Spin-Pumping-Induced Inverse Spin Hall Effect in Nb/Ni80Fe20 Bilayers and its Strong Decay Across the Superconducting Transition Temperature

Abstract

We quantify the spin Hall angle θ SH and spin-diffusion length l sd of Nb from inverse spin Hall effect (ISHE) measurements in Nb / Ni 80 Fe 20 bilayers under ferromagnetic resonance. By varying the Nb thickness t Nb and comparing to a Ni 80 Fe 20 / Pt reference sample, room temperature values of θ SH and l sd for Nb are estimated to be approximately −0.001 and 30 nm, respectively. We also investigate the ISHE as a function of temperature T for different t Nb . Above the superconducting transition temperature T c of Nb , a clear t Nb -dependent T evolution of the ISHE is observed whereas below T c , the ISHE voltage drops rapidly and is below the sensitivity of our measurement setup at a lower T. This suggests the strong decay of the quasiparticle (QP) charge-imbalance relaxation length across T c , as supported by an additional investigation of the ISHE in a different sample geometry along with model calculation. Our finding suggests careful consideration should be made when developing superconductor spin Hall devices that intend to utilize QP-mediated spin-to-charge interconversion.