  Magnetization reversal is a well-studied problem with obvious applicability
in computer hard-drives. One can accomplish a magnetization reversal in at
least one of two ways: application of a magnetic field, or through a spin
current. The latter is more amenable to a fully quantum mechanical analysis. We
formulate and solve the problem whereby a spin current interacts with a
ferromagnetic Heisenberg spin chain, to eventually reverse the magnetization of
the chain. Spin-flips are accomplished through both elastic and inelastic
scattering. A consequence of the inelastic scattering channel, when it is no
longer energetically possible, is the occurrence of a new entity: a
non-equilibrium bound state (NEBS), which is an emergent property of the
coupled local plus itinerant spin system. For certain definite parameter values
the itinerant spin lingers near the local spins for some time, before
eventually leaking out as an outwardly diffusing state. This phenomenon results
in novel spin-flip dynamics and filtering properties for this type of system.
