  At the interface between a ferromagnetic insulator and a superconductor there
is a coupling between the spins of the two materials. We show that when a
supercurrent carried by triplet Cooper pairs flows through the superconductor,
the coupling induces a magnon spin current in the adjacent ferromagnetic
insulator. The effect is dominated by Cooper pairs polarized in the same
direction as the ferromagnetic insulator, so that charge and spin supercurrents
produce similar results. Our findings demonstrate a way of converting Cooper
pair supercurrents to magnon spin currents.
