Probing the Ferromagnetism and Spin Wave Gap in VI3 by Helicity-Resolved Raman Spectroscopy

Evidence of Ferromagnetism and Ultrafast Dynamics of Demagnetization in an Epitaxial FeCl2 Monolayer

The development of two-dimensional (2D) magnetism is driven not only by the interest of low-dimensional physics but also by potential applications in high-density miniaturized spintronic devices. However, 2D materials possessing a ferromagnetic order with a relatively high Curie temperature (Tc) are rare. In this paper, the evidence of ferromagnetism in monolayer FeCl2 on Au(111) surfaces, as well as the interlayer antiferromagnetic coupling of bilayer FeCl2, is characterized by using spin-polarized scanning tunneling microscopy. A Curie temperature (Tc) of ∼147 K is revealed for monolayer FeCl2, based on our static magneto-optical Kerr effect measurements. Furthermore, temperature-dependent magnetization dynamics is investigated by the time-resolved magneto-optical Kerr effect. A transition from one- to two-step demagnetization occurs as the lattice temperature approaches Tc, which supports the Elliott-Yafet spin relaxation mechanism. The findings contribute to a deeper understanding of the underlying mechanisms governing ultrafast magnetization in 2D ferromagnetic materials.

Theoretical Study on the Raman Effect Due to Magnons in Two-Dimensional Magnets

Raman spectroscopy is one of the most useful experimental tools for studying elementary excitations in two-dimensional (2D) materials. The Raman scattering due to phonons was widely employed for detecting structural evolutions, especially those caused by magnetic phase transitions in 2D magnets. A first-principles theory of the Raman scattering effect caused by magnons is still lacking. We theoretically study the magnon Raman effect in 2D magnet CrI3. We propose a first-principles method and have calculated the intensity of circularly polarized Raman signals due to different magnon modes in the CrI3 monolayer and bilayers. The calculated Raman intensities due to magnons in the CrI3 monolayer and the rhombohedral bilayer are consistent with the selection rule deduced from the magnon pseudoangular moment and the parity of magnon modes. We also find that the selection rule is violated in the symmetry-broken monoclinic bilayer due to interlayer coupling.

Atomic structure and large magnetic anisotropy in air-sensitive layered ferromagnetic VI3

We report the air-sensitivity, atomic structure, and magnetic anisotropy of VI₃ single crystals. We find that VI₃ nanocrystals exhibit a large M_R/M_S ratio of around 0.75 and a uniaxial anisotropic constant of an order of 10⁵ erg cc^-1 below the Curie temperature. Furthermore, density functional theory calculations reveal that both the monolayer and bulk VI₃ are ferromagnetic insulators, and the magnetic moment of the system arises mainly from the d orbital of the V atom. These findings open a feasible avenue to fabricating TEM specimens of air-sensitive layered materials, providing an in-depth comprehensive understanding of a layered ferromagnetic VI₃.

Tunable interaction between excitons and hybridized magnons in a layered semiconductor

The interaction between distinct excitations in solids is of both fundamental interest and technological importance. One such interaction is the coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon, a collective spin excitation. The recent emergence of van der Waals magnetic semiconductors¹ provides a platform to explore these exciton-magnon interactions and their fundamental properties, such as strong correlation², as well as their photospintronic and quantum transduction³ applications. Here we demonstrate the precise control of coherent exciton-magnon interactions in the layered magnetic semiconductor CrSBr. We varied the direction of an applied magnetic field relative to the crystal axes, and thus the rotational symmetry of the magnetic system⁴. Thereby, we tuned not only the exciton coupling to the bright magnon, but also to an optically dark mode via magnon-magnon hybridization. We further modulated the exciton-magnon coupling and the associated magnon dispersion curves through the application of uniaxial strain. At a critical strain, a dispersionless dark magnon band emerged. Our results demonstrate an unprecedented level of control of the opto-mechanical-magnonic coupling, and a step towards the predictable and controllable implementation of hybrid quantum magnonics^5-11.

Terahertz Magnetic and Lattice Excitations in van der Waals Ferromagnet VI3

We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI₃. The structural transition at T_S1 = 79 K is accompanied by a large splitting of polar phonon modes. Below T_S1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below T_C = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the temperature T_S2 of the second structural transition associated with the order-order magnetic phase transition. Below T_S2, a magnon in the THz range appears in Raman spectra. The THz magnon observed in VI₃ indicates the application potential of 2D van der Waals ferromagnets in ultrafast THz spintronics, which has previously been considered the exclusive domain of antiferromagnets.

Probing Magnetism in Exfoliated VI3 Layers with Magnetotransport

We perform magnetotransport experiments on VI₃ multilayers to investigate the relation between ferromagnetism in bulk and in exfoliated layers. The magnetoconductance measured on field-effect transistors and tunnel barriers shows that the Curie temperature of exfoliated multilayers is T_C = 57 K, larger than in bulk (T_C,bulk = 50 K). Below T ≈ 40 K, we observe an unusual evolution of the tunneling magnetoconductance, analogous to the phenomenology observed in bulk. Comparing the magnetoconductance measured for fields applied in- or out-of-plane corroborates the analogy, allows us to determine that the orientation of the easy-axis in multilayers is similar to that in bulk, and suggests that the in-plane component of the magnetization points in different directions in different layers. Besides establishing that the magnetic state of bulk and multilayers are similar, our experiments illustrate the complementarity of magnetotransport and magneto-optical measurements to probe magnetism in 2D materials.

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.

Valley splitting and magnetic anisotropy in two-dimensional VI3/MSe2 (M = W, Mo) heterostructures

As a new van der Waals ferromagnetic material, VI₃ can be used to lift the valley degeneracy of transition metal dichalcogenides at the K' and K points. Here, the electronic structure and magnetic anisotropy of the VI₃/MSe₂ (M = W, Mo) heterostructures are studied. The VI₃/WSe₂ heterostructure is semiconducting with a band gap of 0.26 eV, while the VI₃/MoSe₂ heterostructure is metallic. Considering the spin-orbit-coupling, a maximum valley splitting of 3.1 meV appears in the VI₃/WSe₂ heterostructure. The biaxial strain can tune the valley splitting and magnetic anisotropy of VI₃/MSe₂ heterostructures. In the VI₃/WSe₂ heterostructure, which has the most stable stacking, valley splitting can be increased from 1.8 meV at 4% compressive strain to 3.1 meV at 4% tensile strain. At a biaxial strain of -2% to 4%, the VI₃/WSe₂ heterostructure maintains a small perpendicular magnetic anisotropy, while the VI₃/MoSe₂ heterostructure shows in-plane magnetic anisotropy under different strains. The significantly tunable electronic structure and magnetic anisotropy under biaxial strain suggest that the VI₃/MSe₂ heterostructures have potential applications in spintronic devices.

Anomalous Thermal Hall Effect in an Insulating van der Waals Magnet

Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile playground for the discovery and exploration of physical phenomena and new physics. In this Letter, we report the observation of an anomalous thermal Hall effect (THE) with κ_{xy}∼1×10^{-2}  W K^{-1} m^{-1} in an insulating van der Waals ferromagnet VI_{3}. The thermal Hall signal persists in the absence of an external magnetic field and flips sign upon the switching of the magnetization. In combination with theoretical calculations, we show that VI_{3} exhibits a dual nature of the THE, i.e., dominated by topological magnons hosted by the ferromagnetic honeycomb lattice at higher temperatures and by phonons induced by the magnon-phonon coupling at lower temperatures. Our results not only position VI_{3} as the first ferromagnetic system to investigate both anomalous magnon and phonon THEs, but also render it as a potential platform for spintronics-magnonics applications.

Magnetism and Its Structural Coupling Effects in 2D Ising Ferromagnetic Insulator VI3

van der Waals (vdW) magnets have emerged as a tunable platform for exploring a variety of layer-dependent magnetic phenomena. Here we probe the thickness-dependent magnetism of vanadium triiodide (VI₃), a material known as a layered ferromagnetic Mott insulator in its bulk form, using magnetic circular dichroism microscopy. Robust ferromagnetism is observed in all thin layers, down to the monolayer limit with large coercive fields. In contrast to known vdW magnets, the Curie temperature shows an anomalous increase as the layer number decreases, reaching a maximum of 60 K in monolayers. Second harmonic generation measurements reveal broken inversion symmetry in exfoliated flakes, down to trilayers. This observation demonstrates that the exfoliated flakes take a layer stacking arrangement that differed from the inversion-symmetric parent bulk counterpart. Our results suggest a coupling effect between magnetic and structural degrees of freedom in VI₃ and its potential for engineering layer and twist angle-dependent magnetic phenomena.

CrSbS3 monolayer: a potential phase transition ferromagnetic semiconductor

Two dimensional intrinsic ferromagnetic semiconductors with controllable magnetic phase transition are highly desirable for spintronics. Nevertheless, reports on their successful experimental realization are still rare. Herein, based on first principles calculations, we propose to achieve such a functional material, namely CrSbS₃ monolayer by exfoliating from its bulk crystal. Intrinsic CrSbS₃ monolayer is a ferromagnetic half semiconductor with a moderate bandgap of 1.90 eV. It features an intriguing magnetic phase transition from ferromagnetic to antiferromagnetic when applying a small compressive strain (∼2%), making it ideal for fabricating strain-controlled magnetic switches or memories. In addition, the predicted strong anisotropic absorption of visible light and small effective masses make the CrSbS₃ monolayer promising for optoelectronic applications.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.