Modulation On Electronic Structure Of Two-Dimensional Intrinsic Ferromagnetic Semiconductor VSe₂ And CrI₃

Spintronic devices based on the ferromagnetic（FM）semiconductors,such as non-volatile magnetic memories,have the excellent characteristics such as low power consumption,fast speed and high storage density.Therefore,the development of new two dimensional（2D）intrinsic FM semiconductors is of great significance for the research of high-performance ultrathin semiconductor spintronic devices.In this dissertation,the electronic structure and magnetic properties of the 2D intrinsic FM semiconductors VSe₂ and Cr₂I₃X₃（X=Br,Cl）are studied by density functional theory.The effects of interfacial proximity and strains on the electronic structure,magnetic properties and valley characteristics of the 2D VSe₂ and Cr₂I₃X₃（X=Br,Cl）are analyzed.The results provide a theoretical basis for spintronic and valleytronic devices based on 2D FM semiconductors.The electronic structure of monolayer 2H-VSe₂/BiFeO₃（111）triferroic heterostructures is investigated.It is found that the V magnetic moment,spin splitting and valley splitting of monolayer VSe₂ can be tailored by the ferroelectric polarization and G-type antiferromagnetic order of BiFeO₃（111）substrate.Particularly,the reversed orientation of ferroelectric polarization and magnetic order of the BiFeO₃（111）substrate can modulate the magnitude of spin and valley splitting,and change the spin splitting direction and the spin-dependent valley state of monolayer VSe₂.The coupling among ferroelectrics,magnetism and ferrovalley can be realized in 2H-VSe₂/BiFeO₃（111）triferroic heterostructures.The electronic structure and magnetic properties of Janus Cr₂I₃X₃（X=Br,Cl）monolayers are investigated.Janus Cr₂I₃X₃ monolayers are an indirect-band-gap semiconductor with good stability,intrinsic ferromagnetism and electric polarization.Janus Cr₂I₃Br₃ monolayer is a half semiconductor with the perpendicular magnetic anisotropy（PMA）,while Janus Cr₂I₃Cl₃ monolayer is a bipolar magnetic semiconductor with the in-plane magnetic anisotropy（IMA）.The band gap and Cr magnetic moment of Janus Cr₂I₃X₃ monolayers can be tailored by the biaxial strains.Additionally,a strain-induced transition from bipolar magnetic semiconductor to half semiconductor appears in Janus Cr₂I₃X₃ monolayers,where IMA also turns into PMA by applying the biaxial strains.The stacking-dependent electronic structure and magnetic properties of the 2D van der Waals Cr₂I₃X₃（X=Br,Cl）bilayers are investigated.The magnetic ground state and band gap of the Cr₂I₃X₃ bilayers can be influenced by the stacking patterns.In the Janus Cr₂I₃X₃ bilayers,the electrostatic potential and electric polarization can be greatly affected by combination modes,which can be attributed to the parallel or antiparallel built-in electric field between the monolayers.The Janus Cr₂I₃X₃ bilayers show the PMA.The magnetic anisotropy energy of Cr₂I₃Cl₃ bilayer is larger than the Cr₂I₃Br₃ bilayer.Additionally,the Dzyaloshinskii-Moriya interaction of Cr₂I₃Br₃bilayer can also be modulated by the combination modes.