Determining spin-torque efficiency in ferromagnetic metals via spin-torque ferromagnetic resonance

Spin current generated in a ferromagnetic metal (FM) can be divided into two types. While one is magnetization dependent and induced by the well-known anomalous Hall effect, the other is a magnetization-independent spin Hall effect which is similar to that in a paramagnetic heavy metal (HM). Here, we study the magnetization-independent spin Hall current in YIG/FM (NiFe and CoFeB) via spin-torque ferromagnetic resonance (ST-FMR) technique. Our experiments reveal the existence of a magnetization-independent spin current. Although there is a strong exchange interaction in FM, the spin current does not dephase as quickly as expected. Furthermore, we estimate the spin-torque efficiency xi of NiFe was 0.009, which is about 25% of the spin-torque efficiency of Pt. These results indicate that the spin Hall effect of FM should also be taken into account when investigating FM/HM heterostructures, and furthermore this effect can also benefit from the development of spin-orbit torque devices.