Effect of interfacial insertion layers on the spin-orbit torque in W(O)|CoFeB heterostructures

We report on an experimental investigation of spin-orbit torque (SOT) in W(O)|CoFeB heterostructures where a thin insertion layer with negligible spin-orbit coupling is inserted at the W(O)|CoFeB interface. The SOT is found to be suppressed with the addition of the insertion layer, contrary to estimates using the transparency formalism. In addition, the SOT, as quantified by the spin Hall angle remains constant at -50% for W(O) thicknesses down to 2 nm. Our data is thus consistent with an interfacial SOT mechanism in the W(O)|CoFeB system.