First-Principles Study Of Magnetic Properties In Two Dimensional Materials

Since the discovery of graphene,two-dimensional(2D)materials have attracted tremendous attention due to their fascinating properties and potential applications.The investigations on magnetism,especially on the mechanisms,phenomena and applications,of two-dimensional materials are popular in spintronics.According to the constituent elements,the two-dimensional materials can be divided into two classes:nonmetal materials,such as graphene,phosphorene,h-BN,silicene and so on;metal and nonmetal composite materials,such as transition metal disulfides(TMDs),metal carbides and nitrides(MXenes),metal oxides(MOX),metal organic frameworks(MOFs)and so on.The metal and nonmetal composite materials can not only apply in optics,catalysis,sensing,gas storage and other fields,but also exhibit fascinating magnetic properties that can be used in spintronics potentially.The nonmetal materials,such as graphene,display broad application prospects,but the intrinsic properties of nonmagnetic limit their potentials in spintronic devices.Based on the density functional theory,we performed first principles calculations in some two-dimensional materials to explore novel properties for spintronic devices.In this doctoral thesis,the main works are as follows:1.We systematically investigated the electronic and magnetic properties of 2D transition metal phthalocyanine monolayer with kagome lattice(kag-TMPc).The calculations suggest that the 2D kag-MnPc framework is ferromagnetic(FM)with Curie temperature(Tc)of 125K,as high as the material of square lattice MnPc framework.The 2D kag-CrPc framework is a candidate for S=2 kagome antiferromagnetism(AFM).Based on the XXZ Heisenberg model,the calculated anisotropy(??1)suggests that the 2D kag-CrPc is almost isotropy.Match with the phase diagrm from the theoretical study,the ground state of 2D kag-CrPc favors the RT3 state.The optical absorption spectrums indicate that as self-assembled into 2D kag-TMPc frameworks,the optical properties of MPc molecules are preserved and enhanced in degree.2.We systematically investigated the electronic and magnetic properties of 2D metal subphthalocyanine(MsubPc)monolayer with hexagonal lattice.The B-ClsubPc framework is semiconductor with tunable energy gap.With external strain up to 10%,the gap ranges from 0.68eV to 0.93eV.The InsubPc and TlsubPc frameworks are gapless semiconductor with Dirac point at the Fermi level.Including the spin-orbit couplings,a gap of 0.27meV and 0.50meV is opened respectively,extremely larger than that graphene.As 3d/5d metal replaced into the subPc pore,the FesubPc framework is ferromagnetic metal with Curie temperature of 185K.The WsubPc and IrsubPc frameworks are FM half-metal with Curie temperature of 310K and 110K.The maximum perpendicular(Z)magnetic anisotropy energy(MAE)is 31.98meV in IrsubPc,while in parallel(XY)plane is 21.39meV in WsubPc which is tunable under strain.3.We studied the electronic and magnetic properties of 2D metal porphyrazine(TMPz)monolayer with square lattice in detail.The metal free H2Pz framework is nonmagnetic semiconductor with a narrow band gap of 64meV which is sensitive to external strain.As 3d metal introduced into Pz pore,the FePz framework is FM metal and with external strain larger than 4%,a spin crossover effect is happened which changes from low spin state to high spin state.The MnPz framework is AFM metal and can be tuned into FM by regulation of strain.The strongest FM coupling is observed under 3%strain with Tc of 280K.Through the method of doping the same metal atoms in the pore between Pz,the doped MnPz and FePz frameworks present stable long-range FM coupling with Tc of 230K and 930K respectively.As 5d transition metal introduced,the RePz framework can be tuned into FM half-metal under strain,and the easy magnetization axis along Z direction with MAE of about 30meV.While 5d transition metal dimer inserted,the Ir2Pz framework is FM half-metal with Tc of 90K,and the easy magnetization axis along Z direction with MAE of 48.2meV.4.We investigated the electronic and magnetic properties of 2D metal benzenehexathiol monolayer with kagome lattice(TM-BHT).Among the 3d TM-BHTs,Ni-BHT framework is FM metal with high conductivity and the Tc is 350K.The Mn-BHT,Fe-BHT and Co-BHT frameworks display characters of bipolar antiferromagnetic semiconductors.All of them can be tuned into FM half-metal through hole doping,and the Tc are higher than room temperature.With electron doping,Fe-BHT and Co-BHT frameworks are FM metal with high spin polarization.The Fe-BHT framework can be adjusted from AFM to FM ground state under external strain and the highest Tc is 740K with corresponding spin polarization of-67.7%.Through the method of substitution of S atoms by N,the Mn3C6S3N3 framework is FM half-metal with high Tc of 620K.Meanwhile,the Mn3C6N3 framework is as well FM half-metal.Particularly,with the compression larger than 2%,spin crossover effect will happen that the spin state changes from high spin state(HS)to low spin state(LS)combine with the reversal in the spin polarization from 100%to-100%.5.We theoretically explored the spin injection of the phosphorene on the magnetic insulator by utilizing proximity effects.On the EuO(111)surface,the adsorbed phosphorene becomes metallic,with the band gap shifting below the EF,and has the spin polarization at the EF beyond 30%along with an exchange splitting of the CBM at ? point with the splitting energy of?0.184 eV where only one spin channel is conductive.The inplane strain will effectively modulate the energy region and splitting energy of this exchange-splitting.It is also found that the proximity effect makes the deformation potentials,and then the anisotropic mobilities at the CBM of the phosphorene become spin-polarized,but it will not significantly weaken the transport performance of the phosphorene.6.We studied the spin injection of the silicene and germanene on the magnetic insulator by utilizing proximity effects.On the EuO(111)surface,both the silicene and germanene are polarized and the spin polarization at the EF is?10%and?40%,respectively.Through the interactions with the EuO substrate,a gap of-0.5eV is opened at Dirac point in absorbed silicene extremely larger than that opened by SOC in free silicene(0.05meV)and absorbed on BN(30meV).The degeneracy lifting at the Dirac point is spin dependent that an exchange splitting of?0.1 eV is observed in CBM of absorbed silicene.In the inpalne strain,the gap at Dirac point of silicene can be modulated into the gap of EuO substrate while the spin polarization of germanene at the Fermi level is slightly enhanced.7.We systematically investigated the magnetic anisotropy of 5d transition metal atoms and dimers adsorbed on phosphorene(TM/TM2@phosphorene)and single vancancy phosphorene(TM/TM2@SVP).The 5d TM atoms adsorbed on phosphorene are stable that will not diffuse or gather into clusters.The W@SVP has MAE of 25.30meV with easy magnetization axis perpendicular to phosphorene.The Os2?phosphoreneH4 and Ir2@phosphoreneH4 exhibit great MAE of 88.55meV and 112.96meV,which the easy magnetization axis is perpendicular to phosphorene.The external electric field modulation suggests that they exhibit properties of easy writing operation.Moreover,the mixed TM dimers Ru-Os@phosphoreneH4 and Co-Ir@phosphoreneH4 also have large MAE of 63.61meV and 78.59meV with the easy magnetization axis perpendicular to phosphorene.All the conclusions indicate that the 5d transition metal absorbed phosphorene are promising candidate for future atomic scale magnetic storage devices.