OBSERVATION OF SPIN-WAVE RENORMALIZATION EFFECTS IN IRON AND NICKEL

Angular distributions characteristic of the small-angle scattering of neutrons by long-wavelength spin waves have been observed at temperatures up to and above the Curie temperature in iron and nickel. For temperatures below 0·9Tc the spin-wave stiffness D, the spin-wave scattering cross section and the spin-wave lifetime have been obtained directly from the measured angular distributions, and the results are compared with the predictions of spin-wave interaction theories. For temperatures up to 0·42Tc in iron a good fit to the variation of D can be obtained using an expression of the form D0-D1T 2-D2T 5/2 based on the itinerant electron model, but above 0·42Tc the fit becomes progressively poorer and D decreases faster than the fitted expression. The cross section for spin-wave scattering shows a temperature dependence additional to that contained in the thermodynamic factor alone; for iron at temperatures up to 0·7Tc this additional dependence has the form <S z>{1-(0·30±0·08)T/Tc} and is stronger than the variation with <Sz> predicted by Marshall and Murray in a calculation of the effects of kinematic interactions between spin waves in the Heisenberg model. The observed spin-wave lifetimes have the same form of temperature dependence as is obtained from a nearest-neighbour Heisenberg model calculation by Cooke and Gersch. At temperatures above 0·9Tc the observed angular distributions were corrected for critical scattering using calculated cross sections for the latter due to Villain. D is found to fall slowly on passing through the Curie temperature in both iron and nickel, and is still of the order of a quarter of the room temperature value when the spin-wave scattering intensity falls below the threshold of observability.