2D Magnetic Materials Research Articles

Two-Dimensional Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) Monolayers: Half-Metallic Ferromagnets with Tunable Magnetic Properties under Strain.

Two-dimensional (2D) ferromagnetic materials with high spin polarization are highly desirable for spintronic devices. 2D Janus materials exhibit novel properties due to their broken symmetry. However, the electronic structure and magnetic properties of 2D Janus magnetic materials with high spin polarization are still unclear. Inspired by the successful synthesis of a ferromagnetic FeCl2 monolayer and 2D Janus MoSSe and WSSe, we systematically study the electronic structure and magnetic properties of Janus FeXY (X, Y = Cl, Br, and I, X ≠ Y) monolayers. Based on the Goodenough-Kanamori-Anderson theory, the ferromagnetism stems from the superexchange interaction mediated by Fe-X/Y-Fe bonds. The band gaps of spin-up channels are large enough (>4 eV) to prevent spin flipping, which is beneficial for spintronic devices. Additionally, the sizable magnetocrystalline anisotropy energy (MAE) indicates that Janus FeXY monolayers are suitable for information storage. More importantly, the half-metallic character is still kept in Janus FeXY monolayers, and their magnetic properties are enhanced by the biaxial compressive strain. The MAE of FeClI and FeBrI increases by 1 order of magnitude, and the Curie temperature of FeXY monolayers enhances by 100%. These results provide an example of the 2D Janus half-metallic materials and enrich the 2D magnetic material library.

Chiral Phonons and Giant Magneto-Optical Effect in CrBr3 2D Magnet.

Phonons with chirality determine the optical helicity of inelastic light scattering processes due to their nonzero angular momentum. Here it is shown that 2D magnetic CrBr3 hosts chiral phonons at the Brillouin-zone center. These chiral phonons are linear combinations of the doubly-degenerate Eg phonons, and the phonon eigenmodes exhibit clockwise and counterclockwise rotational vibrations corresponding to angular momenta of l = ± 1.Such Eg chiral phonons completely switch the polarization of incident circularly polarized light. On the other hand, the non-degenerate non-chiral Ag phonons display a giant magneto-optical effect under an external out-of-plane magnetic field, rotating the plane of polarization of the scattered linearly polarized light. The corresponding degree of polarization of the scattered light changes from 91% to -68% as the magnetic field strength increases from 0 to 5 T. In contrast, the chiral Eg modes display no field dependence. The results lay a foundation for the study of phonon chirality and magneto-optical phenomena in 2D magnetic materials, as well as their related applications, such as the phonon Hall effect, topological photonics, and Raman lasing.

Crystallographic and magnetic structures of the VI3 and LiVI3 van der Waals compounds

Two-dimensional (2D) layered magnetic materials are generating a great amount of interest for the next generation of electronic devices thanks to their remarkable properties associated with spin dynamics. The recently discovered layered $\mathrm{V}{\mathrm{I}}_{3}$ ferromagnetic phase belongs to this family, although a full understanding of its properties is limited by the incomplete understanding of its crystallographic structure. The motivation of this work is to address this issue. Here, we investigate the $\mathrm{V}{\mathrm{I}}_{3}$ crystal structures at low temperature using both synchrotron x-ray and neutron powder diffraction and provide structural models for the two structural transitions occurring at 76 and 32 K. Moreover, we confirm by magnetic measurements that $\mathrm{V}{\mathrm{I}}_{3}$ becomes ferromagnetic at 50 K and we question the establishment of a long-range magnetic structure by neutron diffraction. We equally determined the magnetic properties of our recently reported $\mathrm{LiV}{\mathrm{I}}_{3}$ phase, which is like the well-known $\mathrm{Cr}{\mathrm{I}}_{3}$ ferromagnetic phase in terms of electronic and crystallographic structures and found an antiferromagnetic behavior with a N\'eel temperature of 12 K. Such a finding provides extra clues for a better understanding of magnetism in these low-dimension compounds. Finally, the easiness of preparing Li-based 2D magnetic materials by chemical/electrochemical means opens wide the opportunity to design materials with exotic properties.

Theoretical prediction of topological insulators in two-dimensional ternary transition metal chalcogenides (MM'X4, M = Ta, Nb, or V; M'= Ir, Rh, or Co; X = Se or Te)

Ternary transition metal chalcogenides (TTMCs) have attracted interest due to the discovery of their Weyl semimetallic property and the recent synthesis of layered TTMCs which are regarded as potential candidates for two-dimensional (2D) topological insulators. Here, employing first-principles calculations, we predicted the emergence of non-trivial band topologies in the monolayer MM'X4 family (M= V, Nb, or Ta; M' = Co, Rh, or Ir; and X = Se or Te) within hybrid functional calculations. Five of eighteen 2D materials were found to be topological insulators, while four of them are magnetic thin films. The nontrivial topologies were verified via the calculated Z2 topological invariant and topologically protected edge states. Further calculations showed a strain-induced phase transition in VCoTe4 from a magnetic phase to a nonmagnetic topological insulating phase. Our comprehensive study revealed a diverse family of monolayer ternary transition metal chalcogenides adding new members to the current catalog of 2D topological insulators and 2D magnetic materials.

Intrinsic Room-Temperature Ferromagnetism in V2C MXene Nanosheets

Two-dimensional (2D) materials with intrinsic magnetic properties are intensively explored due to their potential applications in low-power-consumption electronics and spintronics. To date, only a handful of intrinsic magnetic 2D materials have been reported. Here, we report a realization of intrinsic ferromagnetic behavior in 2D V2C MXene nanosheets through layer mismatch engineering. The V2C MXene nanosheets with a small-angle twisting show a robust intrinsic ferromagnetic response with a saturation magnetic moment of 0.013 emu/g at room temperature. An in-depth study has been performed by X-ray absorption spectroscopy as well as electron paramagnetic resonance (EPR) and photoelectron spectroscopy analyses. It has been revealed that the symmetry-broken interlayer twisting reduced the degeneracy of V 3d states and the van Hove singularity. This led to a redistribution of the density of electronic states near the Fermi level and consequently activated the Stoner ferromagnetism with improved density of itinerant d electrons. This work highlights V2C MXene as a promising intrinsic room-temperature ferromagnetic material with potential applications in spintronics or spin-based electronics.

Manipulating Ferromagnetism in Few-Layered Cr2 Ge2 Te6.

The discovery of magnetism in 2D materials offers new opportunities for exploring novel quantum states and developing spintronic devices. In this work, using field-effect transistors with solid ion conductors as the gate dielectric (SIC-FETs), wehave observed a significant enhancement of ferromagnetism associated with magnetic easy-axis switching in few-layered Cr2 Ge2 Te6 . The easy axis of the magnetization, inferred from the anisotropic magnetoresistance, can be uniformly tuned from the out-of-plane direction to an in-plane direction by electric field in the few-layered Cr2 Ge2 Te6 . Additionally, the Curie temperature, obtained from both the Hall resistance and magnetoresistance measurements, increases from 65 to 180 K in the few-layered sample by electric gating. Moreover, the surface of the sample is fully exposed in the SIC-FET device configuration, making further heterostructure-engineering possible. This work offers an excellent platform for realizing electrically controlled quantum phenomena in a single device.

Controlled vapor growth of 2D magnetic Cr2Se3 and its magnetic proximity effect in heterostructures* *Project supported by the National Natural Science Foundation of China (Grant Nos. 52022029, 91850116, 51772084, 62090035, and U19A2090), Hunan Provincial Natural Science Foundation of China (Grant Nos. 2018RS3051 and 2018WK4004), and the Key Program of the Hunan Provincial Science and Technology Department (Grant No. 2019XK2001).

Two-dimensional (2D) magnetic materials have aroused tremendous interest due to the 2D confinement of magnetism and potential applications in spintronic and valleytronic devices. However, most of the currently 2D magnetic materials are achieved by the exfoliation from their bulks, of which the thickness and domain size are difficult to control, limiting the practical device applications. Here, we demonstrate the realization of thickness-tunable rhombohedral Cr2Se3 nanosheets on different substrates via the chemical vapor deposition route. The magnetic transition temperature at about 75 K is observed. Furthermore, van der Waals heterostructures consisting of Cr2Se3 nanosheets and monolayer WS2 are constructed. We observe the magnetic proximity effect in the heterostructures, which manifests the manipulation of the valley polarization in monolayer WS2. Our work contributes to the vapor growth and applications of 2D magnetic materials.

Magnetic and electronic properties of two-dimensional metal-organic frameworks TM3(C2NH)12 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 62074053, 61901161, 21906041, and 11774079), the Natural Science Foundation of Henan Province, China (Grant Nos. 202300410226, 202300410237, and 202300410100), Henan Overseas Expertise Introduction Center for Discipline Innovation (Grant No. CXJD2019005), and key scientific research projects of Colleges and

The ferromagnetism of two-dimensional (2D) materials has aroused great interest in recent years, which may play an important role in the next-generation magnetic devices. Herein, a series of 2D transition metal-organic framework materials (TM-NH MOF, TM = Sc–Zn) are designed, and their electronic and magnetic characters are systematically studied by means of first-principles calculations. Their structural stabilities are examined through binding energies and ab-initio molecular dynamics simulations. Their optimized lattice constants are correlated to the central TM atoms. These 2D TM-NH MOF nanosheets exhibit various electronic and magnetic performances owing to the effective charge transfer and interaction between TM atoms and graphene linkers. Interestingly, Ni- and Zn-NH MOFs are nonmagnetic semiconductors (SM) with band gaps of 0.41 eV and 0.61 eV, respectively. Co- and Cu-NH MOFs are bipolar magnetic semiconductors (BMS), while Fe-NH MOF monolayer is a half-semiconductor (HSM). Furthermore, the elastic strain could tune their magnetic behaviors and transformation, which ascribes to the charge redistribution of TM-3d states. This work predicts several new 2D magnetic MOF materials, which are promising for applications in spintronics and nanoelectronics.

Strain modulation of the spin-valley polarization in monolayer manganese chalcogenophosphates alloys

Inspired by the profound physical connotations and potential applications of the spintronics and valleytronics, two-dimensional (2D) monolayer manganese chalcogenophosphates alloys are constructed, and the strain modulated spin-valley characteristics are investigated through the first principles calculations. For both the MnFePS3 and MnFePSe3, the conductivity can be tuned reversibly between semiconductive and half-metallic, while and magnetic stability is controllable between ferromagnetism and antiferromagnetism. Large valley splitting of up to 1000 meV is achieved in MnFePS3 under a −4% strain. Simultaneous spin splitting of 219 meV and valley splitting of 160 meV are acquired in MnFePS3 under a 4% strain. Strain tunable magnetic moment and interaction between Mn, Fe and S/Se atoms are revealed as the internal mechanisms of controlling the magnetic stability, spin and valley polarizations in the two structures. All the findings in this work provide a strategy for the manipulation of spin and valley degrees of freedom in 2D magnetic materials.

Spin Dynamics Slowdown near the Antiferromagnetic Critical Point in Atomically Thin FePS3.

Two-dimensional (2D) magnetic materials have attracted much recent interest with unique properties emerging at the few-layer limit. Beyond the reported impacts on the static magnetic properties, the effects of reducing the dimensionality on the magnetization dynamics are also of fundamental interest and importance for 2D device development. In this report, we investigate the spin dynamics in atomically thin antiferromagnetic FePS3 of varying layer numbers using ultrafast pump-probe spectroscopy. Following the absorption of an optical pump pulse, the time evolution of the antiferromagnetic order parameter is probed by magnetic linear birefringence. We observe a strong divergence in the demagnetization time near the Néel temperature. The divergence can be characterized by a power-law dependence on the reduced temperature, with an exponent decreasing with sample thickness. We compare our results to expectations from critical slowing down and a two-temperature model involving spins and phonons and discuss the possible relevance of spin-substrate phonon interactions.

Insight into interlayer magnetic coupling in 1T -type transition metal dichalcogenides based on the stacking of nonmagnetic atoms

The interlayer coupling in two-dimensional (2D) magnetic materials is significantly important in determining the properties of 2D materials and applications of related devices. However, the mechanism that determines the interlayer magnetic coupling has only been comprehensively studied in $\mathrm{Cr}{\mathrm{I}}_{3}$ and that of transition metal dichalcogenides is still blurred. In this work, through first-principle calculations, we find in 2D magnetic bilayer that the interlayer magnetic coupling is determined by the stacking order of interlayer nonmagnetic atoms, accompanied by the transition between half metal and semiconductor in $\mathrm{Mn}{\mathrm{S}}_{2}$. The nonmagnetic atoms bridge the interlayer coupling and the stacking order of nonmagnetic atoms determines the interlayer coupling by altering the interlayer ${p}_{z}$-orbital bonding. Adjusting the structure of interlayer nomagnetic atoms by biaxial strain can also tune the interlayer coupling. The perspective proposed in our work, from the stacking order of nonmagnetic atoms, to understand the interlayer magnetic coupling is also applicable in other 2D materials.

Morphology-Tunable Synthesis of Intrinsic Room-Temperature Ferromagnetic γ-Fe2O3 Nanoflakes.

Intrinsic two-dimensional (2D) magnetic materials with room-temperature ferromagnetism and air stability are highly desirable for spintronic applications. However, the experimental observations of such 2D or ultrathin ferromagnetic materials are rarely reported owing to the scarcity of these materials in nature and for the intricacy in their synthesis. Here, we report a successful controllable growth of ultrathin γ-Fe2O3 nanoflakes with a variety of morphologies tunable by the growth temperature alone using a facile chemical vapor deposition method and demonstrate that all ultrathin nanoflakes still show intrinsic room-temperature ferromagnetism and a semiconducting nature. The γ-Fe2O3 nanoflakes epitaxially grown on α-Al2O3 substrates take a triangular shape at low temperature and develop gradually in lateral size, forming eventually a large-scale γ-Fe2O3 thin film as the growth time increases due to a thermodynamic control process. The morphology of the nanoflakes could be tuned from triangular to stellated, petaloid, and dendritic crystalloids in sequence with the rise of precursor temperature, revealing a growth process from thermodynamically to kinetically dominated control. Moreover, the petaloid and dendritic nanoflakes exhibit enhanced coercivity compared with the triangular and stellated nanoflakes, and all the nanoflakes with diverse shapes possess differing electrical conductivity. The findings of such ultrathin, air-stable, and room-temperature ferromagnetic γ-Fe2O3 nanoflakes with tunable shape and multifunctionality may offer guidance in synthesizing other non-layered magnetic materials for next-generation electronic and spintronic devices.

Two‐dimensional magnetic transition metal chalcogenides

AbstractThe field of two‐dimensional (2D) magnets has expanded rapidly during the past several years since the first demonstration of intrinsic 2D magnetism in atomically thin CrI3 and Cr2Ge2Te6 in 2017. 2D transition metal chalcogenides (TMCs), a class of strongly correlated materials, have exhibited a wide variety of novel electronic and optical properties, and more recently magnetism. Here, we review recent experimental progress achieved in the growth of 2D magnetic TMC materials using chemical vapor deposition and molecular beam epitaxy methods. Outstanding examples include the demonstration of room temperature intrinsic and extrinsic ferromagnetism in monolayer VSe2, MnSe2, Cr3Te4, V‐doped WSe2, and so on. A brief discussion on the origin of the exotic magnetic properties and emergent phenomena is also presented. Finally, we summarize the remaining challenges and future perspective on the development of 2D magnetic materials for next‐generation spintronic devices.

Quarter-filled Kane-Mele Hubbard model: Dirac half metals

Recent experimental success in the realization of two-dimensional (2D) magnetism has stimulated the search for new magnetic 2D materials with strong magnetic anisotropy and high Curie temperature. One promising subgroup of 2D magnetic systems are Dirac half-metals (DHM) which have gained a lot of interest recently, as they host a high-temperature quantum anomalous Hall effect (QAHE). This article discusses predictions for intrinsic DHMs and identifies them as realizations of the Kane-Mele Hubbard model at quarter filling. This proposed unification contributes to a firmer understanding of these materials and suggests pathways for the discovery of new DHM systems.

Recent developments on 2D magnetic materials: challenges and opportunities

The emergence of two-dimensional (2D) magnetic materials exhibiting strong magnetization at ultrathin limits above room temperature is promising for miniaturization of devices beyond Moore’s law for future energy efficient nanoelectronic devices. Here, the current status, different mechanisms for the existence of magnetism, spin current injection, and other magnetic properties of monolayer to few layers of various 2D magnetic materials are reviewed. Some of the promising applications of these materials are spintronics devices such as spin valves, spin tunnel field-effect transistors, and spin-filtering magnetic tunnel junctions. Due to the tunable electronic properties of these 2D materials, it is quite interesting to inject the spin current with suitable ferromagnetic contacts. For instance, black phosphorus is a layered material with a small Schottky barrier height capable of injecting spin current. This review includes many recently explored 2D magnetic materials ranging from exfoliated 2D crystals to CVD grown materials from single to several layers, demonstrating tunable layer-dependent magnetic properties. We also explore some of the promising theoretical study based on 2D magnetic compounds such as 2D alkali-based chromium chalcogenides, which shows ferromagnetic as well as semiconducting behavior. The layer-dependent magnetic ordering has been observed in layered compounds like 1T-CrTe2, VSe2, CrI3, and Fe3GeTe2, which have great potential for the future applications in magnetic-based electronic devices. Finally, we emphasize the challenges, opportunities, and future directions of the 2D magnetic materials, where new discoveries might have outstanding impact in transformational scientific breakthroughs towards memory, spintronics, optoelectronics, and other multifunctional device applications.

Largely Tunable Magneto-Coloration of Monolayer 2D Materials via Size Tailoring.

Magnetically influenced light-matter interaction provides a contactless, noninvasive and power-free way for material characterization and light modulation. Shape anisotropy of active materials mainly determines the sensitivity of magneto-optic response, thereby making magnetic two-dimensional (2D) materials suitable in achieving the giant magneto-birefringence effect as discovered recently. Consequently, relationship between magneto-birefringence response and shape anisotropy of 2D materials is critical but has remained elusive, restricting its widespread applications. Here, we report the highly sensitive and largely tunable magneto-coloration via manipulating the shape-anisotropy of magnetic 2D materials. We reveal a quadratic increasing relationship between the magneto-optic Cotton-Mouton coefficient and the lateral size of 2D materials and achieve a more than one order of magnitude tunable response. This feature enables the engineerable transmissive magneto-coloration of 2D materials by tailoring their shape anisotropy. Our work deepens the understanding of the tunability of magneto-optic response by size effect of active materials, offering various opportunities for their applications in vast areas where color is concerned.

Nanodevices engineering and spin transport properties of MnBi2Te4 monolayer

Two-dimensional (2D) magnetic materials are essential for the development of the next-generation spintronic technologies. Recently, layered van der Waals (vdW) compound MnBi2Te4 (MBT) has attracted great interest, and its 2D structure has been reported to host coexisting magnetism and topology. Here, we design several conceptual nanodevices based on MBT monolayer (MBT-ML) and reveal their spin-dependent transport properties by means of the first-principles calculations. The pn-junction diodes and sub-3-nm pin-junction field-effect transistors (FETs) show a strong rectifying effect and a spin filtering effect, with an ideality factor n close to 1 even at a reasonably high temperature. In addition, the pip- and nin-junction FETs give an interesting negative differential resistive (NDR) effect. The gate voltages can tune currents through these FETs in a large range. Furthermore, the MBT-ML has a strong response to light. Our results uncover the multifunctional nature of MBT-ML, pave the road for its applications in diverse next-generation semiconductor spin electric devices.

Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator.

Two-dimensional (2D) semiconductors enable the investigation of light-matter interactions in low dimensions1,2. Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials3,4. Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS3. The exciton exhibits a narrow photoluminescence linewidth of roughly 350 μeV with near-unity linear polarization. When we reduce the sample thickness from five to two layers, the photoluminescence is suppressed and eventually vanishes for the monolayer. This suppression is consistent with the calculated bandgap of NiPS3, which is highly indirect for both the bilayer and the monolayer5. Furthermore, we observe strong linear dichroism (LD) over a broad spectral range. The optical anisotropy axes of LD and of photoluminescence are locked to the zigzag direction. Furthermore, their temperature dependence is reminiscent of the in-plane magnetic susceptibility anisotropy. Hence, our results indicate that LD and photoluminescence could probe the symmetry breaking magnetic order parameter of 2D magnetic materials. In addition, we observe over ten exciton-A1g-phonon bound states on the high-energy side of the exciton resonance, which we interpret as signs of a strong modulation of the ligand-to-metal charge-transfer energy by electron-lattice interactions. Our work establishes NiPS3 as a 2D platform for exploring magneto-exciton physics with strong correlations.

Multiple Weyl fermions and tunable quantum anomalous Hall effect in 2D half-metal with huge spin-related energy gap

Topological states of matter in two-dimensional (2D) system, especially those combined with magnetism have received special attention currently, because of their new physics and the potential applications in spintronics and nanoscale. At present, realizing multiple topological phases in a single 2D Magnetic material and studying their potential entanglement still face great challenges. Here, we predict a novel 2D topological material namely monolayer EuO2, which can realize multiple types of topological states, namely, type-I Weyl point (WP), type-II WP, critical-type WP, and Quantum anomalous Hall (QAH) effect. Specifically, Without considering Spin-orbit coupling (SOC), these Weyl points perfectly possess the 100% spin-polarization because monolayer EuO2 is a half metal phase with huge spin-related energy gap as large as 6.2 eV. Remarkably, With considering SOC, the topological states in monolayer EuO2 can be tuned by direction of magnetization. Under out-plane magnetization, monolayer EuO2 can realize the QAH effect with single chiral edge state occurring inside the energy gap. Under in-plane magnetization, the system can remain 2D Weyl states at special directions. In other in-plane magnetization, the system shows the in-plane QAH effect with the Chern number of −1 or 1. Therefore, Our work promotes the realization of tunable topological phase in 2D half metals.

Enhancing magneto-optic effects in two-dimensional magnets by thin-film interference

The magneto-optic Kerr effect is a powerful tool for measuring magnetism in thin films at microscopic scales, as was recently demonstrated by the major role it played in the discovery of two-dimensional (2D) ferromagnetism in monolayer CrI3 and Cr2Ge2Te6. These 2D magnets are often stacked with other 2D materials in van der Waals heterostructures on a SiO2/Si substrate, giving rise to thin-film interference. This can strongly affect magneto-optical measurements but is often not taken into account in experiments. Here, we show that thin-film interference can be used to engineer the magneto-optical signals of 2D magnetic materials and optimize them for a given experiment or setup. Using the transfer matrix method, we analyze the magneto-optical signals from realistic systems composed of van der Waals heterostructures on SiO2/Si substrates, using CrI3 as a prototypical 2D magnet, and hexagonal boron nitride to encapsulate this air-sensitive layer. We observe a strong modulation of the Kerr rotation and ellipticity, reaching several tens to hundreds of milliradians, as a function of the illumination wavelength, and the thickness of SiO2 and layers composing the van der Waals heterostructure. Similar results are obtained in heterostructures composed by other 2D magnets, such as CrCl3, CrBr3, and Cr2Ge2Te6. Designing samples for the optimal trade-off between magnitude of the magneto-optical signals and intensity of the reflected light should result in a higher sensitivity and shorter measurement times. Therefore, we expect that careful sample engineering, taking into account thin-film interference effects, will further the knowledge of magnetization in low-dimensional structures.