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Creating low dimensional ferromagnetic (FM) semiconductors or half metals with strong FM orders is promising to meet the requirement for next-generation spintronics. However, most of the demonstrated FM semiconductors or half metals suffer from low Curie temperatures (TCs). Here, by first-principles calculations, we predict that the two-dimensional (2D) M3XSe4 (M = V, Cr; X = S, Te) monolayers are a type of intrinsic 2D ferromagnets with thermodynamical stability. Our results show that V3XSe4 (X = S, Te) monolayers are FM semiconductors with indirect bandgaps of 0.60 and 0.50 eV, respectively. Particularly, both structures are revealed to have high TCs of 387 and 770 K and suppress the application limit of room-temperature. In addition, Cr3XSe4 (X = S, Te) monolayers are FM half metals with 100% spin-polarized currents. Moreover, the electronic and magnetic properties of these M3XSe4 monolayers can be modulated by biaxial strains. V3TeSe4 monolayer can be tuned to be room temperature direct bandgap semiconductor under biaxial 1% tensile strain, and TC of V3SSe4 can be largely enhanced under compressive strains. Our results suggest that M3XSe4 monolayers are promising candidates for spintronic devices. [ABSTRACT FROM AUTHOR]

The valley-polarized quantum anomalous Hall (VP-QAH) materials, which combine valley polarization and the quantum anomalous Hall (QAH) effect, are of both fundamental and technological importance due to their potential applications for nanoscale devices. Here, we explored the stability, magnetic, and topological properties of two-dimensional MnTiX2 (X = F/Cl/Br) monolayers based on first-principles calculations. Our results show that all the structures have robust antiferromagnetic orders with large magnetic anisotropic energies and high transition temperatures (480–670 K). In the absence of spin–orbital coupling (SOC), the MnTiX2 monolayers represent antiferromagnetic quadratic crossing semimetals. In the presence of SOC, the quadratic crossing points of the systems are opened with sizable bandgaps (> 0.5 eV), and they are transformed to be QAH insulators with |C| = 1. Moreover, the MnTiF2 monolayer is found to be a spontaneous VP-QAH semiconductor due to the time reversal symmetry and inversion symmetry being broken. These insights provide an ideal platform for achieving VP-QAH materials for dissipationless transport and quantum computing. [ABSTRACT FROM AUTHOR]

Realizing the rational control of two-dimensional valleytronics is appealing for potential applications in valleytronic devices. Using density functional theory method, we systematically study the magnetoelectronic coupling properties in ferrovalley/ferroelectricity van der Waals (vdW) heterostructure, ScI2/Ga2STe2. Interestingly, an on/off transition of the valley nature is found for this multiferroic ScI2/Ga2STe2 heterostructure. Under the Ga2STe2-P↑ polarization state, the heterostructure shows a ferromagnetic (FM) semiconducting feature with valley polarization, while it turns out to be FM metal when the ferroelectric (FE) polarization is switched to Ga2STe2-P↓; therefore, a switchable valleytronic device can be realized by altering the FE polarization direction. Furthermore, it is found that such switchable valleytronic behaviors of the ScI2/Ga2STe2 heterostructure can be modulated by applying biaxial strains for both FE polarization directions. The research of nonvolatile electrical control of valley splitting in the two-dimensional ScI2/Ga2STe2 multiferroic heterostructure provides a way for designing all-in-one valleytronic devices. [ABSTRACT FROM AUTHOR]

To realize the quantum anomalous Hall (QAH) effect in two-dimensional (2D) intrinsic magnetic materials, which combines insulating bulk states and metallic edge channel states, is still challenging in experiment. Here, based on first-principles calculations, we predicted two stable kagome-latticed QAH insulators: Cr 3 Se 4 and Fe 3 S 4 monolayers, with the Chern number C = 1. It is found that both structures exhibit a large magnetic anisotropy energy and sizable band gaps, and a topological phase transition from C = -1 to C = 1 occurs when the magnetization orientation changes from the z -axis to the - z -axis. Remarkably, the non-trivial topological properties are robust against biaxial strains of up to ±6%. Furthermore, a variable high Chern number of C = 2 or C = 3 can be observed by stacking two or three layers of the QAH monolayer with an MoS 2 insulator. Our results signify that such layered kagome materials can be promising platforms for exploring novel QAH physics.