  Time-resolved coherent X-ray diffraction is used to measure the spatially
resolved magnetization structure within FePt nanoparticles during laser-induced
ultrafast demagnetization. The momentum-dependent X-ray magnetic diffraction
shows that demagnetization proceeds at different rates at different X-ray
momentum transfer. We show that the observed momentum-dependent scattering has
the signature of inhomogeneous demagnetization within the nanoparticles, with
the demagnetization proceeding more rapidly at the boundary of the
nanoparticle. A shell region of reduced magnetization forms and moves inwards
at a supermagnonic velocity. Spin-transport calculations show that the shell
formation is driven by superdiffusive spin flux mainly leaving the nanoparticle
into the surrounding carbon. Quantifying this non-local contribution to the
demagnetization allows us to separate it from the local demagnetization.
