We have computed the optical diffraction pattern for linearly polarize
d light incident normal to the twist axis (phase grating mode) of a ma
gnetically doped cholesteric (ferrocholesteric). The intrinsic Faraday
rotation of the magnetic grains results in extra orders of diffractio
n. Also we find diffraction for any azimuth of the incident vibration.
Further, using the Jones N matrices we have worked out the optical pr
operties for light propagation along the twist axis on the very low wa
velength side of the reflection band. We find that the medium behaves
very differently from a normal cholesteric.