Magnetization-Independent Spin Hall Effect in Ferromagnetic Trilayers

The anomalous spin Hall effect, which creates charge-to-spin conversion in metallic ferromagnets, offers an opportunity to generate spin currents for which the spin direction is controllable. Here we report spin-torque ferromagnetic resonance measurements of current-induced torques in ferromagnet-spacer-ferromagnet trilayers consisting of NiFe and Co as our magnetic layers with a Cu spacer, allowing us to clearly separate signals from the different ferromagnetic layers. We observe signals with the symmetry of a magnetization-independent spin Hall effect with spin-to-charge efficiencies as large as 10% in NiFe, which is comparable to heavy metal-ferromagnet bilayers. We also investigate how coupling between the two magnetic layers can affect the resonance lineshapes. Our research provides insight into some of the experimental complications that must be considered when measuring spin-torques in ferromangetic trilayer heterostructures.