EXTINCTION EFFECTS IN POLARIZED NEUTRON-DIFFRACTION FROM MAGNETIC CRYSTALS .1. HIGHLY PERFECT MNP AND YIG SAMPLES

Wavelength-dependent flipping-ratio data obtained with polarized neutrons are analysed with the help of two extinction models: Kato's [Acta Cryst. (1980), A36, 763-769, 770-776] statistical dynamical theory (SDT) and the random elastic deformation (RED) approach [Kulda (1987). Acta Cryst. A43, 167-173; (1991). Acta Cryst. A47, 775-779]. Because the crystals used were nearly perfect, Pendellosung oscillations are present in all cases and the extinction theories are thus tested under extreme conditions. Both treatments give fair agreement with the experimental data in the short-wavelength range and there is no significant difference between them in terms of the goodness of fit, but the parameters involved in the RED approach have a more straightforward physical meaning. There are, however, inconsistencies in the values of the SDT parameters obtained, as well as probable deviations at large wavelength and crystal thickness, which could be related to a new validity condition for SDT.