Neutron Small-Angle Scattering by Spin Waves in Iron

The scattering of neutrons by spin waves in iron has been observed in virtual isolation at an intensity of several thousand counts per minute. The theoretical distribution of neutrons scattered near the primary beam hy spin waves is given, assuming perfect collinnation. By rotating the applied magnetic field through 90 deg , the expected spinwave pattern was tanned about the optical axis. The results of this were: (1) intensity having the anticipated angular dependence was present; (2) the expected curve of intensity variation was obtained; (3) absolute intensities were below those calculated for a simple HeisenbergBloch ferromagnet; (4) an absorption curve in silver indicated that the intensily differertial cross section has the form lambda /sup 2/; and (5) for some 54 deg K above room temperature the intensity was proportional to the absolute temperature, and the temperature increase was independent of the crystul setting. (B.O. G.)