Band-Structure Spin-Filtering in Vertical Spin Valves Based on Chemical Vapor Deposited WS₂

Abstract

We report on spin transport in WS2-based 2D-magnetic tunnel junctions (2D-MTJs), unveiling a band structure spin filtering effect specific to the transition metal dichalcogenides (TMDCs) family. WS2 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 WS2 layers are then integrated in complete Co/Al2O3/WS2/Co MTJ hybrid spin-valve structures. We make use of a tunnel Co/Al2O3 spin analyzer to probe the extracted spin-polarized current from the WS2/Co interface and its evolution as a function of WS2 layer thicknesses. For monolayer WS2, our technological approach enables the extraction of the largest spin signal reported for a TMDC-based spin valve, corresponding to a spin polarization of P Co/WS2 = 12%. Interestingly, for bi- and trilayer WS2, the spin signal is reversed, which indicates a switch in the mechanism of interfacial spin extraction. With the support of ab initio calculations, we propose a model to address the experimentally measured inversion of the spin polarization based on the change in the WS2 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.

Spin filtering by proximity effects at hybridized interfaces in spin-valves with 2D graphene barriers

“…The expanding 2D materials family is now the focus of several on-going vertical spin transport studies, taking advantage of, for instance, tunneling (h-BN, etc.) 8 , 9 , semiconducting (MoS 2 10 , 11 , WS 2 12 , 13 , etc.) and magnetic (CrI 3 14 , etc.)…”

Naturally occurring van der Waals materials

“…Tungstenite, similar to molybdenite, is an n-type indirect band gap semiconductor in bulk (~1.3 eV), which becomes a direct band gap semiconductor when thinned down to a single-layer (with a gap value of 2 eV) 71 . The spin-orbit splitting of the valence band in single-layer tungstenite reaches 430 meV, which is approximately three times larger than in single-layer molybdenite (150 meV), and thus this material is very interesting for the incipient field of spinorbitronics [72][73][74][75] . Withers et al 9 isolated WS 2 flakes with thickness ranging from single-layer up to four layers by exfoliation of natural tungstenite and studied its intrinsic electronic properties by fabricating field-effect devices on hexagonal boron nitride, showing a marked n-type behavior and mobilities up to 80 cm 2 V −1 s −1 at room temperature.…”

Computational Atomistic Modeling in Carbon Flatland and Other 2D Nanomaterials

“…Over the last decade, significant research efforts have been dedicated to introduce 2D materials to control and tailor the magnetoresistive response of the interfaces [76]. Boosted by the theoretical prediction of perfect spin filtering by few of its layers [77][78][79][80], graphene has been the first 2D material to be investigated [81][82][83], rapidly followed by insulating h-BN [84][85][86] and semi-conducting transition metal dichalcogenides (TMDCs) [87][88][89][90]. While very promising experimental results have yet been reported, first-principles simulations provide a complementary tool to unravel the full potential of 2DMs in tunneling junctions.…”