  We present an extension to simple s-d models, aiming at simulating ultrafast
magnetization dynamics and spin transport in metallic heterostructures. In
particular, we consider an alternative spin dissipation channel due to a finite
exchange splitting of the s band. From this theory, we show three different
mechanisms governing the dynamics of spin accumulation. On top of the already
widely discussed "-dM/dt" electron-magnon mechanism, we study the role of a
dynamic change of exchange splitting (of conduction electrons) as well as the
rotation of spins reflected at an interface with a ferromagnet. Finally, we use
the presented theory to explain the recent observation of subpicosecond
reversal of a ferromagnet in rare-earth free spin-valves. Our conclusion agrees
with the one of reference [1] favoring magnetization reversal due to the
rotation of the spin polarization of a reflected spin current.
