Direct Observation of the Angular Distribution of Neutrons Scattered at Small Angles by Spin Waves in Fe–Ni Alloys

Abstract

Extensive measurements of the small‐angle scattering of neutrons by spin waves in Fe☒Ni alloys using polycrystalline samples and a polyenergetic beam have been made by Hatherly et al. Their spin‐wave stiffness D was found not to agree with later measurements using the diffraction method on Fe0.5Ni0.5 and Fe0.2Ni0.8 single crystals. In an effort to resolve this disagreement we employed a new technique to observe the small‐angle scattering by spin waves in these materials that uses a double‐Si‐crystal spectrometer, monoenergetic neutrons, and single‐crystal samples. Our result for D for the Fe0.5Ni0.5 sample agrees with the previous single‐crystal data. However our data on Fe0.2Ni0.8 is in closer agreement with the earlier small‐angle scattering measurements. The effect of the dipole‐dipole interaction on the spin‐wave energy has been clearly observed in Fe0.5Ni0.5. Two discontinuities are predicted and observed to occur in the scattering profiles. However, an additional discontinuity is observed in our Fe0.7Ni0.3 single crystal, which may indicate another scattering surface of unknown origin