  Recent ferromagnetic resonance experiments and theory of Pt/Nb/Ni8Fe2
proximity-coupled structures strongly suggest that spin-orbit coupling (SOC) in
Pt in conjunction with a magnetic exchange field in Ni8Fe2 are the essential
ingredients to generate a pure spin supercurrent channel in Nb. Here, by
substituting Pt for a perpendicularly magnetized Pt/Co/Pt spin-sink, we are
able to demonstrate the role of SOC, and show that pure spin supercurrent
pumping efficiency across Nb is tunable by controlling the magnetization
direction of Co. By inserting a Cu spacer with weak SOC between Nb and
Pt/(Co/Pt) spin-sink, we also prove that Rashba type SOC is key for forming and
transmitting pure spin supercurrents across Nb. Finally, by engineering these
properties within a single multilayer structure, we demonstrate a prototype
superconducting spin-wave (SW) device in which lateral SW propagation is
gateable via the opening or closing of a vertical pure spin supercurrent
channel in Nb.
