Neutron Scattering Investigation of the Magnetic Excitations in Iron

The spin-wave spectrum of iron has been measured by neutron inelastic scattering using the triple-axis technique. A large crystal of ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$ (4-at.% Si) was used in most of the measurements to avoid nonmagnetic scattering processes, although a small amount of data was obtained on pure iron. The spin-wave dispersion curves rise nearly quadratically according to the relation $E=D{q}^2$, where $D$ was found to be about 260 meV ${\mathrm{\AA }}^2$ for Fe (4-at.% Si) and 280 meV ${\mathrm{\AA }}^2$ for pure iron. The spin-wave intensity was found to decrease slowly with increasing energy until about 85 meV and then to drop rapidly by an order of magnitude. This sudden decrease in the spin-wave intensity is interpreted as resulting from the intersection of the spin-wave dispersion curve with a continuum band of Stoner excitations.