Tunable Electronic Structure And Magnetic Properties Of Two-dimensional Intrinsic Ferromagnetic Monolayer VTe₂ And VX₃(X=Cl,Br,I)

Two-dimensional materials have extensive applications in the fields of nano-electronics,opto-electronics and energy due to its unique and excellent properties.However,most two-dimensional materials are limited in applications such as high-density nonvolatile data storage and spintronics due to they are nonmagnetic.Although a large number of studies have shown that the introduction of magnetism can be realized by magnetic doping,adsorption or vacancy defects in two-dimensional materials,the magnetism introduced by these methods is localized and difficult to control.Therefore,searching for the 2D materials with intrinsic magnetism is the key to solve the practical application problems.In recent years,two-dimensional intrinsic room temperature ferromagnetic materials 2H-VTe₂ and VX₃(X=Cl,Br,I)have attracted extensive attention due to their spontaneous ferromagnetism,valley polarization and high Curie temperature.The electronic structure and magnetic regulation of 2H-VTe₂and VX₃(X=Cl,Br,I)monolayers are systematically investigated by first principle calculation in this paper.1.Effects of strain and stacking patterns on the electronic structure,valley polarization and magnetocrystalline anisotropy energy(MAE)of layered 2H-VTe₂ are studied by first principles calculations.The results show that the 2H-VTe₂ monolayer possesses a large spontaneous valley polarization of 156.5 me V and in-plane ferromagnetic ground state with Curie temperature of 377 K.The large valley polarization mainly originates from the spontaneous magnetic exchange field rather than the SOC effect based on the effective Hamiltonian k·p model.The orbitalresolved MAE of V atoms shows that the observed in-plane MAE mainly originates from the contribution of d_xy and d²²_x-y orbitals of V atoms.With the increase of strain from-6%to 6%,the in-plane MAE and valley polarization of 2H-VTe₂ monolayer firstly increases and then gradually decreases,which are strongly related to the magnetic moment of V atoms.The stacking patterns can remarkably influence the valley polarization of layered 2H-VTe₂.The AB stacking pattern retains the space inversion symmetry,which leads to the decrease of valley polarization.On the contrary,the large valley polarization in AA stacking pattern can be attributed to the spatial inversion symmetry breaking,which leads to the inequivalence of two valleys.2.Based on the first principle calculation of density functional theory,the effects of in-plane strain and charge on the electronic structure,Curie temperature and magnetism of VX₃(X=Cl,Br,I)monolayer are systematically investigated.The results show that VCl₃ monolayers exhibit out off-plane ferromagnetism in the ground state,while VBr₃ and VI₃ monolayers exhibit in-plane ferromagnetism.Monte Carlo simulation based on Ising model shows that the Curie temperatures are VCl₃(94 K),VBr₃(103 K)and VI₃(116 K).It is worth noting that the three systems have half-metallic Dirac points caused by V-d states around the Fermi level.Under different biaxial strains,VX₃(X=Cl,Br,I)monolayer will change to antiferromagnetic state under tensile strain of more than 4%,and has tunable Curie temperature and MAE,which is related to the occupation of d_xy and d²²_x-y orbits of V atoms near Fermi level.Finally,we find that the magnetic phase transition of VX₃monolayer can be effectively adjusted under the electric field,which opens up new possibilities for broadening two-dimensional magnetic materials and developing high-performance electronic or spin electronic devices.