  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.
