Band-Structure Spin-Filtering in Vertical Spin Valves Based on Chemical Vapor Deposited WS

We report on spin transport in WS-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS mono-, bi-, and trilayers are derived by a chemical vapor deposition process and further characterized by Raman spectroscopy, atomic force microscopy (AFM), and photoluminescence spectroscopy. The WS layers are then integrated in complete Co/AlO/WS/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/AlO spin analyzer to probe the extracted spin-polarized current from the WS/Co interface and its evolution as a function of WS layer thicknesses. For monolayer WS, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of = 12%. Interestingly, for bi- and trilayer WS, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS band structure while going from monolayer (direct bandgap) to bilayer (indirect bandgap). These experiments illustrate the rich potential of the families of semiconducting 2D materials for the control of spin currents in 2D-MTJs.