Dependence of spin pumping and spin transfer torque upon Ni81Fe19 thickness in Ta/Ag/Ni81Fe19/Ag/Co2MnGe/Ag/Ta spin-valve structures

Spin pumping has been studied within Ta / Ag / ${\text{Ni}}₈₁{\mathrm{Fe}}₁₉$ (0--5 nm) / Ag (6 nm) / ${\mathrm{Co}}₂\mathrm{MnGe}$ (5 nm) / Ag / Ta large-area spin-valve structures, and the transverse spin current absorption of ${\mathrm{Ni}}₈₁{\mathrm{Fe}}₁₉$ sink layers of different thicknesses has been explored. In some circumstances, the spin current absorption can be inferred from the modification of the ${\mathrm{Co}}₂\mathrm{MnGe}$ source layer damping in vector network analyzer ferromagnetic resonance (VNA-FMR) experiments. However, the spin current absorption is more accurately determined from element-specific phase-resolved x-ray ferromagnetic resonance (XFMR) measurements that directly probe the spin transfer torque (STT) acting on the sink layer at the source layer resonance. Comparison with a macrospin model allows the real part of the effective spin mixing conductance to be extracted. We find that spin current absorption in the outer Ta layers has a significant impact, while sink layers with thicknesses of less than 0.6 nm are found to be discontinuous and superparamagnetic at room temperature, and lead to a noticeable increase of the source layer damping. For the thickest 5-nm sink layer, increased spin current absorption is found to coincide with a reduction of the zero frequency FMR linewidth that we attribute to improved interface quality. This study shows that the transverse spin current absorption does not follow a universal dependence upon sink layer thickness but instead the structural quality of the sink layer plays a crucial role.