Magnetic Van Der Waals Materials Research Articles

Progress and challenges in the synthesis of two-dimensional Van der Waals ferroic materials and heterostructures

Abstract Two-dimensional ferroelectric and magnetic van der Waals materials are emergent platforms for the discovery of novel cooperative quantum phenomena and development of energy-efficient logic and memory applications as well as neuromorphic and topological computing. This review presents a comprehensive survey of the rapidly growing 2D ferroic family from the synthesis perspective, including brief introductions to the top-down and bottom-up approaches for fabricating 2D ferroic flakes, thin films, and heterostructures as well as the important characterization techniques for assessing the sample properties. We also discuss the key challenges and future directions of the field, including scalable growth, property control, sample stability, and integration with other functional materials.

Enhanced magnetism derived from pore-edge spins in thin Fe3GeTe2 nanomeshes

The growth of two-dimensional van der Waals magnetic materials presents attractive opportunities for exploring new physical phenomena and valuable applications. Among these materials, Fe3GeTe2(FGT) exhibits a variety of remarkable properties and has garnered significant attention. Herein, we have for the first time created a nanomesh structure-a honeycomb-like array of hexagonal nanopores-with the zigzag pore-edge atomic structure on thin FGT flakes with and without oxidation of the pore edges. It is revealed that the magnitude of ferromagnetism (FM) significantly increases in both samples compared with bulk flakes without nanomeshes. Critical temperature annealing results in the formation of zigzag pore edges and interpore zigzag-edge nanoribbons. We unveil that the non-oxide (O) termination of the Fe dangling bonds on these zigzag edges enhances FM behavior, while O-termination suppresses this FM by introducing antiferromagnetic behavior through edge O-Fe coupling. FGT nanomeshes hold promise for the creation of strong FM and their effective application in magnetic and spintronic systems.

Magnon Sensing of NO, NO2 and NH3 Gas Capture on CrSBr Monolayer.

Air pollution and greenhouse emissions are significant problems across various sectors, urging the need for advanced technologies to detect and capture harmful gases. In recent years, two-dimensional (2D) materials have attracted increasing attention due to their large surface-to-volume ratio and reactivity. Herein, we investigate the potential of single-layer CrSBr for gas sensing and capturing by means of first-principles calculations. We explore the adsorption behaviour of different pollutant gases (H2S, NH3, NO, NO2, CO and CO2) on this 2D ferromagnet and the impact of intrinsic defects on its magnetic properties. Interestingly, we find that Br vacancies enhance the adsorption of NH3, NO and NO2 and induce a selective frequency shift on the magnon dispersion. This work motivates the creation of novel magnonic gas sensing devices based on 2D van der Waals magnetic materials.

Magnetically propagating Hund’s exciton in van der Waals antiferromagnet NiPS3

Magnetic van der Waals (vdW) materials have opened new frontiers for realizing novel many-body phenomena. Recently NiPS3 has received intense interest since it hosts an excitonic quasiparticle whose properties appear to be intimately linked to the magnetic state of the lattice. Despite extensive studies, the electronic character, mobility, and magnetic interactions of the exciton remain unresolved. Here we address these issues by measuring NiPS3 with ultra-high energy resolution resonant inelastic x-ray scattering (RIXS). We find that Hund’s exchange interactions are primarily responsible for the energy of formation of the exciton. Measuring the dispersion of the Hund’s exciton reveals that it propagates in a way that is analogous to a double-magnon. We trace this unique behavior to fundamental similarities between the NiPS3 exciton hopping and spin exchange processes, underlining the unique magnetic characteristics of this novel quasiparticle.

Topological magnon–polaron transport in a bilayer van der Waals magnet

The stacking of intrinsically magnetic van der Waals materials provides a fertile platform to explore tunable transport effects of magnons, presenting significant prospects for spintronic applications. The possibility of having topologically nontrivial magnons in these systems can further expand the scope of exploration. In this work, we consider a bilayer system with intralayer ferromagnetic exchange and a weak interlayer antiferromagnetic exchange and study the topological magnon-polaron excitations induced by magnetoelastic couplings. Under an applied magnetic field, the system features a metamagnetic transition, where the magnetic ground state changes from antiparallel layers to parallel. We show that the metamagnetic transition is accompanied by a transition of the topological structure of the magnon polarons, which results in discernible changes in the topology induced transport effects. The magnetic-field dependence of the thermal Hall conductivity and spin Nernst coefficient is analyzed with linear response theories.

Orbital-Ordering Driven Simultaneous Tunability of Magnetism and Electric Polarization in Strained Monolayer VCl3

Two-dimensional (2D) van der Waals magnetic materials have promising and versatile electronic and magnetic properties in the 2D limit, indicating a considerable potential to advance spintronic applications. Theoretical predictions thus far have not ascertained whether monolayer VCl3 is a ferromagnetic (FM) or anti-FM monolayer; this also remains to be experimentally verified. We theoretically investigate the influence of potential factors, including C 3 symmetry breaking, orbital ordering, epitaxial strain, and charge doping, on the magnetic ground state. Utilizing first-principles calculations, we predict a collinear type-III FM ground state in monolayer VCl3 with a broken C 3 symmetry, wherein only the former two of three t 2g orbitals (a 1g, eg2π and eg1π ) are occupied. The atomic layer thickness and bond angles of monolayer VCl3 undergo abrupt changes driven by an orbital ordering switch, resulting in concomitant structural and magnetic phase transitions. Introducing doping to the underlying Cl atoms of monolayer VCl3 without C 3 symmetry simultaneously induces in- and out-of-plane polarizations. This can achieve a multiferroic phase transition if combined with the discovered adjustments of magnetic ground state and polarization magnitude under strain. The establishment of an orbital-ordering driven regulatory mechanism can facilitate deeper exploration and comprehension of magnetic properties of strongly correlated systems in monolayer VCl3.

Highly Efficient Room-Temperature Nonvolatile Magnetic Switching by Current in Fe3GaTe2 Thin Flakes.

Effectively tuning magnetic state by using current is essential for novel spintronic devices. Magnetic van der Waals (vdW) materials have shown superior properties for the applications of magnetic information storage based on the efficient spin torque effect. However, for most of known vdW ferromagnets, the ferromagnetic transition temperatures lower than room temperature strongly impede their applications and the room-temperature vdW spintronic device with low energy consumption is still a long-sought goal. Here, the highly efficient room-temperature nonvolatile magnetic switching is realized by current in a single-material device based on vdW ferromagnet Fe3GaTe2. Moreover, the switching current density and power dissipation are about 300 and 60000 times smaller than conventional spin-orbit-torque devices of magnet/heavy-metal heterostructures. These findings make an important progress on the applications of magnetic vdW materials in the fields of spintronics and magnetic informationstorage.

Magnetism of MoS2 bilayers with intercalated and surface adsorbed Fe

Magnetic thin films composed of two-dimensional van der Waals materials will play an important role in future spintronics devices. Recently, Tuli et al. performed in-situ MOKE measurements of Fe deposited on an MoS2 substrate and found that a magnetic dead layer (MDL) exists up to approximately 2 Å (Tuli et al. (2021)). To understand the origin of the MDL, we investigate MoS2 bilayers with intercalated and surface adsorbed Fe using first-principles calculations. The results show that the Fe-intercalated bilayer MoS2 is more stable than the bilayer MoS2 with surface adsorbed Fe. The Fe-intercalated bilayer MoS2 is non-magnetic and behaves as the MDL. The systematic investigation of variation in the inter-layer distance between the intercalated Fe layer and surface of MoS2 layer reveals that the strong hybridization between Fe and S is the origin of the MDL. We also show that further intercalation of Fe makes the film magnetic. The results provide a deep understanding of the MDL in two-dimensional van der Waals magnetic materials.

Current-induced switching of a van der Waals ferromagnet at room temperature

Recent discovery of emergent magnetism in van der Waals magnetic materials (vdWMM) has broadened the material space for developing spintronic devices for energy-efficient computation. While there has been appreciable progress in vdWMM discovery, a solution for non-volatile, deterministic switching of vdWMMs at room temperature has been missing, limiting the prospects of their adoption into commercial spintronic devices. Here, we report the first demonstration of current-controlled non-volatile, deterministic magnetization switching in a vdW magnetic material at room temperature. We have achieved spin-orbit torque (SOT) switching of the PMA vdW ferromagnet Fe3GaTe2 using a Pt spin-Hall layer up to 320 K, with a threshold switching current density as low as Jsw=\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${J}_{{{{{{\\rm{sw}}}}}}}=$$\\end{document}1.69 ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes$$\\end{document} 106 A cm−2 at room temperature. We have also quantitatively estimated the anti-damping-like SOT efficiency of our Fe3GaTe2/Pt bilayer system to be ξDL=0.093\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\xi }_{{{{{{\\rm{DL}}}}}}}=0.093$$\\end{document}, using the second harmonic Hall voltage measurement technique. These results mark a crucial step in making vdW magnetic materials a viable choice for the development of scalable, energy-efficient spintronic devices.

Understanding and Tuning Magnetism in Layered Ising‐Type Antiferromagnet FePSe3 for Potential 2D Magnet

AbstractRecent developments in 2D magnetic materials have motivated the search for new van der Waals magnetic materials, especially Ising‐type magnets with strong magnetic anisotropy. Fe‐based MPX3 (M = transition metal, X = chalcogen) compounds such as FePS3 and FePSe3 both exhibit an Ising‐type magnetic order, but FePSe3 receives much less attention compared to FePS3. This work focuses on establishing the strategy to engineer magnetic anisotropy and exchange interactions in this less‐explored compound. Through chalcogen and metal substitutions, the magnetic anisotropy is found to be immune against S substitution for Se whereas tunable only with heavy Mn substitution for Fe. In particular, Mn substitution leads to a continuous rotation of magnetic moments from the out‐of‐plane direction toward the in‐plane. Furthermore, the magnetic ordering temperature displays non‐monotonic doping dependence for both chalcogen and metal substitutions but due to different mechanisms. These findings provide deeper insight into the Ising‐type magnetism in this important van der Waals material, shedding light on the study of other Ising‐type magnetic systems as well as discovering novel 2D magnets for potential applications in spintronics.

Glovebox-assisted magnetic force microscope for studying air-sensitive samples in a cryogen-free magnet.

Most known two-dimensional magnets exhibit a high sensitivity to air, making direct characterization of their domain textures technically challenging. Herein, we report on the construction and performance of a glovebox-assisted magnetic force microscope (MFM) operating in a cryogen-free magnet, realizing imaging of the intrinsic magnetic structure of water and oxygen-sensitive materials. It features a compact tubular probe for a 50 mm-diameter variable temperature insert installed in a 12T cryogen-free magnet. A detachable sealing chamber can be electrically connected to the tail of the probe, and its pump port can be opened and closed by a vacuum manipulator located on the top of the probe. This sealing chamber enables sample loading and positioning in the glove box and MFM transfer to the magnet maintained in an inert gas atmosphere (in this case, argon and helium gas). The performance of the MFM is demonstrated by directly imaging the surface (using no buffer layer, such as h-BN) of very air-sensitive van der Waals magnetic material chromium triiodide (CrI3) samples at low temperatures as low as 5K and high magnetic fields up to 11.9T. The system's adaptability permits replacing the MFM unit with a scanning tunneling microscope unit, enabling high-resolution atomic imaging of air-sensitive surface samples.

Near-room temperature ferromagnetism and a tunable anomalous Hall effect in atomically thin Fe4CoGeTe2.

Itinerant ferromagnetism at room temperature is a key factor for spin transport and manipulation. Here, we report the realization of near-room temperature itinerant ferromagnetism in Co doped Fe5GeTe2 thin flakes. The ferromagnetic transition temperature TC (∼323 K-337 K) is almost unchanged when the thickness is as low as 12 nm and is still about 284 K at 2 nm (bilayer thickness). Theoretical calculations further indicate that the ferromagnetism persists in monolayer Fe4CoGeTe2. In addition to the robust ferromagnetism down to the ultrathin limit, Fe4CoGeTe2 exhibits an unusual temperature- and thickness-dependent intrinsic anomalous Hall effect. We propose that it could be ascribed to the dependence of the band structure on thickness that changes the Berry curvature near the Fermi energy level subtly. The near-room temperature ferromagnetism and tunable anomalous Hall effect in atomically thin Fe4CoGeTe2 provide opportunities to understand the exotic transport properties of two-dimensional van der Waals magnetic materials and explore their potential applications in spintronics.

Efficient Thermo-Spin Conversion in van der Waals Ferromagnet FeGaTe.

Recent discovery of 2D van der Waals magnetic materials has spurred progress in developing advanced spintronic devices. A central challenge lies in enhancing the spin-conversion efficiency for building spin-logic or spin-memory devices. Here, the anomalous Hall and Nernst effects are systematically investigated to uncover significant spin-conversion effects in above-room-temperature van der Waals ferromagnet FeGaTe with perpendicular magnetic anisotropy. The anomalous Hall effect demonstrates an efficient electric spin-charge conversion with a notable spin Hall angle of over 6%. In addition, the anomalous Nernst effect produces a significant transverse voltage at room temperature without a magnetic field, displaying unique temperature dependence with a maximum transverse Seebeck coefficient of 440 nVK-1 and a Nernst angle of ≈62%. Such an innovative thermoelectric signal arises from the efficient thermo-spin conversion effect, where the up-spin and down-spin electrons move in opposite directions under a temperature gradient. The present study highlights the potential of FeGaTe to enhance thermoelectric devices through efficient thermo-spin conversion without the need for a magneticfield.

Probing van der Waals magnetic surface and interface via circularly polarized X-rays

Advances in research of magnetic two-dimensional van der Waals (2D vdW) materials have opened up new opportunities in miniaturization of spintronic devices at the atomically thin limit. One major research thrust, which is the subject of this review, is that the magnetism of 2D vdW materials and their derived hetero-interface may be significantly affected by the local atomic geometry and environment. As a result, there is a pressing need for powerful advanced technique to characterize magnetic vdW surface and interface. Circularly polarized X-rays from synchrotron radiation light sources are key to this endeavor, as they can probe the microscopic magnetism of a specific element, for their atomic sensitivity, element-specificity, core-level excitation as well as capability of separation for the spin and orbit moments. We review a series of representative experimental achievements, exploiting circularly polarized X-rays, in (1) 2D vdW magnet, (2) magnetic metal/vdW interface, (3) molecule/vdW interface as well as (4) vdW/2D interface. On the basis of the review, advantages of probing magnetic vdW surface and interface by circularly polarized X-rays are presented, and challenges and opportunities are also discussed.

Intrinsic magnetic properties of the layered antiferromagnet CrSBr

van der Waals magnetic materials are an ideal platform to study low-dimensional magnetism. Opposed to other members of this family, the magnetic semiconductor CrSBr is highly resistant to degradation in air, which, in addition to its exceptional optical, electronic, and magnetic properties, is the reason the compound is receiving considerable attention at the moment. For many years, its magnetic phase diagram seemed to be well-understood. Recently, however, several groups observed a magnetic transition in magnetometry measurements at temperatures of around 40 K that is not expected from theoretical considerations, causing a debate about the intrinsic magnetic properties of the material. In this Letter, we report the absence of this particular transition in magnetization measurements conducted on high-quality CrSBr crystals, attesting to the extrinsic nature of the low-temperature magnetic phase observed in other works. Our magnetometry results obtained from large bulk crystals are in very good agreement with the magnetic phase diagram of CrSBr previously predicted by the mean-field theory; A-type antiferromagnetic order is the only phase observed below the Néel temperature at TN = 131 K. Moreover, numerical fits based on the Curie–Weiss law confirm that strong ferromagnetic correlations are present within individual layers even at temperatures much larger than TN.

Compositional engineering of magnetic anisotropy in Cr2SixGe2-xTe6

Magnetic van der Waals (vdW) materials are highly sensitive to their chemical compositions and atomic structures, which presents rich opportunities for synthetic control of vdW ferromagnets. Here, we synthesized the quaternary alloys Cr2SixGe2-xTe6 using the flux method and discovered that the Ge:Si source ratio should be designed deliberately higher than the expected in resultant crystals due to the stronger affinity of Si than Ge to be involved in Cr2SixGe2-xTe6 reactions. Temperature-dependent magnetization and magnetic hysteresis measurements revealed that as the Si content increases, the Curie temperature decreases while the out-of-plane anisotropy increases monotonically. When x increases from 0 to 2 in Cr2SixGe2-xTe6, the out-of-plane saturation fields remain approximately unchanged at ∼0.2 T, while the in-plane saturation fields increase monotonically from 0.5 T to 1.2 T. The distinct behaviors between out-of-plane and in-plane saturation fields arise from the different mechanisms underpinning the two fields – the out-of-plane saturation field is determined by the competition of exchange interaction, magnetic anisotropy, and dipolar interaction, whereas the in-plane saturation field by magnetic anisotropy. Our compositional engineering provides a fundamental understanding of the layered magnetic materials and insightful guidance for the future design of vdW magnets.

1D Magnetic MX3 Single-Chains (M = Cr, V and X = Cl, Br, I).

Magnetic materials in reduced dimensions are not only excellent platforms for fundamental studies of magnetism, but they play crucial roles in technological advances. The discovery of intrinsic magnetism in monolayer 2D van der Waals systems has sparked enormous interest, but the single-chain limit of 1D magnetic van der Waals materials has been largely unexplored. This paper reports on a family of 1D magnetic van der Waals materials with composition MX3 (M = Cr, V, and X = Cl, Br, I), prepared in fully-isolated fashion within the protective cores of carbon nanotubes. Atomic-resolution scanning transmission electron microscopy identifies unique structures that differ from the well-known 2D honeycomb lattice MX3 structure. Density functional theory calculations reveal charge-driven reversible magnetic phase transitions.

Metal–metal bonding, electronic excitations, and strong resonance Raman effect in 2D layered α-MoCl3

Covalent bonding between transition metal atoms is a common phenomenon in honeycomb lattices of layered materials, which strongly affects their electronic and magnetic properties. This work presents a detailed spectroscopic study of α-MoCl3, 2D van der Waals material with covalently bonded Mo2 dimers, with a particular focus on the Mo–Mo bonding. Raman spectra of α-MoCl3 were studied with multiple excitation laser lines chosen in different parts of the absorption spectrum, while polarization measurements aided in the symmetry assignment of the observed modes. Furthermore, far-IR measurements and (Density Functional Theory) DFT phonon computations were performed to complete vibrational assignment. Polarized absorption, PL, and photoelectron spectroscopy supported by DFT calculations were employed to understand the consequences of the Mo–Mo bonding for the electronic structure and the localization/delocalization balance in d3–d3 interactions. A coupling of dimerization-related structural and electronic properties was revealed in the strong resonance Raman enhancement of the Mo–Mo stretching mode at 153 cm−1 when the excitation laser matched the electronic transition between σ-bonding and antibonding orbitals of the Mo2 dimer (σ → σ*). The deeper understanding of the metal–metal bonding and identification of the vibrational and electronic spectroscopic signatures of the dimerization will be of great use for the studies of electron delocalization in magnetic van der Waals materials.

Real-time observation of magnetization and magnon dynamics in a two-dimensional topological antiferromagnet MnBi2Te4

Atomically thin van der Waals magnetic materials have not only provided a fertile playground to explore basic physics in the two-dimensional (2D) limit but also created vast opportunities for novel ultrafast functional devices. Here we systematically investigate ultrafast magnetization dynamics and spin wave dynamics in few-layer topological antiferromagnetic MnBi2Te4 crystals as a function of layer number, temperature, and magnetic field. We find laser-induced (de)magnetization processes can be used to accurately track the distinct magnetic states in different magnetic field regimes, including showing clear odd–even layer number effects. In addition, strongly field-dependent AFM magnon modes with tens of gigahertz frequencies are optically generated and directly observed in the time domain. Remarkably, we find that magnetization and magnon dynamics can be observed in not only the time-resolved magneto-optical Kerr effect but also the time resolved reflectivity, indicating strong correlation between the magnetic state and electronic structure. These measurements present the first comprehensive overview of ultrafast spin dynamics in this novel 2D antiferromagnet, paving the way for potential applications in 2D antiferromagnetic spintronics and magnonics as well as further studies of ultrafast control of both magnetization and topological quantum states.

Multiferroics in two dimensions: The coupling of mechanical, magnetic and ferroelectric subsystems in van der Waals materials

In this short review the coupling of various subsystems of crystal in van der Waals magnetic materials are considered: the magnetoelectric effect mediated by antisymmetric antiferromagnetic ordering, the ferrovalley-mediated coupling of antiferromagnetism and electric polarization as well as the effects induced by flexural deformations: the spin flexoelectric effect as a curvature induced Dzyaloshinskii-Moriya interaction and flexomagnetic effect as a curvature-induced decompensation of layer magnetization in antiferromagnetic bilayers.