  Ferromagnetism and superconductivity are antagonistic phenomena. Their
coexistence implies either a modulated ferromagnetic order parameter on a
lengthscale shorter than the superconducting coherence length or a weak
exchange coupling between the itinerant superconducting electrons and the
localized ordered spins. In some iron based pnictide superconductors the
coexistence of ferromagnetism and superconductivity has been clearly
demonstrated. The nature of the coexistence, however, remains elusive since no
clear understanding of the spin structure in the superconducting state has been
reached and the reports on the coupling strength are controversial. We show, by
a direct optical pump-probe experiment, that the coupling is weak, since the
transfer of the excess energy from the itinerant electrons to ordered localized
spins is much slower than the electron-phonon relaxation, implying the
coexistence without the short-lengthscale ferromagnetic order parameter
modulation. Remarkably, the polarization analysis of the coherently excited
spin wave response points towards a simple ferromagnetic ordering of spins with
two distinct types of ferromagnetic domains.
