Dynamical diffraction effects in anomalous X-ray magneto-optics of spiral magnetic structures

Strong X-ray resonant magnetic scattering near the M absorption edges of rare earth elements results in a drastic decrease of the magnetic extinction length which becomes comparable with the absorption length. Under such conditions it may be possible to observe dynamical effects in direct (non-satellite) X-ray diffraction on spiral magnetic structures in rare earth magnets. A dynamical theory of X-ray diffraction on a simple magnetic spiral is presented. Diffraction properties of spiral magnets are shown to be similar to the well-known unique properties of liquid crystals (e.g. cholesterics), but the magnetic nature of the spiral leads to more complex diffraction phenomena. Diffraction on spiral magnets shows both “cholesteric” (non-magnetic) and “gyrotropic” (magnetic) features: two types of reflections with different polarization properties at different Bragg angles may be observed. The region of magnetic reflection is shown to have a complex angular and polarization structure. Specific “elliptical” standing waves are formed in a magnetic crystal over this region.