Tunable valley polarization, magnetic anisotropy and Dzyaloshinskii-Moriya interaction in two-dimensional intrinsic ferromagnetic Janus 2H-VSeX (X = S, Te) monolayers

The electronic and magnetic properties modulated by ferroelectric polarization switching in two-dimensional VSeTe/Sc2CO2 van der Waals heterostructures

Exploring multiferroic materials that combine magnetic and ferroelectric properties is scientifically interesting and has important technical implications for many functions of nanoscale devices. In this work, spintronics and magnetoelectric coupling devices are proposed in two-dimensional (2D) layered ferromagnetic (FM)/ferroelectric (FE) van de Waals (vdW) heterostructures, VSeTe/Sc2CO2, employing density functional theory (DFT) calculations. The results indicate that the VSeTe/Sc2CO2 vdW heterostructure changes from a metal to a semiconductor in Sc2CO2-P↑ and Sc2CO2-P↓ polarization states. At the same time, the charge at the interface of the VSeTe/Sc2CO2 heterostructure will also be redistributed with the transformation of the ferroelectric polarization state, resulting in the change of the distribution of the electronic states near the Fermi level, and thus the change in the magnetic anisotropy energy (EMAE) of the heterostructure. Interestingly, biaxial strain brings reversibility and non-volatile regulation to the heterostructure of semiconductors and metals. The results provide an effective way to fabricate magnetoelectric coupling devices with 2D multiferroic heterostructures.

Machine Learning-Assisted Exploration of Intrinsically Spin-Ordered Two-Dimensional (2D) Nanomagnets

The existence of spontaneous spin-ordering in two-dimensional (2D) nanomagnets holds significant importance due to their several unique and promising properties that distinguish them from conventional 2D materials. In recent times, machine learning (ML) has emerged as a powerful tool for effectively exploring and identifying the optimal 2D materials for specific applications or properties within a limited span of time. Here, we have introduced ML-accelerated approaches to specifically estimate the properties, such as the HSE bandgap and magnetoanisotropic energy (MAE) of 2D magnetic materials. Supervised ML algorithms were employed to derive the descriptors that are capable of predicting the properties of intrinsic 2D magnetic materials. Furthermore, the feature selection score is also calculated to reduce the feature space complexity and improve the model accuracy. The input features were obtained from the C2DB database, and models were constructed using linear regression, Lasso, decision tree, random forest, XG Boost, and support vector machine algorithms. The random forest model predicted the HSE band gaps with an unprecedented low root-mean-square error (RMSE) of 0.22 eV, while the linear regression gives the best fit with RMSEs of 0.25 and 0.22 meV for the MAE(x) and MAE(y), respectively. Therefore, the integration of interpretable analytical models with density functional theory offers a swift and reliable approach for uncovering the properties of intrinsic 2D magnetic materials. This collaborative methodology not only ensures speed in analysis but also enriches the material space.

Charge doping and electric field tunable ferromagnetism and Curie temperature of the MnS2 monolayer

Two-dimensional ferromagnets with a long-range ferromagnetic ordering at finite temperature present a bright prospect for their potential applications in nanoscale spintronic devices. The tuning of their intrinsic ferromagnetism and Curie temperature is essential for the development of next-generation data storage and spintronic devices. In this work, the electronic structures, ferromagnetism and Curie temperature of two-dimensional MnS2 monolayer are controlled by charge doping and electric field using first principles calculations. The results show that the dynamic and thermal stability of monolayer MnS2 for all of the cases can be still maintained. Moreover, there is no existence of phase transition and all MnS2 monolayers at any charge doping concentrations and electric field intensities favor ferromagnetic coupling. For the manipulation of electron doping, the calculated total magnetic moment Mtot of the MnS2 monolayer exhibits an increase from 3.112 to 3.491μB per unit cell. Further analysis indicates that a transition from half-metal to metal occurs by introducing the charge doping and vertical electric field, and the Mn 3d electronic states are the major determinants of ferromagnetism. Additionally, the charge doping enables the magnetic anisotropy energy to transform from an in-plane easy axis to the magnetization direction out of the plane. The Curie temperature Tc of the MnS2 monolayer can be moderately enhanced above room temperature by hole doping and application of a vertical electric field. Remarkably, Tc reaches its peak at 767 K at a hole doping concentration of -0.8e. This work enriches the microscopic understanding of the tuning mechanism of ferromagnetism and supplies a sound theoretical basis for subsequent experimental studies.

A graphene/Janus B2P6 heterostructure with a controllable Schottky barrier via interlayer distance and electric field

Lowering the Schottky barrier at the metal-semiconductor interface remains a stern challenge in the field of field-effect transistors. Herein, an in-depth investigation was conducted to explore the formation mechanism of the Schottky barrier via interlayer distance and external electric field, utilizing the first-principles approach. Attributed to the vertical asymmetric structure of B2P6, ohmic contact forms at the interface of a graphene/B2P6(001) heterostructure, and an n-type Schottky contact with a Schottky barrier of 0.51 eV forms at the interface of a graphene/B2P6(001̄) heterostructure. Furthermore, the Schottky barrier height and the contact type can be changed by adjusting the interlayer spacing or applying an electric field along the Z direction. A high carrier concentration of 4.65 × 1013 cm-2 is obtained in the graphene/B2P6(001) heterostructure when an external electric field of 0.05 V Å-1 is applied. Verifiably, alterations in the energy band structure are attributed to the redistribution of charges at the interface. The new findings indicate that GR/B2P6 heterostructures are a key candidate for next-generation Schottky field-effect transistor development.

Intrinsic ferromagnetism in two-dimensional 1T-MX2 monolayers with tunable magnetocrystalline anisotropy

Two-dimensional (2D) ferromagnetic materials with tunable magnetocrystalline anisotropy (MCA) provide unique opportunities for developing the next-generation data-storage and information devices. Herein we systematically investigate the electronic and magnetic properties of the 1T-MX2 (M = Cr, Mn, Fe, Co; X = As, Sb) monolayers, and identify the stable 2D ferromagnets as well as their MCA energies. Notably, the results demonstrate that the biaxial strain and carrier doping effects have a significant influence on their magnetic behaviors. In addition to the robust FM states, three FM monolayers yield tunable MCA depending on the applied strain type and carrier doping values. The dominant contributions to these complicated modifications in MCA are mainly attributed to the strain or carrier doping induced alterations of specific M-derived 3d states, which in turn lead to the changes of their spin-orbit coupling (SOC) energies. These findings show effective approaches to control 2D magnetism and suggest that these 2D FM materials may be promising candidates to design highly efficient memory devices.

MoS2 nanosheets supported on anodic aluminum oxide membrane: An effective interface for label-free electrochemical detection of microRNA

The interesting adsorption affinity of two-dimensional nanosheets to single stranded over double stranded nucleic acids have stimulated the exploration of these materials in biosensing. Herein, MoS₂ nanosheets decorated anodic aluminum oxide (AAO) membrane was simply prepared by suction filtration. The MoS₂/AAO hybrid membrane was initially applied to the electrochemical detection of microRNA using let-7a as the model. When let-7a was incubated with its complementary DNA, double stranded DNA-RNA formed and which displayed weak adsorption capability to the hybrid membrane. And thus the steric effect combining the electrostatic repulsion of the backbone phosphate of nucleic acids for [Fe(CN)₆]^3- transport across the hybrid membrane varied with the concentration of let-7a. In this way, a label-free electrochemical detection method for microRNA was established by monitoring the change of the redox current of [Fe(CN)₆]^3-. To further improve the detection sensitivity of the method, we proposed two separate strategies focusing on the amplification of the target-induced steric hindrance with DNA nanostructure and the magnification of the electrode sensitivity for [Fe(CN)₆]^3- by electrode modification. By using the two strategies, the hybrid membrane based-detection method exhibited broad linear range, low detection limit and good selectivity as well as reproducibility. Therefore, this study provided a proof-of-concept for the application of two-dimensional material to nucleic acids detection.

Combined piezoelectricity, valley splitting and Dzyaloshinskii-Moriya interaction in Janus GdXY (X, Y = Cl, Br, I) magnetic semiconductors

Janus materials, as a family of multifunctional materials with broken mirror symmetry, have played a great role in piezoelectric, valley-related, and Rashba spin-orbit coupling (SOC) applications. Using first-principles calculations, it is predicted that monolayer 2H-GdXY (X, Y = Cl, Br, I) will combine giant piezoelectricity, intrinsic valley splitting and a strong Dzyaloshinskii-Moriya interaction (DMI), resulting from the intrinsic electric polarization, spontaneous spin polarization and strong spin-orbit coupling. Opposite Berry curvatures and unequal Hall conductivities at the K- and K'-valleys of monolayer GdXY are promising for storing information through the anomalous valley Hall effect (AVHE). Through construction of the spin Hamiltonian and micromagnetic model, we obtained the primary magnetic parameters of monolayer GdXY as a function of the biaxial strain. Due to the dimensionless parameter κ having strong tunability, monolayer GdClBr is promising to host isolated skyrmions. The present results are expected to enable the application of Janus materials in piezoelectricity, spin- and valley-tronics and the formation of chiral magnetic structures.

Two-dimensional magnetic Janus monolayers and their van der Waals heterostructures: a review on recent progress

A magnetic Janus monolayer, a special type of material which has asymmetric arrangements of its surface at the nanoscale, has been shown to present rather exotic properties for applications in spintronics and its intersections. This review aims to offer a comprehensive review of the emergent physical properties of magnetic Janus monolayers and their van der Waals heterostructures from a theoretical point of view. The review starts by introducing the theoretical methodologies composed of the state-of-the-art methods and the challenges and limitations in validations for the descriptions of the magnetic ground states and thermodynamic properties in magnetic materials. The built-in polarization field induced physical phenomena of magnetic Janus monolayers are then presented. The tunable electronic and magnetic properties of magnetic Janus monolayer-based van der Waals heterostructures are discussed. Finally, the conclusions and future challenges in this field are prospected. This review serves as a complete summary of the two-dimensional magnetic Janus library and emergent electronic and magnetic properties in magnetic Janus monolayers and their heterostructures, and provides guidelines for the design of electronic and spintronic devices based on Janus materials.

2D electrene LaH2monolayer: an ideal ferrovalley direct semiconductor with room-temperature ferromagnetic stability

In developing nonvolatile valleytronic devices, ferromagnetic (FM) ferrovalley semiconductors are critically needed due to the existence of spontaneous valley polarization. At present, however, the known real materials have various drawbacks towards practical applications, including the in-plane FM ground state, low Curie temperature (TC), small valley polarization, narrow energy window with clean polarized valley, and indirect bandgap. From first-principles calculations, here we predict anideal ferrovalley semiconductor, honeycomb LaH₂monolayer (ML), whose intrinsic properties can overcome all these shortcomings. We demonstrate that LaH₂ML, having satisfied structural stability, is a FM half-semiconducting electrene (La3+2H-⋅e-) with its magnetic moments localized at the lattice interstitial sites rather than La atoms. At the same time, LaH₂ML holds the following desired attributes: a robust out-of-plane FM ground state with a highTC(334 K), a sizable valley polarization (166 meV), a wide energy window (137 meV) harboring clean single-valley carriers, and a direct bandgap. These results identify a much needed ideal ferrovalley semiconductor candidate, holding the promising application potential in valleytronics and spintronics devices.

The impacts of molecular adsorption on antiferromagnetic MnPS3 monolayers: enhanced magnetic anisotropy and intralayer Dzyaloshinskii-Moriya interaction

In two-dimensional (2D) magnetic systems, significant magnetic anisotropy is required to protect magnetic ordering against thermal fluctuation. In this paper, we explored the effect of molecular adsorption on the magnetic anisotropy and intralayer Dzyaloshinskii-Moriya interaction (DMI) of monolayer MnPS₃, combining the first-principles calculation and theoretical analysis. We find that molecular adsorption can break the spatial inversion symmetry in a 2D magnet, and results in a significant DMI, which is rare in pristine 2D magnets. For example, in an MPS-NO system, the magnitude of the asymmetric DMI vector increases 9 times, and the magnetocrystalline anisotropy increases 600 times compared with the pristine MPS monolayer. It is found the DMI mainly comes from the structural deformation after adsorption, whereas the increase of magnetocrystalline anisotropy mainly originates from a new 'bridge' super-exchange interaction between Mn ions and NO gas molecules. The calculated Mn-NO-Mn 'bridge' super-exchange coupling strength is much higher than the Mn-S-Mn coupling strength. Our findings offer a new strategy to increase the magnetic anisotropy and induce chiral magnetic structures in 2D magnets.

Janus 2H-VSSe monolayer: two-dimensional valleytronic semiconductor with nonvolatile valley polarization

Valleytronic as a hot topic in recent years focuses on electrons' valley degree of freedom as a quantum information carrier. Here, by combining two-bandk.pmodel with high-throughput density functional theory (DFT) calculations, the valley states of Janus 2H-VSSe monolayer are studied which have spontaneous polarization. Nonvolatile valley polarization state is mainly arises from intrinsic ferromagnetism contributed by V-3d electronic configuration and not the spontaneous out-of-plane dipole moment of VSSe monolayer. The effective Hamiltonian model and DFT calculations both showed that the valley splitting mainly originates from the smaller spin splitting coming from the spin-orbit coupling effect rather than the spin splitting of magnetic exchange field. By using the effective Dirac Hamiltonian and Kubo formula, we further calculated the longitudinal and transversal conductivities and absorption spectra of VSSe monolayer which exhibits an anomalous valley Hall effect and clear valley-selective circular dichroism. Our calculations indicate that the modification of valley and spin splitting related to Berry curvature by applying an external strain is more noticeable than by the change of the magnetic moment orientation and electric field. We found that carriers accumulation with particular spin and valley label can be manipulated by tuning effective Hamiltonian parameters. The coexistence of robust in-plane magnetic ordering and spontaneous valley polarization of 2H-VSSe monolayer supports the possibility of applications in spintronics, valleytronics and optoelectronics devices.

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.