Charge-Spin Conversion In Van Der Waals Heterostructures And Novel Phenomena At The Interfaces

2D materials like graphene have drawn great attention for their high mobility,high conductivity,etc.As the highly potential substitution of silicon,2D materials show as thin as one atom layer and are perfect material candidates for the next-generation electronics devices with high-integrate,high-performance and low power consumption.Spintronics technology requires efficient methods for the creation and utilization of spins for faster and low power consumption electronics.With the discovery of new 2D materials,like Weyl semimetals and transition metal dichalcogenides(TMDC),more and more novel physics properties have been reported.As a versatile platform for investigation of various phenomena,Van der Waals heterostructures have been studied very intensively,in which one can realize the spin injection,manipulation,transport,and detection.Also,it offers the opportunity to control the charge-spin conversion.In this thesis,the Van der Waals heterostructures of WTe2/Graphene and Co/TiO2/Graphene have been studied.We also investigated the charge-spin conversion and related novel phenomena at the interfaces,which may offer insights for the design of the next-generation spintronics devices.A brief of the studies are listed as follows.(1)The spin Hall effect were studies in the WTe2/Graphene Van der Waals heterostructure.This study reveals that the spin Hall angle of WTe2 is comparable to that of the conventional heavy metals.And the spin Hall signals are two orders of magnitude larger than those measured in metals with Cu as the channel,which benefits from graphene,the excellent spin carries channel.It also shows that the spin Hall signal can be enhanced up to 600%by the gate voltage,which is likely due to the enhancement of the spin absorption by WTe2 under the electric field.The DFT calculations show that the spin Hall conductivity is highly dependent on the Fermi level.Many factors can change the Fermi level,like thickness,strain,etc.These novel properties are related to the spin splitting of WTe2 at the Fermi level,in which the electron and hole pockets coexist with the Fermi surface states.Moreover,these special structures are originated from the Td phase of WTe2 with strong spin orbital coupling(SOC)and the broken space inversion symmetry.(2)A new Edelstein effect is observed in the WTeZ/Gr Heterostructure.Compared to conventional surface states dominated Rashba-Edelstein effect,the Edelstein effect in type-? Weyl semimetal WTe2 is attributed to the spin-polarized bulk states.This novel band structure is originated from the Td phase of WTe2 with the broken inversion symmetry and its strong spin-orbit coupling.Based on this effect,spin injection and detection are realized by WTe2 individually.Some previous theory works show that WTe2 may not be Weyl semimetal any more even if there is a minor change in the crystal lattice caused by temperature.The temperature dependence of the Edelstein signal shows that such spin-polarized bulk states are robust from 10 K up to room temperature.Moreover,the Edelstein effect and its inverse effect are following the Onsager reciprocity relation.Both the Edelstein effect and its inverse effect show good linear bias dependence,while the conventional ferromagnetic electrodes spin injection suffers the impedance mismatch a lot.The lower limit of the Edelstein length(charge-spin conversion efficiency)in WTe2 is up to 0.72nm,which is comparable with the records of previously reported works.(3)An anomalous Hanle effect is discovered in the low contact resistance graphene non-local spin valve.While the sign change of the spin polarization can be observed in Co/TiO2/Gr heterostructure with both low and high contact resistance.Phenomenally,this sign change process of the spin valve signal can be decomposed into one increasing component and another decreasing one.After ruling out the thermal related spin signals and spin lifetime anisotropy,our further DFT calculations show that magnetic proximity effect at the Co/TiO2/Gr interface very likely plays a key role in the sign change of the spin polarization.The highly Fermi energy-dependent density of states can be tuned by bias current/voltage and gate voltage.Such a reversal of the spin polarization is much easier with a critical current density as low as 103 A/cm2,which is different from the spin transfer torque(STT)and spin orbital torque(SOT)magnetic moments switching mechanism.This can provide a new insight for future spintronics devices.