First-principle Calculations On Two-dimensional Hexagonal Magnetic Semiconductors

The emergence of novel properties and potential applications of two-dimensional crystals has recently stimulated considerable worldwide attention.A particularly interesting class of candidate two-dimensional crystals are the layered magnetic semiconductors which can simultaneously display semiconductor characteristics and magnetic ordering that are advantageous for nano-spintronics applications.Nonetheless,Mermin-Wagner theory demonstrates that there is an absence of spontaneous polarization in an two-dimensional isotropy Heisenberg model.Along with the fact that the unsatisfactory compatibility between semiconductivity and magnetism,at present,the intrinsic layered van der Waals ferromagnetic semiconductors are rare.In this paper,based on first-principle calculations,we give a systematically investigation on the stability,electronic and magnetic properties of transition metal chalcogenide layered CrGeS₃,Janus VXY（X,Y=Cl,Br and I,X≠ Y）and transition metal iodide V₂Cr₂I₉.Besides,the reason for high TC and for their external-strain-dependent character are investigated.We also find effective strategy to control the ferromagnetism by biaxial strain and electric field.First we predict that the electric property and magnetism of monolayer CrGeS₃,which shows a ferromagnetic ground state with an exchange splitting.The ferromagnetism mainly originates from the superexchange interaction between nearest-neighboring Cr atoms mediated by S atoms.The exchange splitting is calculated as 0.67 eV,which originates from bonding d pσ hybridized states of Cr eg-S p and unoccupied Cr t2g-Ge p hybridization.In addition,it is found that the exfoliation energy of monolayer CrGeS₃ is close to graphene,so we believe that CrGeS₃ can be experimentally exfoliated.Also,the monolayer CrGeS₃ possesses the TC of 161 K by mean-field theory.Second,based on the recently experimentally synthesized monolayer VI3,we construct Janus structure VXY by breaking the out-of-plane symmetry.The TC of monolayer VXY is estimated as 240 K by Monte Carlo simulations with Heisenberg module.And it is discussed that the reason for high TC in Janus structure originates from the lower virtual exchange gap between t2g and eg states of nearest-neighbor V atoms.Besides,TC can be further substantially enhanced by tensile strain due to the increasing ferromagnetism driven by rapidly quenched direct exchange interaction.At last,we design monolayer V₂Cr₂I₉ by stacking monolayer VI3 onto CrI3 with a middle-layer I atoms discarded.After that,the stable and metastable phases are determined among 7 possible phases by first-principles calculations.It is illustrated that both the two phases have Curie temperature～6（4）times higher than monolayer CrI3 and VI3.The reason can be partly attributed to their large magnetic anisotropy energy（the maximum value reaches 412.9 μeV/atom）.More importantly,the Curie temperature shows an electric field and strain dependent character and can even surpass room temperature under a moderate strain range.