Properties Of 2D Magnets Research Articles

Hybrid spin-orbit exciton-magnon excitations in FePS3

FePS3 is a layered van der Waals (vdW) Ising antiferromagnet that has recently been studied in the context of true 2D magnetism and emerged as an ideal material platform for investigating strong spin-phonon coupling, and non-linear magneto-optical phenomena. In this work, we demonstrate an important unresolved role of spin-orbit coupling (SOC) in the ground state and excitations of this compound. Combining first-principles calculations with linear flavor wave theory (LFWT), we find strong mixing and spectral overlap of different spin-orbital single-ion states. Low-lying excitations form hybrid spin-orbit exciton/magnon modes. Complete parameterization of the low-energy model requires nearly half a million coupling constants. Despite this complexity, such a model can be inexpensively derived using local many-body-based approaches, which yield quantitative agreement with recent experiments. The results highlight the importance of SOC even in first-row transition metals and provide essential insight into the properties of 2D magnets with unquenched orbital moments.

Impact of Spin-Entropy on the Thermoelectric Properties of a 2D Magnet.

Heat-to-charge conversion efficiency of thermoelectric materials is closely linked to the entropy per charge carrier. Thus, magnetic materials are promising building blocks for highly efficient energy harvesters as their carrier entropy is boosted by a spin degree of freedom. In this work, we investigate how this spin-entropy impacts heat-to-charge conversion in the A-type antiferromagnet CrSBr. We perform simultaneous measurements of electrical conductance and thermocurrent while changing magnetic order using the temperature and magnetic field as tuning parameters. We find a strong enhancement of the thermoelectric power factor at around the Néel temperature. We further reveal that the power factor at low temperatures can be increased by up to 600% upon applying a magnetic field. Our results demonstrate that the thermoelectric properties of 2D magnets can be optimized by exploiting the sizable impact of spin-entropy and confirm thermoelectric measurements as a sensitive tool to investigate subtle magnetic phase transitions in low-dimensional magnets.

Magneto‐Optical Interactions in Layered Magnets

AbstractThe rapidly emerging field of 2D magnetic materials has garnered significant attention due to its fascinating physical properties and wide‐ranging potential applications. This review highlights the importance of magneto‐optical interactions as a crucial tool for both studying and modulating 2D magnets. It offers a comprehensive survey of current research concerning magneto‐optical interactions in 2D magnetic materials, encompassing the magneto‐optical Kerr effect, reflection magnetic circular dichroism, second‐harmonic generation, photoluminescence, inelastic light scattering, and time‐resolved spectroscopy. This review discusses how these techniques provide insights into the properties of 2D magnets, enabling exploration of magnetic phase transitions, lattice alterations, spin dynamics, as well as their responses to external fields. Moreover, it emphasizes the modulation of magnetic properties by photo‐stimulation and offers a brief outlook on this swiftly developing field.

Effects of simultaneous loading of temperature and biaxial stress on the 1&2D magnetic properties of non-oriented electrical steel sheets

As the effect of factors like temperature and stress on the magnetic properties of electrical steel materials are gradually being emphasized, many research teams are carrying out related tests. Currently, most studies focus only on the effect of a single factor on the magnetic properties, while in reality, multiple factors exist simultaneously in electrical equipment. Therefore, accurate data of alternating (1D) and rotational (2D) magnetic properties under simultaneous loading of temperature and stress are very important. In this paper, a system based on a vertical rotational single sheet tester (VRSST) with temperature and stress loading parts is designed and constructed. Then, the 1D and 2D magnetic properties under many combinations of different temperatures, uniaxial and biaxial stress are measured and analyzed.

Tuning magnetic properties of two-dimensional antiferromagnetic <i>M</i>P<i>X</i><sub>3</sub> by organic cations intercalation

Electrical control of magnetism of two-dimensional (2D) antiferromagnetic (AFM) materials combines the advantages of controlling magnetism by purely electrical means, compatibility with semiconductor process, low energy consumption, heterogeneous integration of 2D materials with van der Waals (vdW) interface, and AFM materials with no stray field, resistance to external magnetic field interference, and high intrinsic frequency, and thus becomes a research focus in the field. The carrier concentration control is the main mechanism of electrical control of magnetism, and has been proved to be an effective way to control the magnetic properties of materials. The intralayer-antiferromagnetic materials have net-zero magnetic moments, and it is a challenging task to measure their regulated magnetic properties. Therefore, there is limited research on the electrical control of magnetism of intralayer-antiferromagnetic materials, and their potential mechanisms are not yet clear. Based on the diversity of organic cations, the present work systematically modulates the carrier concentrations of 2D intralayer-antiferromagnetic materials <i>M</i>P<i>X</i><sub>3</sub> (<i>M</i> = Mn, Fe, Ni; <i>X</i> = S, Se) by utilizing organic cations intercalation, and investigates the influence of electron doping on their magnetic properties. Phase transitions between AFM-ferrimagnetic (FIM)/ferromagnetic (FM) depending on carrier concentration changes are observed in <i>M</i>P<i>X</i><sub>3</sub> materials, and the corresponding regulation mechanism is revealed through theoretical calculations. This research provides new insights into the carrier-controlled magnetic phase transition of 2D magnetic materials, and opens up a pathway for studying the correlation between the electronic structure and magnetic properties of 2D magnets, and designing novel spintronic devices as well.

Proximity coupling induced significantly enhanced Curie temperature in van der Waals CrSBr/MoTe2 heterostructure

CrSBr monolayer is a promising ferromagnetic (FM) semiconductor with stable magnetic ground state, large bandgap, and high carrier density. However, its Curie temperature (TC) of about 146 K is still below room temperature. Herein, electronic and magnetic properties of a CrSBr/MoTe2 heterostructure are explored to boost the TC via the proximity effect using first-principles calculations. The long-range FM ordering in the CrSBr layer is enhanced both through an extra spin superexchange channel (Cr–Te–Cr) afforded by the MoTe2 substrate and the reduced degeneracy t2g orbitals of Cr atoms. Directly owing to the enhanced FM coupling, TC increases to 225 K. The strong p–d hybridizations increase the interorbital hopping between the t2g states through the Cr–Te–Cr superexchange channel. The hopping is related to the off diagonal matrix element of the velocity operator, which suggests that is necessary for not only the increased TC but also the nonzero Berry curvature. Additionally, the Fermi level (EF) is pushed into higher energy levels with the electron-doped CrSBr layer due to the charge transfer. The synergic effect of the EF shift from the transferred electron filling and the interorbital hopping from the Cr–Te interaction results in nonzero anomalous Hall conductivity in the heterostructure. Our work reveals that an interface is an uplifting way for engineering the magnetic and transport properties of 2D magnets, providing opportunities for fantastic spintronic devices.

Revealing 2D Magnetism in a Bulk CrSBr Single Crystal by Electron Spin Resonance

Abstract2D magnets represent material systems in which magnetic order and topological phase transitions can be observed. Based on these phenomena, novel types of computing architectures and magnetoelectronic devices can be envisaged. Unlike conventional magnetic films, their magnetism is independent of the substrate and interface qualities, and 2D magnetic properties manifest even in formally bulk single crystals. However, 2D magnetism in layered materials is rarely reported often due to weak exchange interactions and magnetic anisotropy, and low magnetic transition temperatures. Here, the electron spin resonance (ESR) properties of a layered antiferromagnetic CrSBr single crystal are reported. The W‐like shape angular dependence of the ESR linewidth provides a signature for room temperature spin–spin correlations and for the XY spin model. By approaching the Néel temperature the arising of competing intralayer ferromagnetic and interlayer antiferromagnetic interactions might lead to the formation of vortex and antivortex pairs. This argument is inferred by modeling the temperature dependence of the ESR linewidth with the topological Berezinskii‐Kosterlitz‐Thouless phase transition. These findings together with the chemical stability and semiconducting properties, make CrSBr a promising layered magnet for future magneto‐ and topological‐electronics.

Spin–orbit coupling proximity effect in MoS2/Fe3GeTe2 heterostructures

Layered two-dimensional (2D) magnet/semiconductor heterostructures combine spintronic and optoelectronic properties of constituent materials, leading to new magneto-optical and magnetoelectric phenomena such as spontaneous emission of helical light and enhanced Zeeman splitting in single photon emission. While prior focus was mostly on the magnetic proximity effect, where properties of 2D magnets are transferred to nonmagnetic 2D materials, the inverse effect of 2D semiconductors altering 2D magnets is much less understood. Here, we fabricated and studied van der Waals (vdW) heterostructures of 2D magnet Fe3GeTe2 (FGT) and 2D semiconductor MoS2. With reflectance magnetic circular dichroism, we found that the coercive field of MoS2-covered FGT reduces compared with uncovered FGT, agreeing well with our first-principles calculations. With its strong spin–orbit coupling (SOC), MoS2 effectively alters the crystal field of the adjacent FGT and its magnetic anisotropy. Furthermore, an unconventional two-step hysteresis loop emerges in MoS2/FGT as a result of the superposition of two regions of FGT: at the interface and away from the interface. Our experimental elucidation of the SOC proximity effect that MoS2 exerts on FGT provides fundamental understanding for the rational development of 2D magnet/semiconductor heterostructures.

Giant Biquadratic Exchange in 2D Magnets and Its Role in Stabilizing Ferromagnetism of NiCl2 Monolayers

Two-dimensional (2D) van der Waals (vdW) magnets provide an ideal platform for exploring, on the fundamental side, new microscopic mechanisms and for developing, on the technological side, ultra-compact spintronic applications. So far, bilinear spin Hamiltonians have been commonly adopted to investigate the magnetic properties of 2D magnets, neglecting higher order magnetic interactions. However, we here provide quantitative evidence of giant biquadratic exchange interactions in monolayer NiX2 (X=Cl, Br and I), by combining first-principles calculations and the newly developed machine learning method for constructing Hamiltonian. Interestingly, we show that the ferromagnetic ground state within NiCl2 single layers cannot be explained by means of bilinear Heisenberg Hamiltonian; rather, the nearest-neighbor biquadratic interaction is found to be crucial. Furthermore, using a three-orbitals Hubbard model, we propose that the giant biquadratic exchange interaction originates from large hopping between unoccupied and occupied orbitals on neighboring magnetic ions. On a general framework, our work suggests biquadratic exchange interactions to be important in 2D magnets with edge-shared octahedra.

Tuning of 2D magnets Curie temperature via substrate

We have investigated the substrate influence on the Curie temperature of 2D magnet using the simple Ising model on a square lattice and the Frenkel–Kontorova potential for the substrate. We have found that the change in the phase transition temperature is directly proportional to the gradient of the exchange integral and the period of the substrate. The incommensurate arrangement of atoms with respect to the substrate potential leads to complex effects, such as the influence of the substrate potential and the coverage coefficient on the phase transition in the spin system. Moreover, the phase transition temperature can be increased or decreased, either the coverage coefficient is θ<1 or θ>1. Our results will help to understand deeply the effect of substrate on magnetic properties of 2D magnets.

Structure Engineering of 2D Materials toward Magnetism Modulation

2D magnetic materials have shown great potentials in fundamental research and spintronic devices. However, the discovered 2D magnetic materials are still limited and their monotonous magnetic properties cannot meet the needs of practical applications. Recently, the modulation of magnetism in 2D materials attracted tremendous attention, which can enrich the 2D magnetic material family and extend their application prospects. Structure engineering is one of the most effective ways to realize tunable 2D magnetic properties. Herein, the recent progress on 2D magnetism modulation via structure engineering is summarized. The basic theory of 2D magnetism is introduced at first. Then, the magnetism modulation by structure engineering is classified into two ways: inducing long‐range magnetic ordering into nonmagnetic materials and regulating the native magnetism of intrinsic magnetic materials. Moreover, recent progress on magnetism modulation by intercalation, chemical doping, defect engineering, and heterostructure construction is discussed in detail. Finally, the challenges and prospects in the future of 2D magnetism are also provided.

Ternary FePSe3 Atomic Layers with Competitive Temperature Coefficient of Resistance for Uncooled Infrared Bolometers

AbstractThe semiconducting metal phosphorus trichalcogenides (MPX3, X = S, Se), a new family of layered atomic materials similar to the transition‐metal dichalcogenides, have recently attracted great attention owing to their 2D magnetic properties as well as their wide range of tunable bandgaps, which can lead to promising applications in spintronic and optoelectronic devices. Herein, an uncooled bolometer based on FePSe3 atomic layers is reported, on which a competitive temperature coefficient of resistance (TCR) of ≈−2.96% K−1 at room temperature is observed. In detecting infrared radiation (980–1550 nm) at room temperature using the FePSe3 bolometer, high responsivity exceeding 108 V W−1 and specific detectivity up to 109 Jones are achieved, which can be attributed to the combined advantages of the competitive room‐temperature TCR and natural thermal isolation between the inner layers of FePSe3 and the environment. This result reveals a remarkable property of FePSe3 atomic layers and paves the way toward new types of bolometers with high sensitivities.

Visualizing atomic structure and magnetism of 2D magnetic insulators via tunneling through graphene

The discovery of two-dimensional (2D) magnetism combined with van der Waals (vdW) heterostructure engineering offers unprecedented opportunities for creating artificial magnetic structures with non-trivial magnetic textures. Further progress hinges on deep understanding of electronic and magnetic properties of 2D magnets at the atomic scale. Although local electronic properties can be probed by scanning tunneling microscopy/spectroscopy (STM/STS), its application to investigate 2D magnetic insulators remains elusive due to absence of a conducting path and their extreme air sensitivity. Here we demonstrate that few-layer CrI3 (FL-CrI3) covered by graphene can be characterized electronically and magnetically via STM by exploiting the transparency of graphene to tunneling electrons. STS reveals electronic structures of FL-CrI3 including flat bands responsible for its magnetic state. AFM-to-FM transition of FL-CrI3 can be visualized through the magnetic field dependent moiré contrast in the dI/dV maps due to a change of the electronic hybridization between graphene and spin-polarised CrI3 bands with different interlayer magnetic coupling. Our findings provide a general route to probe atomic-scale electronic and magnetic properties of 2D magnetic insulators for future spintronics and quantum technology applications.

Remarkably enhanced Curie temperature in monolayer CrI3 by hydrogen and oxygen adsorption: A first-principles calculations

Two-dimensional (2D) materials unique properties and their promising applications in post-silicon microelectronics have attracted significant attention in the past decade. Recently, ferromagnetic order with out-of-plane easy axis in a monolayer of CrI3 has been observed and reported, with a Curie temperature of 45 K. Here we study, using density functional theory (DFT) calculations, how hydrogen and oxygen adsorption affects the structural, electronic, and magnetic properties of a CrI3 monolayer. Our results show that while the structure remains almost unchanged by the adsorption of hydrogen, adsorption of oxygen completely distorts it. We have also found that both the adsorption of hydrogen and oxygen atoms significantly influences the electronic and magnetic properties of the monolayer. While hydrogen quenches the magnetic moments of Cr atoms, oxygen introduces an impurity band in the gap. Interestingly, we find a strong enhancement of the Curie temperature by full hydrogenation, while the results are not conclusive for O. This result suggests a simple and effective approach to manipulate the electronic and magnetic properties of 2D magnets for spintronics applications.

Orbitally-resolved ferromagnetism of monolayer CrI3

Few-layer CrI3 is the most known example among two-dimensional (2D) ferromagnets, which have attracted growing interest in recent years. Despite considerable efforts and progress in understanding the properties of 2D magnets both from theory and experiment, the mechanism behind the formation of in-plane magnetic ordering in chromium halides is still under debate. Here, we propose a microscopic orbitally-resolved description of ferromagnetism in monolayer CrI3. Starting from first-principles calculations, we construct a low-energy model for the isotropic Heisenberg exchange interactions. We find that there are two competing contributions to the long-range magnetic ordering in CrI3: (i) Antiferromagnetic Anderson’s superexchange between half-filled t2g orbitals of Cr atoms; and (ii) Ferromagnetic exchange governed by the Kugel–Khomskii mechanism, involving the transitions between half-filled t2g and empty eg orbitals. Using numerical calculations, we estimate the exchange interactions in momentum-space, which allows us to restore the spin-wave spectrum, as well as estimate the Curie temperature. Contrary to the nearest-neighbor effective models, our calculations suggest the presence of sharp resonances in the spin-wave spectrum at 5–7 meV, depending on the vertical bias voltage. Our estimation of the Curie temperature in monolayer CrI3 yields 55–65 K, which is in good agreement with experimental data.

Emerging 2D-ferromagnetism and strong spin-orbit coupling at the surface of valence-fluctuating EuIr2Si2

The development of materials that are non-magnetic in the bulk but exhibit two-dimensional (2D) magnetism at the surface is at the core of spintronics applications. Here, we present the valence-fluctuating material EuIr2Si2, where in contrast to its non-magnetic bulk, the Si-terminated surface reveals controllable 2D ferromagnetism. Close to the surface the Eu ions prefer a magnetic divalent configuration and their large 4f moments order below 48 K. The emerging exchange interaction modifies the spin polarization of the 2D surface electrons originally induced by the strong Rashba effect. The temperature-dependent mixed valence of the bulk allows to tune the energy and momentum size of the projected band gaps to which the 2D electrons are confined. This gives an additional degree of freedom to handle spin-polarized electrons at the surface. Our findings disclose valence-fluctuating rare-earth based materials as a very promising basis for the development of systems with controllable 2D magnetic properties which is of interest both for fundamental science and applications.

Examination of measurement apparatus for 2D magnetic properties

AbstractThe development of highly efficient electrical rotating machines is an important issue in energy conservation. To realize this goal, we must consider the details of magnetic phenomena in rotating machines. Therefore, an accurate evaluation of 2D magnetic properties is indispensable. However, it is very difficult to measure these properties, especially under high magnetic induction, due to the difficulty in measuring rotating magnetic properties in electrical steel sheets. In this paper, a new apparatus with an eight‐tooth‐core, distributed windings, and interpoles, is proposed in order to measure 2D magnetic properties. We evaluate the uniformity of magnetic conditions in specimens by finite element analysis. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(2): 57–64, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10403

Modeling of Two-Dimensional Magnetic Properties Based on One-Dimensional Magnetic Measurements

This paper proposes a modeling method of two-dimensional (2D) magnetic properties based on magnetic energy obtained from several one-dimensional (1D) magnetic properties with different inclination angles from rolling direction. We compare the measurement results of non-oriented electrical steel sheets by 2D and 1D single sheet testers (SSTs) and confirm that almost the same magnetic properties can be obtained from each tester. Based on several 1D magnetic properties measured by the 1D-SST, we evaluate a magnetic energy by using a Fourier series expansion. By differentiating the estimated magnetic energy, we can obtain the perpendicular components of magnetic field with respect to the magnetic flux density which cannot be measured by the 1D-SST. In order to confirm the validity of the proposed method, we compare the estimated 2D magnetic properties with measured ones. Finally, the calculation procedure of the Jacobian matrix is presented and the proposed method is applied to the finite element analysis.

Cube-textured Si-steel sheets by oxide-separator-induced decarburization and growth mechanism of cube grains

Oxide-separator-induced decarburization causes a pronounced cube-texture development in Si-steel sheets. Cube grains nucleate prior to the decarburization, and they then selectively grow when a diffusion-induced transformation occurs. The 0.35 mm thick cube-textured strips show 2D magnetic properties with an almost ideal four-fold symmetry as well as surprisingly low core losses in the easy magnetizing directions.

Structures and magnetism of alkali tetrafluoromanganates(III)

The AMnF 4 compounds (A = alkali atom) have Jahn-Teller-distorted layered structures deriving from the TIAIF 4 type. Thus, the compounds are interesting models for studies of 2D magnetic properties and of the concurring ferromagnetic σ- and antiferromagnetic π-superexchange interactions dependent on the bridge angles within the layers. Although most of these compounds are known since several years (1,2) the detailed crystal structures were determined only for the LiMnF 4 compound recently (3). In our single crystal investigations we found that the different size of the alkali counter cations gives rise to three different structure variants: 1. NaMnF 4 , isotypic with LiMnF 4 (3), space group P2 1/c, a = 573,6(2), b = 489,2(1), c = 574,8(2) pm, β = 108.07(2)° (see fig. below). 2. KMnF 4 , space group P2 1/a, a = 769.9, b = 764.4, c = 576.9 pro, β = 90.54°. RbMnF 4 , isotypic, a = 780.6, b = 776.6, c = 604.6 pm, β = 90.69°. 3. CsMnF 4 , space group P4/n, a = 794.4, c = 633.8 pm, merohedral (110) twins, seeming P4/nmm space group (2). Other variants are found for the corresponding TI (4) and NH 4 (5) compounds. The structural classes differ in the Mn-F-Mn bridge angles and in the puckering type of the [MnF 4] - layers. The correlations between these geometric features and the magnetic properties, investigated by magnetic measurements and neutron diffraction, are discussed.