Temperature-dependent spin-transport and current-induced torques in superconductor/ferromagnet heterostructures

TL;DR

This study explores how temperature affects spin currents and current-induced torques in superconductor/ferromagnet heterostructures, revealing suppression of damping-like torque and emergence of large field-like torque below the superconducting transition.

Contribution

It provides the first quantitative analysis of temperature-dependent spin transport and torques in superconductor/ferromagnet heterostructures using phase-sensitive microwave detection.

Findings

Suppression of damping-like torque below T_c due to changes in spin current transport.

Observation of a large field-like torque below T_c.

Temperature-dependent variation of inverse spin Hall effect signals.

Abstract

We investigate the injection of quasiparticle spin currents into a superconductor via spin pumping from an adjacent FM layer.$\;$To this end, we use NbN/\ch{Ni80Fe20}(Py)-heterostructures with a Pt spin sink layer and excite ferromagnetic resonance in the Py-layer by placing the samples onto a coplanar waveguide (CPW). A phase sensitive detection of the microwave transmission signal is used to quantitatively extract the inductive coupling strength between sample and CPW, interpreted in terms of inverse current-induced torques, in our heterostructures as a function of temperature. Below the superconducting transition temperature $T_{\mathrm{c}}$, we observe a suppression of the damping-like torque generated in the Pt layer by the inverse spin Hall effect (iSHE), which can be understood by the changes in spin current transport in the superconducting NbN-layer. Moreover, below $T_{\mathrm{c}}$ we find a large field-like current-induced torque.