Theoretical Design And Manipulation Of Low-Dimensional Magnetic Materials Based On First-Principles Calculation

Spintronic,utilizing both charge and spin freedoms of electrons for information storage,transfer and processing,can significantly increase information storage density and reduce energy consumption with advantages of non-volatile and fast response.Therefore,it is considered as one of the most important technologies to solve the bottleneck of electronic devices in the post-Moore era.However,there are three fundamental scientific problems for the development of spintronic devices which are spin generation and injection,spin long-range transport,and spin regulation and detection.The solution to the above-mentioned problems relies on the development of spintronic materials.Low-dimensional magnetic materials are the material basis for the development of nano-spintronic devices.Considering the practical device applications,it is necessary to construct low-dimensional magnetic materials with high spin polarization ratio,high magnetic transition temperature and large magneto-crystal anisotropy.Also,the chemical stability and response to external field manipulation of low-dimensional magnetic materials need to be considered.Theoretical design of low-dimensional magnetic materials with room-temperature magnetism and exploration of the external field manipulation of their magnetism is an important part in the research of low-dimensional magnetic materials,which can provide theoretical guidance for related experiments.Combining the first-principles calculation and Monte Carlo simulations,our works have predicted a series of low-dimensional ferromagnetic materials with high Curie temperature via "top down" and "bottom up" design strategies,respectively,and explored manipulation strategies of spin degeneracy and spin polarization in low-dimensional antiferromagnets via outer electric-field.The main studies are as follows:1.We reported four two-dimensional iron silicides with room-temperature magnetism namely Fe4Si2-hex,Fe3Si2,Fe4Si2-orth and FeSi2 utilizing "top-down"design strategy for dimensional reduction.Ab initio molecular dynamics(AIMD)calculation indicates Fe4Si2-hex,Fe3Si2,Fe4Si2-orth and FeSi2 can maintain their lattices at 1000,400,1000 and 900K,respectively.First-principles calculation and Monte Carlo simulation indicate FeSix process ferromagnetic metallic ground states with high spin polariton ratios up to 96.4%.The Curie temperatures of four FeSix are higher than room temperature,among which the Curie temperature of Fe4Si2-orth reaches up to 971 K.Furthermore,the magnetic anisotropy energies of Fe4Si2-hex,Fe3Si2,Fe4Si2-orth and FeSi2 are 58,627,529 and 936 μeV/Fe,respectively.This work provides theoretical guidance for the search of two-dimensional ferromagnetic materials with room-temperature magnetism from magnetic iron-silicon alloys.2.Based on the magnetic cluster Fe6Se8Cl2,we theoretically designed a twodimensional bipolar magnetic semiconductor metal organic framework(MOF),namely Fe6Se8Cl2-2(4,4’-bpy)with room-temperature magnetism via "bottom up"strategy.Theoretical calculations show that two-dimensional Fe6Se8Cl2-2(4,4’-bpy)are bipolar magnetic semiconductor(BMS)with ferromagnetic ground state.The band gaps of the two spin channels are 0.897 and 1.017 eV,respectively.Furthermore,the conductivity of the two spin channels can be tuned by electron or hole doping with Fe6Se8Cl2-2(4,4’-bpy)transformed into ferromagnetic half metal with 100%spin polarization.Monte Carlo simulation demonstrates that the Curie temperature of two-dimensional Fe6Se8Cl2-2(4,4’-bpy)is 981 K,which is higher than room temperature.This study provides theoretical guidance for the design of two-dimensional MOFs with room-temperature magnetism starting from magnetic clusters,and provides a new bipolar magnetic semiconductor for the realization of electronic-field-controlled spin polarization.3.Our works have proposed a strategy to manipulate the spin polarization of two-dimensional bilayer A-type antiferromagnetic(A-type AFM)materials via perpendicular electric field and achieved room-temperature magnetism and electricfield-controllable spin polarization at the same time.The energy band theory model and the first-principles calculations show that by applying perpendicular electric field,two-dimensional van der Waals(vdW)bilayer A-type antiferromagnetic material can be transformed into an asymmetric antiferromagnetic unipolar magnetic semiconductor(AFM UMS)or an antiferromagnetic bipolar magnetic semiconductor(AFM BMS).To be specific,we investigated two vdW bilayer systems with A-type antiferromagnetic ground state,bilayer NiBi2Te4 and bilayer Cr(pyz)2.When the applied vertical electric field less than 0.05 V/?,bilayer NiBi2Te4 transformed into AFM BMS,while bilayer Cr(pyz)2 transforms from Atype antiferromagnets to asymmetric AFM UMS under vertical electric field less than 0.17 V/?.Electric structure analysis indicates the transformation depends on the real-space distribution of spin charge densities at valence band maximum and conduction band minimum in two-dimensional van der Waals bilayer.This study has proposed electric-field modulation of the magnetic properties in two-dimensional bilayer A-type antiferromagnetic materials and provided material systems and theoretical scheme for electric-field-controlled antiferromagnetic spintronic devices.4.Based on the research of electric-field response of two-dimensional van der Waals bilayer A-type antiferromagnetic system,we proposed the electric-field manipulation strategy on single-layer A-type antiferromagnetic materials with roomtemperature magnetism and illustrated by the case of MnCl single layer and Cr2CF2 single layer.Our results show that MnCl single layer and Cr2CF2 single layer are Atype antiferromagnetic semiconductor and TN are 766 and 675 K for MnCl and Cr2CF2 nanosheets,respectively.By applying an external electric-field,the spin degeneracy of MnCl single layer and Cr2CF2 single is broken and 100%spin polarization are induced around Fermi level.Meanwhile,the TN of 2D MnCl and Cr2CF2 decreases to 314 and 365 K when the applied external electric field reaches 0.6 V/?,respectively,which are above room temperature.Further study has demonstrated the spin splitting gaps between two opposite spin channels are related to the spatial distribution of spin charge density in 2D A-type AFM semiconductors.This work proposed an electric-field manipulation strategy for obtaining spinpolarized carriers and room-temperature magnetism based in single-layer A-type antiferromagnetic semiconductor,and provided theoretical scheme for the application of two-dimensional antiferromagnetic materials in electric-fieldcontrolled spintronic devices.