  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.
