Computational Study Of Two 2D Ferromagnetic Structures At Room Temperature And Their Novel Quantum States

In recent years,the experimental discovery of the intrinsic magnetism of 2D materials has aroused great interest in 2D magnetic materials.Among them,2D ferromagnetic materials with high spin-polarization（e.g.,ferromagnetic semiconductor,ferromagnetic half-metal）are considered to be the key to the next generation of spintronic devices.However,the ferromagnetic materials found in the current experiment have the defects of high cost and low T_C.This greatly limits the application of 2D materials in the field of spintronics.With the improvement of computing hardware and the development of theoretical algorithms,first-principles calculation has become an effective way to explore new room-temperature ferromagnetic materials.Based on this,two types of 2D ferromagnetic materials（ferromagnetic HM and ferromagnetic semiconductor）with room temperature stability were designed and obtained by using first-principles calculation in this paper.The specific content and research results are as follows:（1）The structure,stability,electronic properties and magnetism of the ferromagnetic HM MnNBr nano-monolayer（ML）have been studied by the first-principles and the tight-bound approximation.The results show that the system not only has good energy,mechanical,kinetic and thermal stability（900 K）,but also has a T_C of up to 910 K for the ferromagnetic ground state.The calculation of electronic properties shows that the type-Ⅰ and type-Ⅱ Weyl states exist in the Fermi level of the 100%spin-polarized ferromagnetic HM MnNBr ML.Across nodes form continuous lines running a closed loop inside the BZ.More importantly,the Weyl states near the nodes are protected symmetrically by sliding mirrors,and remain unchanged under the action of out-of-plane magnetization of spin-orbit-coupling（SOC）,so they are SOC resistant.In addition,in order to investigate the anisotropy of Weyl fermion transport,we take type-Ⅱ Weyl points as the center to calculate the band dispersion and corresponding Fermi velocities in different k directions.The results show that the system has strong anisotropy:the maximum Fermi velocity is6.86×10⁵ m/s along theΓ→X direction.However,along theΓ→Y direction,the Fermi velocity is close to 0,and a quasi-flat band with negligible dispersion appears.（2）In recent years,ferromagnetic semiconductor has become an ideal platform for people to explore the coupling of ferromagnetism and ferrovalley.Based on this,a2D H-LaH₂ ML semiconductor material with both intrinsic ferromagnetic and ferrovalley properties,which is symmetric broken by central inversion,is designed and obtained in this paper.First-principles calculation results show that 2D H-LaH₂ML has ideal mechanical,kinetic and thermal stability.Its magnetic ground state is ferromagnetic order,and the T_C is higher than room temperature（476 K）.Due to the broken symmetry of center inversion and time inversion,the electron band has an intrinsic valley polarization the energy difference between K and K’valley valence band reaches 145 meV.Due to the reverse Berry curvature of K and K’,the Valley Hall effect can be produced under the action of an in-plane electric field,and the 100%valley polarized carrier can be obtained.Interestingly,H-LaH₂ ML is transformed from a ferrovalley semiconducting material to a single-spin half-valley Weyl nodal line semimetal（K is a semiconductor,K’is a Weyl nodal semimetal）under the action of a weak stress field（less than–3.5%）.Because of the weak magnetic anisotropy,H-LaH₂ ML can be transformed into single-node Weyl semimetals and anomalous quantum Hall insulators when the magnetization direction of the external magnetic field is changed.It can be seen that a variety of quantum states related to spin,energy valley and topology have been realized in LaH₂ ML,which is expected to be an ideal platform for exploring spintronics,valley electronics and topological physics.