the vdW magnet, CrBr₃.

Atomic Wires of Transition Metal Chalcogenides: A Family of 1D Materials for Flexible Electronics and Spintronics

As analogues of two-dimensional (2D) layered materials, searching for one-dimensional (1D) van der Waals wired materials as 1D Lego blocks for integration and device applications has been pursued. Motivated by the recently synthesized atomic wires of molybdenum chalcogenide, here we explored the structures and stability of 66 atomic wires of 3d, 4d, and 5d transition metal chalcogenides in the M₆X₆ stoichiometry (M = transition metal, X = chalcogen). After high-throughput first-principles calculations, 53 unprecedented and experimentally feasible M₆X₆ wires have been identified. Diverse functionalities are found in these 1D materials, including semiconductors, metals, and ferromagnets with high Young's modulus and large fracture strain. Notably, six kinds of M₆X₆ wires are robust ferromagnets with Curie temperatures up to 700 K, which can be further elevated under axial strains. Moreover, these M₆X₆ atomic wires possess high stability and resistance to oxidation, humidity, and aggregation; both merits are desirable for device applications. This large family of 1D materials with definite structures and rich properties allows atomically precise integration for flexible electronics and spintronics.

Anomalous thickness dependence of Curie temperature in air-stable two-dimensional ferromagnetic 1T-CrTe2 grown by chemical vapor deposition

The discovery of ferromagnetic two-dimensional van der Waals materials has opened up opportunities to explore intriguing physics and to develop innovative spintronic devices. However, controllable synthesis of these 2D ferromagnets and enhancing their stability under ambient conditions remain challenging. Here, we report chemical vapor deposition growth of air-stable 2D metallic 1T-CrTe₂ ultrathin crystals with controlled thickness. Their long-range ferromagnetic ordering is confirmed by a robust anomalous Hall effect, which has seldom been observed in other layered 2D materials grown by chemical vapor deposition. With reducing the thickness of 1T-CrTe₂ from tens of nanometers to several nanometers, the easy axis changes from in-plane to out-of-plane. Monotonic increase of Curie temperature with the thickness decreasing from ~130.0 to ~7.6 nm is observed. Theoretical calculations indicate that the weakening of the Coulomb screening in the two-dimensional limit plays a crucial role in the change of magnetic properties.

Here, the authors report chemical vapor deposition growth of metallic 1T-CrTe₂ ultrathin crystals with controlled thickness and long-range ferromagnetic ordering, and observe a monotonic increase of the Curie temperature with decreasing thickness.

Synthesis of emerging 2D layered magnetic materials

van der Waals atomically thin magnetic materials have been recently discovered. They have attracted enormous attention as they present unique magnetic properties, holding potential to tailor spin-based device properties and enable next generation data storage and communication devices. To fully understand the magnetism in two-dimensions, the synthesis of 2D materials over large areas with precise thickness control has to be accomplished. Here, we review the recent advancements in the synthesis of these materials spanning from metal halides, transition metal dichalcogenides, metal phosphosulphides, to ternary metal tellurides. We initially discuss the emerging device concepts based on magnetic van der Waals materials including what has been achieved with graphene. We then review the state of the art of the synthesis of these materials and we discuss the potential routes to achieve the synthesis of wafer-scale atomically thin magnetic materials. We discuss the synthetic achievements in relation to the structural characteristics of the materials and we scrutinise the physical properties of the precursors in relation to the synthesis conditions. We highlight the challenges related to the synthesis of 2D magnets and we provide a perspective for possible advancement of available synthesis methods to respond to the need for scalable production and high materials quality.

Tunable Tunneling Magnetoresistance in van der Waals Magnetic Tunnel Junctions with 1-CrTe2 Electrodes

Two-dimensional (2D) van der Waals (vdW) heterostructures have opened new avenues for spintronic applications with novel properties. Here, by density functional theory calculations, we investigated the spin-dependent transport in vdW magnetic tunnel junctions (MTJs) composed of 1T-CrTe₂ ferromagnetic electrodes. Meanwhile, graphene and h-BN are employed as tunnel barriers. It has been found that the tunneling magnetoresistance (TMR) effects of two types of vdW MTJs present analogous trends: thicknesses of barriers have a great influence on the TMR ratios, which reach up to the maximum when barriers increase to five monolayers. However, despite the similarity, the graphene-barrier junction is more promising for optimization. Through observing the energy-resolved transmission spectra of vdW MTJs, we noticed that TMR ratios of graphene-barrier junctions are tunable and could be enhanced through tuning the position of Fermi energy. Therefore, we successfully realized the TMR optimization by substitutional doping. When substituting one carbon atom with one boron atom in the graphene barrier, TMR ratios are drastically improved, and a TMR ratio as high as 6962% could be obtained in the doped seven-monolayer-barrier junction. Our results pave the way for vdW MTJ applications in spintronics.