Spin current injection via equal-spin Cooper pairs in ferromagnet/superconductor heterostructures

TL;DR

This paper demonstrates that equal-spin Cooper pairs enable spin transport in superconductor/ferromagnet heterostructures, with the spin injection process governed by the inverse proximity effect, advancing superconducting spintronics understanding.

Contribution

It provides the first theoretical evidence of the role of equal-spin Cooper pairs in spin transport within S/F heterostructures using a non-equilibrium Green function approach.

Findings

Equal-spin Cooper pairs transport spin inside the superconductor.

Spin current increases below the superconducting critical temperature.

Spin injection is governed by the inverse proximity effect.

Abstract

Equal-spin Cooper pairs are pivotal building blocks for superconducting spintronics devices. In recent experiments, unusual behavior was observed in ferromagnet/ferromagnet/superconductor devices when a precession of the magnetization was induced by ferromagnetic resonance. By using a non-equilibrium Usadel Green function formalism, we study spin transport for such a setup. We solve for spin-resolved distribution functions and demonstrate that the spin injection process in superconductors is governed by the inverse proximity effect in the superconducting layer. We find that equal-spin Cooper pairs, which are produced by the two misaligned ferromagnetic layers, transport spin inside the S layer. This then results in an increase of the injected spin current below the superconducting critical temperature. Our calculations provide the first evidence of the essential role of equal-spin Cooper pairs on spin-transport properties of S/F devices and pave new avenues for the design of superconducting spintronics devices.