holographic gratings recorded in superparamagnetic nanoparticle-polymer composites the diffraction effciency should next to the coherent scattering density modulation amplitude depend on the strength of an external magnetic field and the incident neutron spin state. As a consequence, diffraction gratings should be tunable to act as mirrors for one spin state, while being essentially transparent for the other. Thus, polarizing beam splitters for cold neutrons become feasible.