A Spin Transparency Model for WSe₂/Pt/Py Heterostructures: Unified Analysis of ST-FMR and Magnetoresistance Effects

Transition metal dichalcogenides (TMDCs) show promise as spin sinks for enhancing spin-orbit torque (SOT) efficiency. However, a robust quantitative understanding of their impact on spin backflow suppression remains absent. This gap limits the systematic optimization of spintronic applications. In this work, we provide a quantitative framework by applying the spin transparency model to analyze spin transport in Pt/Py heterostructures with and without an underlying WSe₂ layer. We performed spin-torque ferromagnetic resonance (ST-FMR) measurements to systematically extract the spin-orbit torque efficiencies and the effective spin mixing conductance. By interpreting the ST-FMR data through the spin transparency model, we successfully decoupled the intrinsic Pt/Py interface properties from the spin backflow contribution. Our analysis quantifies the significant reduction (-22.7 %) in the spin backflow induced by the WSe₂ layer, confirming its role as an efficient spin sink while leaving the Pt/Py interface properties unaffected. This conclusion is further corroborated by the enhancement observed in unidirectional spin Hall magnetoresistance (USMR) and anisotropic magnetoresistance (AMR) on the same devices. This model-driven, quantitative approach moves beyond qualitative assessments and offers clear guidelines for optimizing spin current efficiency in future TMDC-based spintronic systems.