  In conventional superconductors, electrons of opposite spins are bound into
Cooper pairs. However, when the superconductor is in contact with a
non-uniformly ordered ferromagnet, an exotic type of superconductivity can
appear at the interface, with electrons bound into three possible spin-triplet
states. Triplet pairs with equal spin play a vital role in low-dissipation
spintronics. Despite the observation of supercurrents through ferromagnets,
spectroscopic evidence for the existence of equal-spin triplet pairs is still
missing. Here we show a theoretical model that reveals a characteristic gap
structure in the quasiparticle density of states which provides a unique
signature for the presence of equal-spin triplet pairs. By scanning tunnelling
spectroscopy we measure the local density of states to reveal the spin
configuration of triplet pairs. We demonstrate that the Al/EuS interface causes
strong and tunable spin-mixing by virtue of its spin-dependent transmission.
