Properties And Control Of Low-dimensional Spintronic Materials From First-principles

Spin,out of charge as an extra intrinsic property in electrons,is the cornerstone of spintronics.Combining with the low dimensional magnetic materials discovered recent years,electronic devices are promising to be furtherly minimized,less power comsuming,and realize multifunctionals.Low dimensional magnetic materials,being as the material base,deserve deeper investigations about their atomic scale magnetic mechanism and electron transportation properties.Magnetic anisotropy is a necessary condition of the existence of long-range-orederd ferromagnetism against thermal fluctuation in two-dimensional（2D）semiconductors.Concerning the magnetic anisotropy and magnetic moments of the M³⁺（M=V,Cr）ions in ferromagnetic MI₃ monolayers,which have a honeycomb pattern of edge-sharing MI₆ octahedra,conflicting observations have been reported in experimental and theoretical studies.We resolve these conflicts by determining the magnetic anisotropy energies for the M³⁺ions of MI₃ monolayers,by analyzing their preferred spin orientations in terms of the selection rules,which are based on the HOMO-LUMO interactions of the MI₆ octahedra,and by discussing whether or not the M³⁺ions are uniaxial.Here we show that the magnetic anisotropy energy for the V³⁺（d²,S=1）ion of VI₃ is greater than that for the Cr³⁺（d³,S=3/2）ion of CrI₃ by more than an order of magnitude（i.e.,～8 vs.～0.6 me V）.The FM monolayer VI₃ is uniaxial,but that of CrI₃ is not.The V³⁺ion exhibits uniaxial magnetism because its orbital quantum number L is not zero（L=1）,in contrast to the Cr³⁺ion（L=0）.We built the effective spin Hamiltonian of MI₃ monolayers,and parametrized by first-principles calculations,then we performed Monte Carlo simulations to predict their Curie temperature,where VI₃ monolayer is predicted to be 60 K.And we determined the minimal model Hamiltonian that can distinguish between the VI₃ and CrI₃ monolayers.In valleytronics,the broken inversion symmetry leads to the non-equivalent valleys at K⁺/K^-in momentum space.Furtherly,by the method of applying external fields or the using of approximation effect,the valley polarization could be excited to induce the properties of circular dichroism etc.,which could be ultilized fordirect information storage.2H-transition metal dichalcogenides（2H-TMD）materials possess honeycomb lattice,and their inversion symmetry is naturally broken,thus they have non-equivalent valleys.Together with the comparably strong SOC effect,their spin and valley are coupled with each other,so we could manupulate valley polarization with magnetic field.Monolayer 1T-VSe2 is a rare example exfioliated in experiment as a 2D room-temperature ferromagnetic conductor.The analoge 2H-VSe₂ is predicted energetically more stable than the 1T phase,and being possiblly a room-temperature ferroamgnet semiconductor and ferrovalley material.Accoding to a recente prediction,monolayer2H-VSSe could show enlarged valley polarization amplitude.Thus in this thesis,we performed first-principles calculations and classical Monte Carlo simulations based on effective spin Hamiltonian to systematically analyze six kinds of monolayer 2H-VXY（X,Y=S,Se,Te）with their electronic structures,magnetic ground states,dynamical stabilities,magnetic anisotropy,magnetic phase change temperature,and valley polarization value.It turns out that monolayer 2H-VXY are all having easy-plane magnetic anisotropy,which will lead to a Berezinskii-Kosterlitz-Thouless quasi-long-range pahse change.When the magnetic moments are oriented in the out-of-plane direction,valley polarization happens and their value are proportion to the average element number of chalcogen ligands.The members of 2H-VXY who include Te ligands are energetically less stable than their corresponding 1T phases,and their large negative next-nearest-neighbor exchange coefficients lessen their phase change temperature,so momnolayer 2H-VSe₂ has the highest room-temperature BKT phase change temperature in its ground state.Quasi-one-dimensional zigzag edged graphene nanoribbon（ZGNR）could be a ferromagnetic half-metal under appropriate conditions,and it was theoretical proposed could be used to build devices with giant magnetoresistance ratio.On the other hand,the basic units of self-assembling 2D-conjugated aromatic polymers are the DDQP molecules which include nitrogen atoms.So in this work we take the former as lead,connecting to the DDQP molecule to build an spin filtering organic molecular magnetic tunnel junction.We performed a series of non-equilibrium Green’s function formalism-density functional theory calculations to investigate the quantum transportations in multiple electronic and structural configurations of the device.We found significant enhancement of spin filter efficiency and magnetoresistance ratio have been obtained in our molecular device in comparison with similar all-carbon n-acene device;The increasing of the ZGNR width could effectively improve the spin filter efficiency toward 100%under low bias voltage with simple parallel spin configuration;Small difference of the contact geometry between ZGNR and center molecule could results in a small difference in current value size.Combining the transmission spectra,real space scattering states,scattering states wavefunctions under different bias voltage,and configuration-barrier analysis,we analyzed the mechanism of above phenomena,and concluded the manuplating and designing strategy of transportation properties in these kind of molecular spintronic deveices.