The magnon excitations near the first $\mathcal{Y}\mathrm{}\left(Q\right)\mathrm{}$ maximum in amorphous ferromagnets have been reexamined by the use of a triple-axis polarized-beam spectrometer. This technique permits us to separate inelastic magnetic cross sections from phonons and all elastic components. We point out that this technique is not useful once the magnon excitation energy becomes smaller than the instrumental width. We have examined three different Fe-based alloy compositions at room temperature. In each case our results are consistent with a mechanism involving "umklapp scattered" long-wavelength spin waves with no energy gap. We compare our results on ${\mathrm{Fe}}_{75}$${\mathrm{P}}_{15}$${\mathrm{C}}_{10}$ with those of Mook and Tsuei who interpreted their results in terms of a 19-meV energy gap in ${\mathrm{Fe}}_{75}$${\mathrm{P}}_{15}$${\mathrm{C}}_{10}$.

• Received 12 February 1982

DOI:https://doi.org/10.1103/PhysRevB.26.2575