For holographic gratings recorded in superparamagnetic nanoparticle-polymer composites the diffraction efficiency should – next to grating spacing, nanoparticle concentration and grating thickness – 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.