First-principles Study On Magnetism And Magnetocrystalline Anisotropy Of Low-dimensional Materials

At present,the two-dimensional?2D?materials represented by graphene and monolayer MoS₂ have attracted extensive attention due to their excellent performance and potential application.The investigations to use 2D semiconductors as spintronic materials have gained high attention and development.In addition,2D ferromagnetic materials can be regarded as atomically thin films.Thus applying 2D ferromagnetic materials with large magnetocrystalline anisotropy?MCA?for magnetic storage will help to further increase the density of data storage.Monolayer Cr S₂ and Be₂ C are direct band-gap semiconductors and possess good stability,which means that they are suitable for applications in nano-spintronics.However,monolayer Cr S₂ and Be₂ C are intrinsically nonmagnetic.For their prospective applications in nano-spintronic devices,it is required to induce and manipulate the magnetism in monolayer Cr S₂ and Be₂ C.As known substitutional doping,strain and cutting nanoribbon can effectively induce the magnetism in 2D materials.In contrast,monolayer TaTe₂ is intrinsically ferromagnetic.More importantly,Ta atom possesses strong spin-orbit coupling interaction,which means that monolayer TaTe₂ holds great potential for the realization of large MCA.In this thesis,first-principles calculations are carried out to investigate the effects of transition metal?TM?and alkaline-earth dopants on the electronic structure and magnetic properties of monolayer Cr S₂,the electronic structure and magnetic properties of Be₂ C nanoribbons,and magnetism and MCA of monolayer TaTe₂ under different strains.Furthermore the mechanisms about these investigations are clarified and the main works are as follows:1.We systematically investigate the effects of a series of 3d and 4d TM and alkaline-earth dopants,including Ca,Sc,Ti,V,Mn,Fe,Co,Ni,Cu,Zn,Ga,Sr,Y,Zr,Nb,Mo,Ru,Rh,Pd,Ag,Cd and In,on the electronic structures and magnetic properties of Cr S₂ monolayer.Our calculations show that the doping at Cr site of monolayer Cr S₂ with numerous TM and alkaline-earth atoms is energetically favorable under the S-rich growth conditions.Nb,Mo,Ru and Rh dopants can not induce magnetism in doped monolayer Cr S₂,whereas ground states of Ca,Sc,Ti,V,Mn,Fe,Co,Ni,Cu,Zn,Ga,Sr,Y,Zr,Pd,Ag,Cd and In-doped systems are magnetic,in which V doped system is easily thermally excited to non-spin polarized state.Furthermore the calculations show that the value and the distribution of the magnetic moment induced by dopant not only relate to the number of valence electrons and the occupancy of the d orbitals of dopant,but also depend on hybridization between dopant and its neighboring atoms.2.We systematically study the geometrical structure,stability,electronic structure and magnetic properties of Be₂ C nanoribbons,including N-b-Be₂C-NR-Be,N-a-Be₂C-NR,N-b-Be₂C-NR-Be-Be and N-b-Be₂C-NR-C.Our calculations show that N-b-Be₂C-NR-Be,N-a-Be₂C-NR and N-b-Be₂C-NR-Be-Be are thermo-dynamically stable at room temperature and are likely to be fabricated in the experiments.Besides,H passivation at the edge greatly enhances the stability of N-b-Be₂C-NR-C due to the reduction of dangling bonds at the edge,and hence N-b-Be₂C-NR-C is thermo-dynamically stable at room temperature.Moreover,the stable nanoribbons N-b-Be₂C-NR-Be,N-a-Be₂C-NR and N-b-Be₂C-NR-Be-Be are all nonmagnetic semiconductors with direct band gaps,whose values are significantly dependent on the edge configuration and the ribbon width.However,H passivated N-b-Be₂C-NR-C is half-metallic with a magnetic ground state.N-b-Be₂C-NR-C-H has a strong intra-edge ferromagnetic coupling interaction in the ground state,and an inter-edge ferromagnetic interaction is found in small-width H passivated nanoribbon 5-b-Be₂C-NR-C-H.The calculated density of states and the spin density distribution show that the intra-edge and inter-edge ferromagnetic coupling in N-b-Be₂C-NR-C-H can be attributed to the p–p hybridization interaction involving polarized electrons.3.We systematically investigate the electronic structures,magnetism and MCA of monolayer TaTe₂ under different strains ranging from-2% to 8%.Our calculations show that monolayer TaTe₂ is a ferromagnetic metal and the magnetic moment is mainly contributed by Ta atom.Moreover,the calculated magnetic anisotropy energy?MAE?of unstrained monolayer TaTe₂ is as high as-4.72 me V per unit cell.The magnetic moment,magnetic coupling and magnetic anisotropy of monolayer TaTe₂ can be significantly enhanced by strain.In particular,the MAE of monolayer TaTe₂ under 8% strain increases by 165% compared with that of unstrained monolayer TaTe₂.By analyzing the density of states and the contribution to MCA from the SOC interaction between different d orbital channels of Ta atom,it is concluded that large in-plane magnetic anisotropy of monolayer TaTe₂ is mainly contributed by the SOC interaction between opposite spin dxy and dx2-y2 orbitals of Ta atom and the significant increase of the negative contribution to MCA from the SOC interactions between opposite spin dxy and dx2-y2 orbitals under strain is the main reason why the MAE of monolayer TaTe₂ is greatly enhanced by strain.