Superconductivity-Enabled Conversion of Ferromagnetic Resonance into Standing Spin Waves

TL;DR

This study demonstrates that conventional superconductors can convert uniform ferromagnetic resonance into standing spin waves in adjacent insulators, enabled by spin-polarized triplet Cooper pairs and vortex-induced fields.

Contribution

It provides the first experimental and theoretical evidence of superconductivity enabling conversion of FMR into PSSWs via interfacial spin-transfer torque and vortex effects.

Findings

Additional resonance feature appears below Nb transition temperature.

Conversion involves spin-polarized triplet Cooper pairs and vortex-induced fields.

Theoretical model reproduces experimental lineshapes.

Abstract

Superconductors can transport spin without Joule dissipation, yet their coherent coupling to short-wavelength magnons in insulating magnets remains largely unexplored. Here we demonstrate experimentally and theoretically that a conventional diffusive superconductor can enable the conversion of the uniform ferromagnetic-resonance (FMR) mode into perpendicular standing spin waves (PSSWs) in an adjacent ferrimagnetic insulator. In Bi-substituted iron-garnet/Nb bilayers, the microwave transmission develops an additional resonance feature that appears only below the Nb transition temperature and lies close to the uniform FMR peak. A microscopic theory that self-consistently couples the quasiclassical Keldysh--Usadel description of the superconducting condensate to the Landau--Lifshitz--Gilbert dynamics shows that the conversion requires two ingredients: (i) an interfacial spin-transfer torque mediated by spin-polarized triplet Cooper pairs and (ii) a depth-dependent effective field produced by Abrikosov vortices (electromagnetic proximity). The resulting susceptibility reproduces the measured lineshapes and establishes superconductivity as an active control knob for exchange standing-wave modes in magnetic insulators.