Neutron diffraction investigation of the nuclear and magnetic extinction in MnP

The absolute values of the reflecting powers ρ are measured for the 200 and 2 ± τ, 0, 0 set of magnetic and nuclear reflections in the helimagnetic phase of a good-quality crystal of MnP as a function of its thickness. Severe and very different extinction effects are observed for the magnetic and nuclear reflections (y_magnetic ~ 0.4, y_nuclear ~ 0.02 for the largest thickness). This corresponds to the spectacular result that the magnetic reflecting powers ρ_± are twice as big as the nuclear one ρ_N, in spite of the fact that the scattering cross sections |F_±|² are about ten times smaller than the nuclear |F_N|². The nuclear results appear consistent with dynamical theory while the magnetic ones are not. They can be explained by Zachariasen's type II secondary extinction model based on the chirality domain pattern. The same measurements were performed in the ferromagnetic phase, yielding y_ferro ≃ 0.03. A model using the relative sizes of the ferromagnetic and chirality domains is presented.