Theoretical Study On The Electronic Structure And Magnetism Of Two-dimensional Ternary Chalcogenides

The successful preparation of graphene has promoted the research of two-dimensional(2D)materials.In recent years,more and more graphene-like materials,such as molybdenum disulfide,hexagonal boron nitride and black phosphorus,have been synthesized in the laboratory.Due to their excellent physical and chemical properties,they have important applications in optoelectronic devices,field effect transistors and other devices.However,most 2D materials are not magnetic,which greatly limits the application in the field of spintronics.Therefore,it is of great significance to search or design 2D magnetic materials with excellent properties.With the preparation of 2D Cr PS₄,Cr Ga Se₃and other materials,2D ternary chalcogenides have gradually attracted people's attention.Based on these findings,we studied the electronic structure and magnetism of 2D ternary chalcogenides MGa₂X₄ through first-principles calculations.The main research contents and conclusions are as follows:(1)Study on the electronic structure and magnetic properties of 2D ternary compound MGa₂X₄.T-Fe Ga₂S₄ and H-Mo Si₂N₄ were obtained by chemical vapor deposition.Inspired by these two materials,we constructed three possible T-type and H-type MGa₂X₄ monolayers,where M represents six elements in 3d transition metals,namely V,Cr,Mn,Fe,Co and Ni,and X represents S,Se and Te.After optimizing all the T-type and H-type MGa₂X₄,the structures with the lowest system energy are selected as the research objects.On the basis,the electronic structure and magnetic properties of these manolayers are calculated.It is found that MGa₂X₄ system covers a variety of electronic and magnetic properties,such as ferromagnetic metal,ferromagnetic half-metal and antiferromagnetic semiconductor.Based on molecular field theory and exchange model,we theoretically analyze the source of magnetic moment and the coupling between magnetic atoms in the system.For the ferromagnetic Cr Ga₂Te₄ monolayer with high spin polarization,the magnetic anisotropy characteristics of Cr Ga₂Te₄ monolayer with spin-orbit coupling are calculated,and it is found that the magnetic anisotropy energy(MAE)is very large.Monte Carlo simulations based on the Heisenberg model predict that the Curie temperature of Cr Ga₂Te₄ monolayer is as high as 442 K,which is higher than that of Cr I₃ monolayer(45 K)and Cr₂Ge₂Te₃(30 K)reported experimentally.Finally,ab initio molecular dynamics simulations and phonon dispersion calculations verify the good thermal and dynamical stabilities of Cr Ga₂Te₄ monolayer.Therefore,2D MGa₂X₄ system has a rich variety of electronic structures and magnetic properties,and has a good development potential in the design of spintronic devices.(2)Study on the electronic structure and magnetic anisotropy of two-dimensional ferromagnetic semiconductor Cr Ga₂Se₄.Using first-principles calculations,we further explored the Cr Ga₂Se₄ monolayer,which has excellent electronic structure,large magnetic moment and high Curie temperature.The calculation of electronic structure based on hybrid functional approximation shows that the band structure of Cr Ga₂Se₄ monolayer belongs to ferromagnetic semiconductor,the valence band maximum(VBM)of spin-up channel band and the conduction band minimum(CBM)of spin-down channel band are just located at the Fermi level,which is a kind of semiconductor known as“spin-gapless”.Ab initio molecular dynamics calculations show that Cr Ga₂Se₄ monolayer maintains good thermal stability at room temperature.The magnetic anisotropy energy of Cr Ga₂Se₄monolayer is further calculated by considering the spin-orbit coupling.It is found that the magnetic anisotropy energy of Cr Ga₂Se₄ monolayer is large,and there is significant magnetic anisotropy characteristics both in the basal plane and the vertical plane.The analysis shows that the phenomenon is mainly caused by the distortion-induced rearrangement of the 3d electrons in the Cr Se₆ octahedron.The Curie temperature of Cr Ga₂Se₄ monolayer is estimated to be about 220 K by Monte Carlo simulations.