Study On Half-Metallicity And Transport Properties Of 2D Transition Metal Halides

Spintronics,which utilizes electronic spin or both electronic spin and charge for information processing and storage,has aroused great research interest,and it has been used in some devices such as the spin filter,the spin valve and the spin field effect transistor.Magnetic half-metals with theoretical 100%spin polarizability are very suitable for spin electron injection.With the development of nanotechnology and2D materials,exploring 2D magnetic half-metallic materials and studying their spin transport properties are of great significance for the development of 2D spintronics.However,according to the Mermin-Wagner theorem,the ferromagnetic order is usually prohibited in a 2D isotropic system at a finite temperature.Therefore,searching for 2D ferromagnetic materials is extremely challenging.Fortunately,ferromagnetic order has been observed in recent experiments in several few-layered2D anisotropic crystals such as Cr I₃,Cr₂Ge₂Te₆,VSe₂,and Fe₃Ge Te₂.Although they are ferromagnetic semiconductors or metals instead of ferromagnetic half-metals,and their Curie temperatures are relatively low,which provides a direction for searching for 2D ferromagnetic half-metals.In this thesis,we investigate the electronic structure and magnetic properties of2D transition metal dichlorides and trihalides by first-principles calculations.We also design the corresponding 2D lateral(ionic bond interactions)and vertical(van der Waals interactions)heterojunctions,and study the spin transport properties by using the first-principles combined non-equilibrium Green's function method.The main results are as follows:1.For the 2D monolayer transition metal dichlorides MCl₂(M=V,Cr,Mn,Fe,Co,Ni),we find that their 1T phase is more stable than the 2H phase,and the monolayer 2H-VCl₂,Cr Cl₂,Co Cl₂ and Ni Cl₂ and 1T-Cr Cl₂,FeCl₂ and Co Cl₂ all exhibit half-metallic properties.FeCl₂,Co Cl₂ and Ni Cl₂ monolayers in both 1T and2H phases have the ferromagnetic ground state,which are consistent with the experimental results.Combined with the crystal field theory,we analyze why monolayer 2H-VCl₂ has a low spin state and 1T-VCl₂ has a high spin state.We also study the transport properties of 1T-FeCl₂,and find the perfect spin filtering effect,negative differential resistance effect,and high magnetic resistance(up to 1.34×10⁵%).This work demonstrates the potential applications of 2D monolayer transition metal dichlorides in spintronics.2.Among 2D transition metal trihalides MX₃(M=V,Cr,Mn,Fe,Co,Ni;X=Cl,Br,I),we find that only Co X₃ monolayers are non-magnetic semiconductors.Monolayer VX₃,Mn X₃ and Ni X₃ are Dirac spin-gapless semiconductors,also known as a special class of Dirac ferromagnetic half-metals.By the Monte Carlo simulation based on 2D Ising model,we predict that the Curie temperature of monolayer VCl₃ is about 100 K.In addition,we design a 2D monolayer lateral magnetic tunnel junction VCl₃/Co Br₃/VCl₃,and find that the current in the zigzag direction is much larger than that in the armchair direction.The magnetic tunnel junction has an excellent spin filtering effect and a high tunneling magnetoresistance(up to 4.5×10¹²%).These transport characteristics can be explained from the spin-polarized energy bands of electrodes and the spin-dependent transmission spectrum of the device.3.We further design a 2D vertical van der Waals magnetic tunnel junction of1T-MoS₂/1T-FeCl₂/2H-MoS₂/1T-FeCl₂/1T-MoS₂,where the electrodes are the 2D non-magnetic metal of 1T-MoS₂,and the central scattering region is the 2D ferromagnetic half-metal of 1T-FeCl₂ and the non-magnetic semiconductor of2H-MoS₂.By calculating the bias voltage dependent currents in the parallel magnetization state and the antiparallel magnetization state of electrodes,we find the spin filtering effect,the negative differential resistance effect and the higher tunneling magnetoresistance.We also analyze the energy-and momentum-resolved transmission spectra,and observe three different transmission channels.These results indicate the 2D spintronic devices applications of the van der Waals magnetic tunnel junction based on 2D ferromagnetic half-metal of 1T-FeCl₂.