First-Principles Design Of Two-dimensional Van Der Waals Magnetic Materials And Heterostructures

Spintronics uses electron spin to realize ultra-fast,ultra-low-power-consumption and ultra-high-density logic manipulation and information storage.Owing to the atomic thickness and flat layered structure,various physical properties,and continuous breakthroughs in experimental preparation,two dimensional van der Waals（vd Ws）magnetic materials are expected to become an ideal platform for the next-generation spintronic devices.Focusing on the structural and performance characteristics of vd Ws magnetic materials and from the perspective of vd Ws interlayer interaction and symmetry,this thesis uses the first-principles calculations as the major research tool to explore effective strategies for the design of two-dimensional magnetic materials and the controlling of magnetic properties and electronic structures.The main researches of this thesis are as follows:（1）The effects of vd Ws stacking order on the electronic structure and magnetism of bilayer H-VSe₂ are explored,and the interlayer magnetic coupling transition and metal-to-semiconductor transition caused by stacking order change are studied.The origin of magnetic transition is explained from the perspective of the competition mechanism between the interlayer double exchange and the super-super exchange,and the reason of metal-to-semiconductor transition is analyzed from the perspective of interlayer quasi-bond formation.On this basis,this work have explored the possibility of realizing ferromagnetic-to-antiferromagnetic transition in bilayer H-VSe₂ by carrier doping or pressure.（2）A ferromagnetic/non-magnetic/ferromagnetic van der Waals spin valve model based on a buckled honeycomb middle layer is proposed,in where the reversible control of metal-semiconductor transition can be realized by controlling the parallel/antiparallel arrangement of the magnetic moments in the two ferromagnetic layers.Based on the first-principles calculations,12 kinds of such vd W spin valves are designed to confirm this model.Besides,taking the Cr I₃/Ge/Cr I₃ and Cr I₃/Sn P₃/Cr I₃heterostructures as examples,the first-principles calculations show that the strong magnetic proximity effect induced by the vd Ws interlayer quasi-bonds is the key to realize this kind of heterostructures.（3）The effect of in-plane uniaxial strain on the magnetic anisotropy of monolayer Re I₃ is studied.It is found that uniaxial strain can transform the monolayer Re I₃ from an in-plane isotropic XY-type ferromagnet to an Ising-type ferromagnet with uniaxial magnetic anisotropy.Based on the first-principles calculation,the numerical relationship between magnetocrystalline anisotropy energy and the strength of uniaxial strain is determined.Besides,based on the second-order perturbation analysis,the microscopic origin of uniaxial in-plane magnetic anisotropy is explained from the perspective of crystal field splitting.（4）A new type of monolayer Janus vd Ws material,VP（X,Y）₃（X,Y=S,Se,Te）is theoretically designed,and their stabilities,electronic structures and magnetism are systematically studied.The monolayer Janus VP（X,Y）₃ are predicted to be two-dimensional indirect-bandgap semiconductors with vertical magnetic anisotropy and the Néel antiferromagnetic ground state.and the non-zero Dzyaloshinskii-Moriya exchange coupling induced by the breaking of the central inversion symmetry are calculated.Based on Monte Carlo simulation,the Néel temperature is calculated,and based on atomistic spin simulations,possible antiferromagnetic skyrmions structures are explored.In two-dimensional magnets,the antiferromagnetic skyrmions are a type of topologically protected magnetic structures,which can be used in the design of novel logic devices and track memories with high integration and stability.