The Electronic Structure,Magnetism Manipulation And Spin Transport Characteristics Of Two-Dimensional Magnetic Materials CrBr₃ And CrSe₂

Recently,the discovery of plenty of two-dimensional（2D）intrinsic magnets has attracted widespread attention from the scientific and industrial circles.The extraordinary physical properties of 2D intrinsic magnets provide an excellent platform for the design and manufacture of high-performance spintronic devices.However,the Curie temperature T_C,which is much lower than room temperature,and the small magnetocrystalline anisotropy energy（MAE）,seriously hinders their practical application.In this research,two rarely studied and chemically stable ferromagnetic（FM）monolayers,semiconducting CrBr₃and metallic CrSe₂,are selected as the research objects.To effectively promote their low T_C（44.1 K and 19.3K respectively）and small perpendicular magnetocrystalline anisotropy（PMA）（197μe V/Cr and 525μe V/Cr respectively）,the manipulation of in-plane biaxial strain,charge doping and van der Waals（vd W）heterostructure engineering on their electronic structures and magnetic behaviors are investigated in detail.The spin-dependent transport characteristics of vd W magnetic tunnel junctions（MTJs）respectively based on monolayer CrBr₃and CrSe₂are also explored,and the structural design of vd W MTJs that can realize huge tunneling magnetoresistance（TMR）and excellent spin filtering is sought.Our work provides important guidance for the developing high-performance spintronic devices based on 2D magnetic materials.This dissertation mainly involves the following three research sections:（1）Based on density functional theory（DFT）calculations,we investigate the effects of biaxial strain and charge doping on the electronic structure and magnetism of monolayer CrBr₃and CrSe₂respectively.The results show that charge doping can much more effectively tune the electrical and magnetic behavior of monolayer CrBr₃compared with biaxial strain,especially hole doping,which can not only realize the transition from semiconductivity to half-metallicity,but also significantly improve T_Cand PMA of monolayer CrBr₃.A doping level of 1.0 h/u.c.can boost T_Cup to220.4 K,and a doping level of 0.8 h/u.c.results in PMA upsurge to 1775μe V/Cr.However,with respect to monolayer CrSe₂,it’s found biaxial strain is more efficient in tailoring magnetism than charge doping.A small compressive strain（1%）can induce magnetic ground state transition from the FM to the antiferromagnetic（AFM）,and a tensile strain of 8%can achieve a highly disrable T_C（1049.3 K）much higher than room temperature;in addition,a tensile stress of 10%also realize a huge and fascinating PMA（5475μe V/Cr）.（2）Two kinds of vd W heterostructures based on monolayer CrBr₃and CrSe₂are designed,namely CrBr₃/Mo S₂vd W heterobilayer and CrSe₂/WSe₂vd W heterobilayer.For CrBr₃/Mo S₂vd W heterobilayer,the calculation shows that the vertical compressive stress significantly increases the T_Cof CrBr₃,which stems from the n-type doping of CrBr₃and reinforced Cr-Br-Cr FM super-exchange interaction induced by structural distortion.At a pressure of 5.69 GPa,TC can be raised to 94.7K.The external electric field can effectively tune the magnetization orientation of CrBr₃.At an electric field of-0.9 V/（?）,the easy magnetic axis of CrBr₃switches from out-of-plane to in-plane;in addition,by tailoring the electric field strength,the band alignment of the heterobilayer can be readily switched among type-I,type-II and type-III,this will facilitate to expand the application prospect of CrBr₃in optoelectronic devices.With regard to CrSe₂/WSe₂vd W heterobilayer,the vertical compressive strain can also effectively improve the T_Cand PMA of CrSe2,the T_Ccan be uplifted to 113 K under a strain of 0.5（?）;moreover,a giant PMA up to 3860μe V/Cr is yielded under a strain of 0.6（?）.In addition,the applied electric field can also remarkably promote the FM coupling and T_Cof CrSe₂,an electric field of-1.6V/（?）can elevate T_Cto 85 K.Interestingly,under the proximity effect of CrSe₂ferromagnet,the valley splitting of WSe₂can be tuned by manipulating the electric field strength,when the electric field varies between-1.6 V/（?）and 1.2 V/（?）,a tunable valley splitting in the range of-18～24 me V can be obtained,indicating a promising valleytronic application of CrSe₂.（3）Using Quantum ATK software based on DFT and nonequilibrium Green’s function（NEGF）,two varieties of vd W MTJs based on FM CrBr₃semiconductor and CrSe₂conductor are designed and explored respectively.For type-I vd W MTJ based on CrBr₃,MTJ4 with 1T-Mo Se₂and 2H-Mo S₂respectively acting as electrodes and nonmagnetic（NM）spacing layer is proposed to be the most sensible design scheme,as it realizes the most excellent spin filtering(P_MTJ=99.98%)and hugest TMR ratio（4.07×10⁵%）;for type-II vd W MTJ based on 1T-CrSe₂,the（MTJ）_dualdesigned with1T-CrSe₂/1T-Mo Se₂dual-electrode also shows fascinating spin filtering(P_MTJ=99.96%)and huge TMR ratio（2.29×10⁵%）.The excellent spin transport characteristics in the two vd W MTJs are originated from the spin-dependent transmission barrier induced by interfacial charge transfer.Moreover,by investigating the influence of U_effon Cr 3d,biaxial strain,vd W functional and bias voltage on TMR,it is found that the most reasonably designed vd W MTJs based on CrBr₃and CrSe₂can well preserve robust high-performance spin transport properties,thus showing their huge application potential in the field of spintronic devices.Our work sheds new light on finding effective strategies to tailor the electronic structure and magnetism of 2D ferromagnets and to design nanoscale high-performance multifunctional spintronic devices.