2D Magnetism Research Articles

First-principles study of the magneto-Raman effect in van der Waals layered magnets

Magneto-Raman spectroscopy has been used to study spin-phonon coupling in two-dimensional (2D) magnets. Raman spectra of CrI3 show a strong dependence on the magnetic order within a layer and between the layers. Here we carry out the first systematic theoretical investigation of the magneto-Raman effect in 2D magnets by performing density functional theory calculations and developing a generalized polarizability model. Our first-principles simulations well reproduce experimental Raman spectra of CrI3 with different magnetic states. The model reveals how the change of spin orientation in each layer is coupled to the layer’s vibration to induce or eliminate the spin-dependent anti-symmetric off-diagonal terms in the Raman tensor for altering the selection rules. We also uncover that the correlation between phonon modes and magnetic orders is a universal phenomenon, which should exist in other phonon modes and 2D magnets. Our predictive simulations and modeling are expected to guide the research in 2D magnets.

The Electronic and Magnetic Properties of Hexagonal and Oblique Carbon Foam Surfaces

The recent observation of 2D magnetism in 3D graphene has generated considerable interest. Herein, three types of carbon foam (CF) models are proposed composed of hybridized carbon atoms ( and ), namely hexagonal A, oblique B, and oblique C. Each CF model can exhibit different surface shapes, showcasing either or terminations, with or without hydrogen saturation. The first‐principles calculations confirm that only the A model exhibits 2D surface magnetism. The A model with bare dangling carbon bond (A‐B) exhibits an antiferromagnetic ground state, while the ‐terminated A model under full H saturation or with bare dangling bonds on the surfaces (A‐S or A‐B) shows a ferromagnetic ground state. In addition, the electronic properties of A, B, and C structures are investigated. The research findings indicate that by modifying the model structure and terminal surfaces, it is possible to achieve a transition from metal to semiconductor. This unique surface magnetism of CFs represents a significant advancement in the field of 2D magnetism.

Medium‐Entropy Engineering of Magnetism in Layered Antiferromagnet CuxNi2(1‐x)CrxP2S6

AbstractAntiferromagnetic van der Waals‐type M2P2X6 compounds provide a versatile material platform for studying 2D magnetism and relevant phenomena. Establishing ferromagnetism in 2D materials is technologically valuable. Though magnetism is generally tunable via a chemical way, it is challenging to induce ferromagnetism with isovalent chalcogen and bimetallic substitutions in M2P2X6. Here, we report co‐substitution of Cu1+ and Cr3+ for Ni2+ in Ni2P2S6, creating CuxNi2(1‐x)CrxP2S6 medium‐entropy alloys spanning a full substitution range (x = 0 to 1). Such substitution strategy leads to a unique evolution in crystal structure and magnetic phases that are distinct from traditional isovalent bimetallic doping, with Cu and Cr co‐substitution enhancing ferromagnetic correlations and generating a weak ferromagnetic phase in intermediate compositions. This aliovalent substitution strategy offers a universal approach for tuning layered magnetism in antiferromagnetic systems, which along with the potential for light‐matter interaction and high‐temperature ferroelectricity, can enable multifunctional device applications.

Monolayer Magnetic Metal with Scalable Conductivity.

2D magnets have emerged as a class of materials highly promising for studies of quantum phenomena and applications in ultra-compact spintronics. Current research aims at design of 2D magnets with particular functional properties. A formidable challenge is to produce metallic monolayers: the material landscape of layered magnetic systems is strongly dominated by insulators; rare metallic magnets, such as Fe3GeTe2, become insulating as they approach the monolayer limit. Here, electron transport measurements demonstrate that the recently discovered 2D magnet GdAlSi - graphene-like AlSi layers coupled to layers of Gd atoms - remains metallic down to a single monolayer. Band structure analysis indicates the material to be an electride, which may stabilize the metallic state. Remarkably, the sheet conductance of 2D GdAlSi is proportional to the number of monolayers - a manifestation of scalable conductivity. The GdAlSi layers are epitaxially integrated with silicon, facilitating applications in electronics.

Coexisting Ferromagnetic-Antiferromagnetic State and Giant Anomalous Hall Effect in Chemical Vapor Deposition-Grown 2D Cr5Te8.

Two-dimensional (2D) magnets, as an important member of the 2D material family, have emerged as a promising platform for spintronic devices. Herein, we report the chemical vapor deposition (CVD) growth of highly crystalline submillimeter-scale self-intercalated metallic 2D ferromagnetic (FM) trigonal chromium telluride (Cr5Te8) flakes on inert mica substrates. Through magneto-optical and magnetotransport measurements, we unveil the exceptional magnetic properties of these 2D flakes. The trigonal Cr5Te8 flakes exhibit a strong anisotropic FM order with a Curie temperature above 220 K. Notably, an emergent antiferromagnetic (AFM) state is observed in the MOKE signal from ultrathin Cr5Te8 flakes around the Curie temperature. The AFM state has a relatively weak interlayer exchange coupling, allowing a switching between the interlayer AFM and FM states by tuning the temperature. Meanwhile, the trigonal Cr5Te8 flakes exhibit a giant anomalous Hall effect (AHE), with an anomalous Hall conductivity of 710 Ω-1 cm-1 and an anomalous Hall angle of 3.5% at zero magnetic field, surpassing typical itinerant ferromagnets. Further analysis suggests that the AHE in trigonal Cr5Te8 is primarily driven by the skew-scattering mechanism rather than the intrinsic or extrinsic side-jump mechanism. These findings demonstrate the potential of CVD-grown ultrathin Cr5Te8 flakes as a promising 2D magnetic material with exceptional AHE properties for future spintronic applications.

Stability of the magnetic subsystem of 2D magnets from the method of the crystal orbital hamilton population

Stability of the magnetic subsystem of 2D magnets from the method of the crystal orbital hamilton population

Bias voltage driven tunneling magnetoresistance polarity reversal in 2D stripy antiferromagnet CrOCl

Atomically thin materials with coupled magnetic and electric polarization are critical for developing energy-efficient and high-density spintronic devices, yet they remain scarce due to often conflicting requirements of stabilizing both magnetic and electric orders. The recent discovery of the magnetoelectric effect in the 2D stripy antiferromagnet CrOCl highlights this semiconductor as a promising platform to explore electric field effects on magnetoresistance. In this study, we systematically investigate the magnetoresistance in tunneling junctions of bilayer and monolayer CrOCl. We observe that the transition from antiferromagnetic to ferrimagnetic phases in both cases induces a positive magnetoresistance at low bias voltages, which reverses to a negative value at higher bias voltages. This polarity reversal is attributed to the additional electric dipoles present in the antiferromagnetic state, as supported by our theoretical calculations. These findings suggest a pathway for the electric control of spintronic devices and underscore the potential of 2D magnets like CrOCl in advancing energy-efficient spintronic applications.

Unconventional Anomalous Hall Effect Driven by Self-Intercalation in Covalent 2D Magnet Cr2Te3.

Covalent 2D magnets such as Cr2Te3, which feature self-intercalated magnetic cations located between monolayers of transition-metal dichalcogenide material, offer a unique platform for controlling magnetic order and spin texture, enabling new potential applications for spintronic devices. Here, it is demonstrated that the unconventional anomalous Hall effect (AHE) in Cr2Te3, characterized by additional humps and dips near the coercive field in AHE hysteresis, originates from an intrinsic mechanism dictated by the self-intercalation. This mechanism is distinctly different from previously proposed mechanisms such as topological Hall effect, or two-channel AHE arising from spatial inhomogeneities. Crucially, multiple Weyl-like nodes emerge in the electronic band structure due to strong spin-orbit coupling, whose positions relative to the Fermi level is sensitively modulated by the canting angles of the self-intercalated Cr cations. These nodes contribute strongly to the Berry curvature and AHE conductivity.This component competes with the contribution from bands that are less affected by the self-intercalation, resulting in a sign change in AHE with temperature and the emergence of additional humps and dips. The findings provide compelling evidence for the intrinsic origin of the unconventional AHE in Cr2Te3and further establish self-intercalation as a control knob for engineering AHE in complex magnets.

Substantially Enhanced Spin Polarization in Epitaxial CrTe2 Quantum Films.

2D van der Waals (vdW) magnets, which extend to the monolayer (ML) limit, are rapidly gaining prominence in logic applications for low-power electronics. To improve the performance of spintronic devices, such as vdW magnetic tunnel junctions, a large effective spin polarization of valence electrons is highly desired. Despite its considerable significance, direct probe of spin polarization in these 2D magnets has not been extensively explored. Here, using 2D vdW ferromagnet of CrTe2 as a prototype, the spin degrees of freedom in the thin films are directly probed using Mott polarimetry. The electronic band of 50 ML CrTe2 thin film, spanning the Brillouin zone, exhibits pronounced spin-splitting with polarization peaking at 7.9% along the out-of-plane direction. Surprisingly, atomic-layer-dependent spin-resolved measurements show a significantly enhanced spin polarization in a 3 ML CrTe2 film, achieving 23.4% polarization even in the absence of an external magnetic field. The demonstrated correlation between spin polarization and film thickness highlights the pivotal influence of perpendicular magnetic anisotropy, interlayer interactions, and itinerant behavior on these properties, as corroborated by theoretical analysis. This groundbreaking experimental verification of intrinsic effective spin polarization in CrTe2 ultrathin films marks a significant advance in establishing 2D ferromagnetic atomic layers as a promising platform for innovative vdW-based spintronic devices.

Recent Progress in Two-Dimensional Magnetic Materials.

The giant magnetoresistance effect in two-dimensional (2D) magnetic materials has sparked substantial interest in various fields; including sensing; data storage; electronics; and spintronics. Their unique 2D layered structures allow for the manifestation of distinctive physical properties and precise performance regulation under different conditions. In this review, we present an overview of this rapidly developing research area. Firstly, these 2D magnetic materials are catalogued according to magnetic coupling types. Then, several vital effects in 2D magnets are highlighted together with theoretical investigation, such as magnetic circular dichroism, magneto-optical Kerr effect, and anomalous Hall effect. After that, we forecast the potential applications of 2D magnetic materials for spintronic devices. Lastly, research advances in the attracting magnons, skyrmions and other spin textures in 2D magnets are discussed.

Single-Step Synthesis of An Ideal Chain Antiferromagnet [H2(4,4'-bipyridyl)](H3O)2Fe2F10 with Spin S=5/2.

One-dimensional (1D) magnets are of great interest owing to their intriguing quantum phenomena and potential application in quantum computing. We successfully synthesized an ideal antiferromagnetic spin S=5/2 chain compound [H2(4,4'-bpy)](H3O)2Fe2F10 (4,4'-bpy=4,4'-bipyridyl) 1, using a single-step low-temperature hydrothermal method under conditions that favors the protonation of the bulky bidentate ligand 4,4'-bpy. Compound 1 consists of well-separated (Fe3+-F-)∞ chains with a large Fe-F-Fe angle of 174.8°. Both magnetic susceptibility and specific heat measurements show that 1 does not undergo a magnetic long-range ordering down to 0.5 K, despite the strong Fe-F-Fe intrachain spin exchange J with J/kB=-16.2(1) K. This indicates a negligibly weak interchain spin exchange J'. The J'/J value estimated for 1 is extremely small (<2.8×10-6), smaller than those reported for all other S=5/2 chain magnets. Our hydrothermal synthesis incorporates both [H2(4,4'-bpy)]2+ and (H3O)+ cations into the crystal lattice with numerous hydrogen bonds, hence effectively separating the (Fe3+-F-)∞ spin chains. This single-step hydrothermal synthesis under conditions favoring the protonation of bulky bidentate ligands offers an effective synthetic strategy to prepare well-separated 1D spin chain systems of magnetic ions with various spin values.

Single‐Step Synthesis of An Ideal Chain Antiferromagnet [H2(4,4′‐bipyridyl)](H3O)2Fe2F10 with Spin S=5/2

AbstractOne‐dimensional (1D) magnets are of great interest owing to their intriguing quantum phenomena and potential application in quantum computing. We successfully synthesized an ideal antiferromagnetic spin S=5/2 chain compound [H2(4,4′‐bpy)](H3O)2Fe2F10 (4,4′‐bpy=4,4′‐bipyridyl) 1, using a single‐step low‐temperature hydrothermal method under conditions that favors the protonation of the bulky bidentate ligand 4,4′‐bpy. Compound 1 consists of well‐separated (Fe3+−F−)∞ chains with a large Fe−F−Fe angle of 174.8°. Both magnetic susceptibility and specific heat measurements show that 1 does not undergo a magnetic long‐range ordering down to 0.5 K, despite the strong Fe−F−Fe intrachain spin exchange J with J/kB=−16.2(1) K. This indicates a negligibly weak interchain spin exchange J′. The J′/J value estimated for 1 is extremely small (&lt;2.8×10−6), smaller than those reported for all other S=5/2 chain magnets. Our hydrothermal synthesis incorporates both [H2(4,4′‐bpy)]2+ and (H3O)+ cations into the crystal lattice with numerous hydrogen bonds, hence effectively separating the (Fe3+−F−)∞ spin chains. This single‐step hydrothermal synthesis under conditions favoring the protonation of bulky bidentate ligands offers an effective synthetic strategy to prepare well‐separated 1D spin chain systems of magnetic ions with various spin values.

Large Magnetic Anisotropy in van der Waals Ferromagnet Fe3GaTe2 above Room Temperature.

Discoveries of above-room-temperature intrinsic ferromagnetism in two-dimensional (2D) van der Waals (vdW) materials offer a platform for studying fundamental 2D magnetism and spintronic devices, especially the recently discovered above-room-temperature 2D vdW Fe3GaTe2 (FGaT). However, the magnetic mechanism in FGaT remains elusive. Here, a detailed investigation using magnetic force microscopy on the thickness-dependent magnetic behavior of FGaT single crystals is reported. The Heisenberg exchange interaction constant (J) at room temperature is determined to be 1.32836 × 10-12 J/m. Our study combining angle-resolved photoemission spectroscopy and density functional theory suggests that the high Curie temperature in FGaT is attributed to the shift of the localized Fe d band toward the Fermi level as well as the enhanced magnetic exchange effect due to the strong itinerant ability of Fe. This work sheds light on the understanding of magnetism in FGaT and provides a promising platform to investigate the mechanisms of 2D magnetic materials.

Strained van der Waals Metallic Magnet for Photomagnetic Modulation and Spin Photodiode Application

AbstractAll‐optical magnetization reversal provides a low‐power approach for investigating spin state manipulation in 2D magnets. However, the ambient observation of photomagnetic coupling presents significant challenges due to the low Curie temperatures exhibited by most 2D magnets. Herein, a mixed‐dimensional heterostructure comprising a surface‐oxidized Fe3GeTe2 nanosheet with enhanced magnetic properties and individual semiconducting ZnO nanorod is proposed to explore proximity photomagnetic modulation and spin‐enhanced photodetection behaviors. The surface curvature of ZnO nanorod induces pronounced strains for Fe3GeTe2 nanosheet, leading to its anomalous Raman polarization and spin ordering at room temperature. Strain‐activated itinerant spin electrons are immobilized on the O‐2p orbitals of adjacent ZnO, thereby facilitating the optical demagnetization process in Fe3GeTe2 without aid of magnetic field. First‐principles calculations together with in situ characterization experiments further confirm that the primary charge transfer channel involves coupling between Fe3+ and oxygen vacancy defects anchored at heterointerfaces. The rapid establishment of magnetization by illumination in ZnO nanorod contributes to spin‐tunneling‐enhanced photocurrent, device response dynamics, polarization detection and ultraviolet imaging capability. These findings offer valuable insights to optimize the optoelectronic properties of conventional semiconductors and advance complex dimensional spin‐optoelectronic devices.

2D magnets filled with lithium.

2D magnets filled with lithium.

Coexistence of ferroelectricity and antiferroelectricity in 2D van der Waals multiferroic

Multiferroic materials have been intensively pursued to achieve the mutual control of electric and magnetic properties. The breakthrough progress in 2D magnets and ferroelectrics encourages the exploration of low-dimensional multiferroics, which holds the promise of understanding inscrutable magnetoelectric coupling and inventing advanced spintronic devices. However, confirming ferroelectricity with optical techniques is challenging in 2D materials, particularly in conjunction with antiferromagnetic orders in single- and few-layer multiferroics. Here, we report the discovery of 2D vdW multiferroic with out-of-plane ferroelectric polarization in trilayer NiI2 device, as revealed by scanning reflective magnetic circular dichroism microscopy and ferroelectric hysteresis loops. The evolution between ferroelectric and antiferroelectric phases has been unambiguously observed. Moreover, the magnetoelectric interaction is directly probed by magnetic control of the multiferroic domain switching. This work opens up opportunities for exploring multiferroic orders and multiferroic physics at the limit of single or few atomic layers, and for creating advanced magnetoelectronic devices.

Sub-THz High Spin Precession Frequency in van der Waals Ferromagnet Fe3GaTe2.

The 2D magnet Fe3GaTe2 has received considerable attention for its high Curie temperature (TC), robust intrinsic ferromagnetism, and significant perpendicular magnetic anisotropy (PMA). In this study, the dynamic magnetic properties of Fe3GaTe2 are systematically investigated using an all-optical pump-probe technique. We find that the spin precession frequency (f) is as high as 351.2 GHz at T = 10 K under a field of H = 70 kOe. However, it decreases to 242.8 GHz at 300 K, mainly due to the reduced effective PMA field (Hkeff). The Gilbert damping factor (α) is modest, which increases from 0.039 (10 K) to 0.075 (300 K) owing to the enhanced scattering rate. Interestingly, when Fe3GaTe2 is coupled with 2 nm of Co, the Hkeff, f, and α just decrease slightly, highlighting the dominant influence of Fe3GaTe2. These findings substantially deepen our understanding of Fe3GaTe2, promoting the development of spintronic devices based on advanced 2D magnetic materials.

Magnetism Measurements of Two-Dimensional van der Waals Antiferromagnet CrPS4 Using Dynamic Cantilever Magnetometry

Abstract Recent experimental and theoretical work has focused on two-dimensional van der Waals (2D vdW) magnets due to their potential applications in sensing and spintronics devises. In measurements of these emerging materials, conventional magnetometry often encounters challenges in characterizing the magnetic properties of small-sized vdW materials, especially for antiferromagnets with nearly compensated magnetic moments. Here, we investigate the magnetism of 2D antiferromagnet CrPS4 with a thickness of 8 nm by using dynamic cantilever magnetometry (DCM). Through a combination of DCM experiment and the calculation based on a Stoner–Wohlfarth-type model, we unravel the magnetization states in 2D CrPS4 antiferromagnet. In the case of H ∥ c, a two-stage phase transition is observed. For H ⊥ c, a hump in the effective magnetic restoring force is noted, which implies the presence of spin reorientation as temperature increases. These results demonstrate the benefits of DCM for studying magnetism of 2D magnets.

Controlling Ultrafast Magnetization Dynamics via Coherent Phonon Excitation in a Ferromagnet Monolayer.

Exploring ultrafast magnetization control in 2D magnets via laser pulses is established, yet the interplay between spin dynamics and the lattice remains underexplored. Utilizing real-time time-dependent density functional theory (rt-TDDFT) coupled with Ehrenfest dynamics and nonadiabatic molecular dynamics (NAMD) simulations, we systematically investigate the laser-induced spin-nuclei dynamics with pre-excited A1g and E2g coherent phonons in the 2D ferromagnet Fe3GeTe2 (FGT) monolayer. Selective pre-excitation of coherent phonons under ultrafast laser irradiation significantly alters the local spin moment of FGT, consequently inducing additional spin loss attributed to the nuclear motion-induced asymmetric interatomic charge transfer. Excited spin-resolved charge undergoes a bidirectional spin-flip between spin-down and spin-up states, characterized by a subtle change in the spin moment within approximately 100 fs, followed by unidirectional spin-flip, which will further contribute to the spin moment loss of FGT within tens of picoseconds. Our results shed light on the coupling of coherent phonons with magnetization dynamics in 2D limit.

Anomalous Raman Response in 2D Magnetic FeTe Under Uniaxial Strain: Tetragonal and Hexagonal Polymorphs.

2D Fe-chalcogenides emerge with rich structures, magnetisms, and superconductivities, which spark the growing research interests in the torturous transition mechanism and tunable properties for their potential applications in nanoelectronics. Uniaxial strain can produce a lattice distortion to study symmetry breaking induced exotic properties in 2D magnets. Herein, the anomalous Raman spectrum of 2D tetragonal (t-) and hexagonal (h-) FeTe is systematically investigated via uniaxial strain engineering strategy. It is found that both t- and h-FeTe keep the structural stability under different uniaxial tensile or compressive strain up to ± 0.4%. Intriguingly, the lattice vibrations along both in-plane and out-of-plane directions exceptionally harden (softened) under tensile (compressive) strain, distinguished from the behaviors of many conventional 2D systems. Further, the difference in thickness-dependent strain effect can be well explained by their structural discrepancy between two polymorphs of FeTe. These results can supply a unique platform to explore the vibrational properties of many novel 2D materials.