Spin-pumping-induced inverse spin-Hall effect in Nb/Ni80Fe20 bilayers and its strong decay across the superconducting transition temperature

TL;DR

This study measures the spin Hall angle and spin diffusion length in Nb/Ni80Fe20 bilayers, revealing a strong decay of the inverse spin-Hall effect below the superconducting transition temperature, impacting superconductor spintronic device design.

Contribution

It provides the first quantitative analysis of the spin Hall angle and spin diffusion length in Nb, and demonstrates the decay of the inverse spin-Hall effect across the superconducting transition.

Findings

Spin Hall angle in Nb is approximately -0.001.

Spin diffusion length in Nb is about 30 nm.

Inverse spin-Hall effect diminishes rapidly below Tc in Nb.

Abstract

We quantify the spin Hall angle {\theta}SH and spin diffusion length lsd of Nb from inverse spin-Hall effect (iSHE) measurements in Nb/Ni80Fe20 bilayers under ferromagnetic resonance. By varying the Nb thickness tNb and comparing to a Ni80Fe20/Pt reference sample, room temperature values of {\theta}SH and lsd 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 tNb. Above the superconducting transition temperature Tc of Nb, a clear tNb-dependent T evolution of the iSHE is observed whereas below Tc, 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 Tc, 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.