  We report elastic and inelastic neutron scattering measurements of the
high-TC ferromagnet Mn(1+delta)Sb. Measurements were performed on a large,
TC=434 K, single crystal with interstitial Mn content of delta~0.13. The
neutron diffraction results reveal that the interstitial Mn has a magnetic
moment, and that it is aligned antiparallel to the main Mn moment. We perform
density functional theory calculations including the interstitial Mn, and find
the interstitial to be magnetic in agreement with the diffraction data. The
inelastic neutron scattering measurements reveal two features in the magnetic
dynamics: i) a spin-wave-like dispersion emanating from ferromagnetic Bragg
positions (H K 2n), and ii) a broad, non-dispersive signal centered at
forbidden Bragg positions (H$\,$K$\,$2$n$+1). The inelastic spectrum cannot be
modeled by simple linear spin-wave theory calculations, and appears to be
significantly altered by the presence of the interstitial Mn ions. The results
show that the influence of the interstitial Mn on the magnetic state in this
system is more important than previously understood.
