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

Equal-spin Cooper pairs are pivotal building blocks for superconducting spintronics devices. In recent experi-ments unusual behavior was observed in ferromagnet/ferromagnet/superconductor devices when a precession of the magnetization was induced by ferromagnetic resonance. By using a nonequilibrium Usadel Green's 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 superconductor layer. This then results in an increase of the injected spin current below the superconducting critical temperature. Our calculations provide evidence of the essential role of equal-spin Cooper pairs in spin-transport properties of superconductor/ferromagnet devices and pave avenues for the design of superconducting spintronics devices.