Structural Design And Properties Study Of Two-Dimensional Intrinsic Boron-based Magnetic Materials

Since graphene was discovered,the research on the magnetic properties of twodimensional materials has never been stopped.With the successful experimental exfoliation of monolayer CrI3 and bilayer Cr2Ge2Te6 and long-range magnetic order observed in 2017,the development of 2D magnetic materials entered a new chapter,and since then more and more intrinsic 2D magnetic materials have been reported theoretically and experimentally.However,there are not many kinds of 2D magnetic materials synthesized in the laboratory,and the transition temperature is also low,which is quite limited for practical spintronic device applications.Therefore,the exploration of more new 2D magnetic material family systems remains an important topic in the field of condensed matter physics.Theoretical calculations have more obvious advantages in finding materials with specific properties than the high cost of experimental trial and error.In this paper,a series of boronbased 2D magnetic materials,including two types of transition metal borides with single atomic layer thickness:TMB3,TMB4,one type of transition metal borides with multiple atomic layer thickness:TM2B3,and one type of ternary transition metal borides:Cr2BC,are designed by using first principles calculations based on density generalized theory,and their physical properties are analyzed in detail,mainly including:(1)TMB3:Monolayers with robust intrinsic magnetism and high spin stabilityThis class of monolayer TMB3(TM=Ti-Fe)system not only has good thermodynamic stability and kinetic stability,but also shows high mechanical anisotropy in terms of Young’s modulus and Poisson’s ratio.In addition the monolayer TMB3 family is rich in electromagnetic properties,among which TiB3 and FeB3 are stable ferromagnetic metals with Curie temperatures of 248 K and 367 K.CrB3 is an antiferromagnetic semiconductor with a direct band gap of 513.6 meV and a magnetic transition temperature of up to 378 K.Furthermore,we found that the magnetic properties could be further tuned by applying external strain.(2)TMB4:Intrinsic magnetism with high critical temperatures and piezoelectricityBased on energy calculations,phonon spectra and ab initio molecular dynamics simulations,it is confirmed that all such TMB4 monolayers we predicted behave stably.Interestingly,the system still has a solid ferromagnetic order at higher critical temperatures of 177 K,229 K,and 325 K when the TMs are Cr,Mn,and Fe,while the TMB4 still exhibit a stable antiferromagnetic ground state at high critical temperatures in 390～755 K range when the TMs are Ti,V,and Co.Compared with CrI3 and Fe3GeTe2,the magnetic anisotropy performance of TMB4 is comparable or greater.In addition,the calculated monolayer CoB4 was determined to be a class of semiconductors with both out-of-plane magnetic susceptibility axis and out-of-plane piezoelectricity.(3)TM2B3:Intrinsic antiferromagnetism and Dirac nodal line semimetalA new class of two-dimensional metal borides,TM2B3(TM=Ti-Ni),was predicted based on the density flooding theory.All compounds exhibit good kinetic,thermodynamic and mechanical stability.Among of them,Ti2B3 monolayer and and V2B3 are found to be antiferromagnetic ground state materials with high magnetic transition temperatures,with V2B3 monolayers and Co2B3 monolayers having transition temperatures above room temperature.Interestingly,the Mn2B3 monolayer was identified as an antiferromagnetic Dirac nodal line semimetal.(4)Cr2BC:A newly designed antiferromagnetic semiconductor 2D metal boro-carbideThe predicted three several configurations of Cr2BC are shown to be a class of dynamic and thermodynamically stable antiferromagnetic semiconductors.By applying strain,the electronic properties of Type-III monolayer Cr2BC can be transformed from an indirect bandgap semiconductor to a direct bandgap semiconductor with a bandgap of up to 0.11 eV under a small range of biaxial compressive strain.With increasing strain,both biaxial and uniaxial y-axis compressive strains induce a transition in the magnetic ground state of the material,and also further increase the magnetic transition temperature of the material.