Dynamics of two ferromagnetic insulators coupled by superconducting spin current

TL;DR

This paper explores how a superconductor between two ferromagnets can sustain a coherent spin current, affecting magnetization dynamics and revealing unique phenomena like mode crossing behaviors.

Contribution

It introduces the current-phase relation for spin supercurrents in superconductor-ferromagnet systems and analyzes their impact on magnetization dynamics and experimental detection methods.

Findings

Superconductor can sustain equilibrium spin current via odd-frequency correlations.

Spin supercurrent influences magnetization dynamics, including hysteresis and resonance.

Mode crossing can be avoided or locked depending on spin current interactions.

Abstract

A conventional superconductor sandwiched between two ferromagnets can maintain coherent equilibrium spin current. This spin supercurrent results from the rotation of odd-frequency spin correlations induced in the superconductor by the magnetic proximity effect. In the absence of intrinsic magnetization, the superconductor cannot maintain multiple rotations of the triplet component but instead provides a Josephson type weak link for the spin supercurrent. We determine the analogue of the current-phase relation in various circumstances and show how it can be accessed in experiments on dynamic magnetization. In particular, concentrating on the magnetic hysteresis and the ferromagnetic resonance response, we show how the spin supercurrent affects the nonequilibrium dynamics of magnetization which depends on a competition between spin supercurrent mediated static exchange contribution and a dynamic spin pumping contribution. Depending on the outcome of this competition, a mode crossing in the system can either be an avoided crossing or mode locking.