First-principles Study Of Two Dimensional Spintronic Material CrX₂

The novel quantum properties and high flexibility make two-dimensional materials ideal carriers for next-generation nanoelectronic devices.Among various two-dimensional materials with unique chemical and physical properties,layered transition metal Cr-based materials,have attracted much attention because of their excellent electromagnetic properties and optical properties,especially their possible structural phase transitions.However,many two-dimensional materials are not magnetic,and the degenerate spins limit their applications in spintronic devices.Therefore,the design of two-dimensional materials with intrinsic magnetic properties is very important for the application of spintronics and the development of nanotechnology.It is shown that modulations such as external electric fields,defects and layers are emerging as effective means to induce the intrinsic magnetic properties in materials.In this thesis,Cr X₂（X=P,As,Sb,S,Se,Te）were investigated for their stability,electronic structure and magnetic properties based on first-principle calculations.The effects of external electric field,vacancy and layer number on their electronic structure and magnetic properties were also investigated.The study is divided into four main areas as follows:First,the stability,electronic structure and magnetic properties of monolayer Cr X₂were investigated.The results show that Cr P₂does not satisfy all stability conditions except mechanical stability,and Cr As₂and Cr Sb₂do not satisfy chemical stability,mechanical stability,kinetic stability,and thermal stability.Cr S₂,Cr Se₂and Cr Te₂satisfy the chemical,mechanical,kinetic and thermal stability,and can maintain their independent planar structure without substrate support.The electronic structure and magnetic properties calculations show that Cr P₂behaves as a ferromagnetic metal,and Cr As₂and Cr Sb₂as antiferromagnetic metals.Cr S₂,Cr Se₂and Cr Te₂are nonmagnetic semiconductors with band gaps of 0.9288 e V,0.7506e V and 0.5303 e V,respectively.Second,the effect of 0～3 V/nm applied electric field on the electronic structure and magnetic properties of Cr X₂was investigated.By analyzing the electronic structure and magnetic properties under 0～3 V/nm applied electric field,it is found that the response of Cr Sb₂to the electric field is larger than that of Cr P₂and Cr As₂.This difference may be due to the larger atomic number of Sb compared to P and As atoms,which enhances the charge transfer ability and thus affects the band gap properties.Apart from this,the contribution of Cr atoms in Cr P₂,Cr As₂and Cr Sb₂with P,As and Sb atoms to the bands near the Fermi energy level does not change with increasing electric field values.The positions of conduction band minimum and valance band maximum of Cr S₂,Cr Se₂and Cr Te₂are slightly shifted under the influence of the external electric field,but the band does not change significantly and their band gaps do not change significantly.This indicates that the applied electric field does not effectively modulate the band gap properties of the monolayer Cr X₂,which has a good resistance to the external electric field effect.In addition,the total and atomic magnetic moments of Cr X₂always remain constant for different electric field values.Again,the effect of vacancy on the electronic structure and magnetic properties of Cr S₂,Cr Se₂and Cr Te₂was investigated by constructing Cr vacancy structures with concentration of11.11%.The results show that after considering Cr vacancy,Cr S₂and Cr Se₂behave as ferromagnetic metals and semiconductors with magnetic moments of 6.7μ_Band 8.0μ_B,respectively,while Cr Te₂behaves as an antiferromagnetic metal.After applying biaxial strain to the system with Cr vacancy,Cr S₂,Cr Se₂and Cr Te₂exhibit possible structural phase transitions.The effects of strain on the electronic structure and magnetic properties of Cr S₂,Cr Se₂and Cr Te₂with Cr vacancy are also elucidated from the perspectives of charge transfer,which may provide valuable clues for the application of Cr S₂,Cr Se₂and Cr Te₂in magnetoelectronic devices.Finally,the electronic structure and magnetic properties of the bilayer Cr X₂were further explored,mainly considering AA stacking and AB stacking.By comparing the total energy of the two stacking methods,it is found that the most stable stacking method for Cr P₂,Cr As₂and Cr Sb₂is AB stacking.While the most stable stacking method for Cr S₂,Cr Se₂and Cr Te₂is AA stacking,and the formation energy is less than zero for all of them,which is expected to be synthesized experimentally.By comparing the total magnetic moments of monolayer and bilayer Cr X₂,it is found that the magnetic properties of Cr X₂are less affected by the number of layers.In summary,Cr X₂（X=P,As,Sb,S,Se,Te）is a potential candidate material for spintronics.The electronic structure and magnetic properties of Cr X₂are well resistant to external electric field and layer modulation.Meanwhile,it is demonstrated through theoretical studies that the electromagnetic properties of Cr X₂,can be greatly modulated by vacancy and strain,providing theoretical guidance for the application of defects and strain engineering on two-dimensional magnetic materials.