Screening and Design of Novel 2D Ferromagnetic Materials with High Curie Temperature above Room Temperature

2D Magnetic Janus Semiconductors with Exotic Structural and Quantum-Phase Transitions

2D magnetic semiconductors are intriguing for their great potential applications in spintronic nanodevices. Despite intensive research for decades, intrinsically 2D magnetic Janus semiconductors are scarce, and their design guidelines remain elusive. Herein we propose new 2D Janus Cr₂O₂XY (X = Cl, Y = Br/I) ferromagnets with asymmetric out-of-plane structural configurations from ab initio calculations. Abnormally, 2D Janus Cr₂O₂XY crystals with Pmm2 structures derived from pristine CrOX compounds are dynamically metastable. By introducing novel structural phase transitions, we generated new Pma2 phases with lower total energy and great dynamical stability. These new Janus Cr₂O₂XY monolayers are intrinsically ferromagnetic semiconductors and could be easily synthesized from experiment. Most interestingly, exotic quantum-phase transitions from the ferromagnetic semiconductor to the antiferromagnetic metal/semiconductor could be achieved in the Cr₂O₂ClI monolayer by applying compressive strains. Our study provides an alternative strategy to design new Janus Cr₂O₂XY monolayers and will inspire further investigations on relevant materials for electronic and spintronic applications.

High-Temperature Ferromagnetism in an Fe3P Monolayer with a Large Magnetic Anisotropy

For the development of high-performance spintronic nanodevices, one of the most urgent and challenging tasks is the preparation of two-dimensional materials with room-temperature ferromagnetism and a large magnetic anisotropic energy (MAE). Through first-principles swarm-intelligence structural search calculations, we identify an ideal ferromagnetic Fe₃P monolayer, in which Fe atoms show a perfect Kagome lattice, leading to strong in-plane Fe-Fe coupling. The predicted Curie temperature of Fe₃P reaches ∼420 K, and its MAE is comparable to those of ferromagnetic materials, such as Fe and Fe₂Si. Moreover, the Fe₃P monolayer remains as an above room-temperature ferromagnet under biaxial strains as large as 10%. Its lattice can be retained at temperatures of ≤1000 K, exhibiting a high thermodynamic stability. All of these desirable properties make the Fe₃P monolayer a promising candidate for applications in spintronic nanodevices.

Valence mediated tunable magnetism and electronic properties by ferroelectric polarization switching in 2D FeI2/In2Se3 van der Waals heterostructures

Exploring two-dimensional (2D) materials with both ferromagnetic and ferroelectric properties is scientifically interesting and of great technical importance to numerous functionalities in nanoscale devices. In this work, we have demonstrated a strong magnetoelectric coupling that appeared in the 2D FeI₂/In₂Se₃ van der Waals heterostructure. FeI₂ layers undergo a transition from ferromagnetic to antiferromagnetic by reversing the direction of ferroelectric polarization. First-principles calculation predicts a new magnetoelectronic coupling mechanism which is completely different from the Dzyaloshinskii-Moriya (DM) effect in multiferroic materials. Because of the polarization discontinuity at the interface, the valence states of Fe ions change between +2 and +3 for two different polarization directions, leading to the magnetic interaction variation between the direct exchange and I ion mediated superexchange. Moreover, metallic 2D electron gas (2DEG) transfers from the surface of FeI₂ to In₂Se₃ when the polarization reverses, which induces the spin polarization of the heterostructure varying from 93% to 0%. Our work is the first realization of manipulation magnetism by an electric field in full 2D van der Waals heterostructures.

Toward Room-Temperature Magnetic Semiconductors in Two-Dimensional Ferrimagnetic Organometallic Lattices

Obtaining room-temperature magnetically ordered two-dimensional (2D) semiconductors is urgently needed for high-speed nanospintronic devices but remains a big challenge. Here, we propose a potential route to solve this issue by constructing ferrimagnetic semiconductors in 2D metal organic frameworks, taking advantage of the high Curie temperature of ferrimagnetic semiconductors and easy tunability of metal organic frameworks. The proposal is confirmed by first-principles design of 2D metal organic frameworks with conjugated electron acceptors diketopyrrolopyrrole (DPP) as organic linkers and transition metal Cr (V) as nodes. The robust ferrimagnetic ordering comes from the strong direct exchange interaction between d-electron magnetic moments on transition metals and charge transfer-induced p-electron magnetic moments on DPPs, which can be modulated facilely by reducing the d-p orbital interaction distance via moderate compressive strain or increasing the d-p orbital charge transfer through introducing electron-withdrawing groups into the DPP moiety.

Large Tunneling Magnetoresistance in VSe2/MoS2 Magnetic Tunnel Junction

Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe₂/MoS₂ heterojunction, where the VSe₂ monolayer acts as a ferromagnet with room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The nonequilibrium study reveals a large tunneling magnetoresistance of 846% at 300 K, identifying significantly its parallel and antiparallel states. Thanks to the strong spin Hall conductivity of MoS₂, SOT is promising for the magnetization switching of VSe₂ free layer. Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe₂/MoS₂ provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.

Strain-tunable electric structure and magnetic anisotropy in monolayer CrSI

Two-dimensional (2D) ferromagnetic semiconductors provide platforms for studying novel physical phenomena in low dimensional materials.