Influence of the spin pumping induced inverse spin Hall effect on spin-torque ferromagnetic resonance measurements

Abstract

Spin-torque ferromagnetic resonance (ST-FMR) has been widely used to determine the spin-orbit torque (SOT) efficiency in ferromagnet/heavy-metal bilayer systems [1]. When a radio frequency (rf) current flows within the HM, the spin Hall effect (SHE) induced SOT and Oersted field act on the FM layer. When the frequency of the rf current and the applied external magnetic field satisfies the ferromagnetic resonance (FMR) condition, the magnetization of the FM layer oscillates with relatively large amplitude around the equilibrium state. This results in a non-negligible resistance oscillation due to the anisotropic magnetoresistance (AMR) of the FM layer. Consequently, a rectified dc voltage is generated as a result of the interplay of the oscillating current and resistance. Importantly, the lineshape of voltage curve due to the damping-like torque is Lorentzian symmetric, while that from the field-like torque (both SHE and Oersted field origin) is anti-symmetric. Thus, one can obtain the SOT efficiency from lineshape analysis of ST-FMR voltage curve. However, the precession of FM moments also acts as the source for an angular momentum flow, which pumps a pure spin current into its neighboring HM [2]. Wherein, an additional contribution to the dc voltage arises from the inverse spin Hall effect (ISHE) [3] which has Lorentzian symmetric lineshape.In this work, we develop a new method to quantitatively separate the SP-ISHE signal from the ST-FMR signal without tedious symmetry analysis. With the precession angles obtained in both setups via the microwave photoresistance measurements [4], we directly obtain the SP-ISHE contribution in the ST-FMR. Our method does not rely on many unknown parameters such as the spin diffusion length, spin Hall angle of HM, and the effective spin-mixing conductance of FM-HM interface etc. Interestingly, we find that the SP-ISHE is in the opposite sign to the symmetric component of ST-FMR for Py(Ni80Fe20)/Pt bilayer layer. Through the Py thickness dependent measurements, we further estimate both the SHE-induced damping-like-torque efficiency and field-like-torque efficiency for Py/Pt bilayer system. **