Theoretical Research On Transport Properties Of Nano-Devices Based On Two-Dimensional Materials

The increasing demand for the storage capacity of larger hard drives has attracted attention andexplosion research in this area.At the end of the last century,researchers discovered the giant magnetoresistance(GMR)effect,and a few years later studied the alumina-based magnetic tunnel junctions(MTJ).MTJ are different form other types of magnetic memories.It is a promising nanostructured device in the field of spin electronics.MTJ has a very high tunnel magnetoresistance(TMR)effect,and it has great advantages in terms of storage capacity,speed,and stability.In addition,because it is a nano-device,its size will be smaller than previous memories.The main research is applied of two fields:hard disk drives and magnetoresistive random access memory.This shows that the magnetic tunnel junctions are important for the futuristic memory devices.The basic element of MTJ is consisted of a ultra thin insulating barrier layer connected to the left and right magnetic electrodes.We need to find suitable electrode and barrier layer materials for the tunnel junction to improve its transport performance.In this paper,mainly by using the ATK(Atomistix Tool Kit)package based on first-principles to systematically investigate and analyze two-dimensional(2D)materials as intermediate barrier layers and magnetic(2D)materials as magnetic electrodes by the non-equilibrium Green's function method in combination with the density functional theory.The effect on the transport properties of the nano-device(magnetic tunnel junction)is analyzed.The obtained results are summarized as following:Detailed systematic exploration of transport properties based on layered WS₂as a barrier layer formagnetic tunnel junctions,the analysis results show that the tunnel magnetoresistance of the Fe/WS₂/Fe junction of the intermediate barrier layer with different layers is calculated.It is found that the magnetic resistance of the tunnel junction with the five-layer barrier is the highest,and reaches the saturation value of the change of the magnetic resistance.Based on practical considerations,we need to add a bias voltage to the tunnel junction,which is higher than 0.4V,and the magnetoresistance will oscillate.We thus know that in order to obtain effective magnetoresistance,the increase in bias voltage needs to be kept within the tunneling effect.We want higher magnetoresistance.To this end,we tried to inject Co spins with different thicknesses into the tunnel junction interface of several barrier layers with different thicknesses.For junctions with Co insertion at the interface of Fe and WS₂,the enhanced magnetoresistance can be observed under zero bias from the interface resonate state.Among them,we choose a tunnel junction with a5-layer barrier layer implanted with 3 layers of Co that has giant magnetoresistance effect.Under the bias,the magnetic resistance is reduced.This shows that the bias voltage is not conducive to the electron spin injection of Co.As a most stable 2D magnetic TMD material,VS₂has attracted much attention in large varietyapplications for its intriguing magnetic behavior.VS₂is used as a tunnel junction electrode and MoS₂to form a transverse magnetic tunnel junction(VS₂/MoS₂/VS₂).To simplify the calculation,direct doping method with electrons to mimic the half metallic characters of F,Cl or Br doped VS₂,and enable it to effectively spin-inject.We explored the magnetoresistance of tunnel junctions with different barrier layer thicknesses,showing that the magnetoresistance increases with increasing barrier layer thickness.The results show that the magnetoresistances of armchair-type and zigzag-type tunnel junctions with five MoS₂barrier layers are as high as 10⁸and 10⁷,respectively,which means that armchair-type devices have achieved higher spin electron injection.Because VS₂and MoS₂have similar and matched energy band structures,the tunnel junction has a perfect spin filtering effect.After adding a bias voltage,the tunnel junction has a certain stability in the magnetoresistance in the tunneling effect,and the tunneling effect ranges from 0 to 0.5V.We divide the tunnel junction into parallel and antiparallel configurations.The results show that the transport properties are mainly contributed by the spin-up channel both in the two configurations,because the spin-up channel conduction band passes through the Fermi level.The results show that VS₂/MoS₂/VS₂has excellent research prospects in 2D spintronic devices.