First-principles Study On Novel Magnetism And Its Tuning In Low-dimensional Materials

Due to the reduction of dimension and size which give rise to quantum confinement,surface and interface effect,low-dimensional materials show unique quantum pheonomena and have drawn great interests in the fields of condensed matter physics and material science.In order to introduce these materials in innovative spintronic devices,the novel magnetic properties and their effective manipulations have to be studied thoroughly.Based on the density functional theory,we have studied the magnetism,magnetocrystalline anisotropy and spin Hall effect of several low-dimensional systems.It is of great significance to introduce magnetism and control it effectively in singlelayer phosphorene for its applications in spintronic devices.We have proposed that by adsorbing transition metal atoms,phosphorene exhibits interesting magnetic properties.Due to the existence of lone pair electrons on P atoms,transition metal atoms bind strongly with phosphorene.The local magnetic moment on phosphorene can be tuned by applying small biaxial strain.New types of memory devices that are based on electric field control of spin orientations are in great request for materials which have large magnetocrystalline anisotropy(MCA)and strong electric field effects.We have found that some monolayer transition metal dichalcogenides are ideal candidate materials for this purpose,and they exhibit exceedingly large magnetocrystalline anisotropy.In particular,the spin orientations in some materials can be switched under a finite electric field,which can be attributed to the band character alteration of transition metal d-states around the Fermi level.Strain also has great effect on MCA,and can help to improve the modulation efficiency while combining with electric field.Spin Hall effect provides a way to generate the spin current via the charge current by the utilization of spin-orbital coupling,while spin Hall angle represents the transfer capability of a material.Thin film of tungsten is observed to have the largest spin Hall angle among all single elemental materials.We have investigated the electronic structure of tungsten and find that near the bands splitting by the spin-orbital coupling,there is a large value of Berry curvature,which induces the large spin Hall conductivity.Together with the small charge conductivity of tungsten,we explain the large spin Hall angle theoretically.By substitution doping,the spin Hall conductivity of ?-W can be further increased.Room-temperature ferromagnetic semiconductor is vital in nonvolatile digital circuits,yet has not been realized till now.We demonstrate that graphene monolayer with Ni substrate can show a room-temperature ferromagnetic semiconductor state.Because of the hybridization with Ni substrate,graphene opens a gap.By tuning the distance between graphene and Ni substrate and changing the charge doping of system,we can get spin-polarized band structure and gap,which is also observed in experiments.