Magnetic Properties Regulation Of Two-dimensional Chromium Chalcogenides Bilayer Through Theoretical Study

Two-dimensional（2D）van der Waals magnetic materials with ultra-thin thickness and dangling-bond-free surfaces are very sensitive to external regulation.Generally,their lattice and electromagnetic properties can be effectively controlled by magnetic field,electric field and carrier,illumination,defects,doping,intercalation,strain,pressure and interlayer stacking.In this paper,based on the first-principles calculation,stain,intercalation and interlayer stacking are used to regulate the magnetic properties of 2D chromium chalcogenide compounds bilayer.The magnetic coupling mechanism of intralayer and interlayer is discussed,and the magnetic peoperties,such as Curie temperature of these materials are also calculated.Firstly,we investigate the electromagnetic properties of the bilayer CrTe₂ and determine that the bilayer CrTe₂ is a metallic magnet with interlayer antiferromagnetism and a Néel temperature of 170K.We find that the magnetic properties of the CrTe₂ bilayer change from intralayer antiferromagnetic coupling to ferromagnetic coupling,and then to interlayer antiferromagnetic coupling when the strain changes from-6%to 4%.This phenomenon of intralayer magnetic transformation is mainly due to the increase of the distance between the two nearest Cr atoms,and the magnetic coupling mechanism changes from Cr-Cr direct exchange to Cr-Te-Cr indirect superexchange mode.The magnetic transition of interlayers is mainly due to the influence of the super-superexchange mechanism of Cr-Te-Te-Cr.At the same time,the easy magnetization axis changes from the in-plane to the out-of-plane,which is mainly due to the influence of the 5p orbital distribution of Te atom.Secondly,we study the influence of intercalation on the magnetic properties of CrTe₂ bilayer.It is found that Cr self-intercalation changes the magnetic properties of the original CrTe₂ bilayer from interlayer antiferromagnetic coupling to ferromagnetic coupling,and the magnetic crystal anisotropy of the structure changes from out-of-plane to in-plane direction.This is mainly because both Cr and Te atoms can affect the magnetic crystal anisotropy of the structure.The density of states distribution of the intercalated structure at 4%strain shows that the magnetic crystal anisotropy will be along the z axis only when the Cr-3d_z² and Te-5p_z orbits occupy the main distribution at the Fermi level at the same time.Then,the magnetic properties of 2D Janus CrSeTe bilayer and the effects of strain on the exchange coupling constant and Curie temperature are investigated.It is found that the AB-stacking CrSeTe bilayer is the most stable structure and is a kind of in-plane ferromagnetic metal material.The intralayer nearest neighbor exchange coupling constant J₁,the intralayer second-neighbor exchange coupling constant J₂and the interlayer nearest neighbor exchange coupling constant J’are all positive,and the Curie temperature is 270K,close to room temperature.Under the strain of-2%to2%,J₁ increases,J₂ decreases slowly,and J’increases slowly.These changes are related to the bond length of atoms and layer spacing.At the same time,the Curie temperature also increases with the increase of strain,and is all close to room temperature.Finally,we study the electromagnetic properties of bilayer Janus Cr STe with different stacking modes.In the antiparallel stack of Cr STe bilayer,AB-stacking have the lowest energy and its stability is verified by phonon spectrum.The band and total density of states verifies that the AB-stacking Cr STe bilayer is a ferromagnetic metal,and its easy magnetization direction is along the b axis.The Curie temperature of275K is simulated by Monte Carlo,which is close to room temperature.The magnetic properties of AB-stacking Cr STe bilayer under strain are also investigated.It is found that the magnetic properties of AB-stacking Cr STe bilayer change from ferromagnetic ground state to interlayer antiferromagnetic ground state.At the same time,the strain will increase the Curie temperature of AB-stacking Cr STe bilayer.In addition,we also found that the AB-stacking and AC-stacking Cr STe bilayer have different magnetic ground states in a certain strain range,indicating that the Cr STe bilayer can be used as a double spin filter.The difference of magnetic ground states between AB-Cr STe and AC-Cr STe can be explained by super-superexchange,the layer spacing of the two structures is different,and the difference of layer spacing will affect the charge distribution and spin density distribution between layers,and then affect the spin distribution of magnetic atoms in the upper and lower layers.At the same time,the magnetic anisotropy of Janus Cr STe bilayer is mainly determined by the 5p orbital distribution of chalcogenide atoms.Since the 5p orbital density of Te+S atom shows that p_y orbitals play a dominant role near the Fermi level,its easy magnetization axes are all along the b axis.