Magnetic excitations in amorphous isotropic ferromagnets

Neutron-scattering techniques have been used to investigate the magnetic excitations in the structurally amorphous ideal isotropic ferromagnet ($T_c=513\mathrm{}$ K) ${\mathrm{Fe}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{P}}_{14}$ ${\mathrm{B}}_6$. Small-angle inelastic-scattering measurements were taken with a triple-axis spectrometer to determine the temperature dependence of the spin-wave dispersion relation at long wavelengths. Most of the measurements, however, were concentrated in the momentum region near the first peak ($Q_0=3.1\mathrm{}$ ${\mathrm{\AA }}^{-1\mathrm{}}$) in the static structure factor, and were made at 295 and 17 K with a pulsed polarized-beam time-of-flight spectrometer that uses the cross-correlation technique of data collection to obtain a high signal-to-noise ratio. It is found that there is a "cone" of scattering as a function of energy with its apex at $Q_0$, in general agreement with the powder-averaged model proposed by Shirane et al. However, not all of the features of the data can be explained by this model; in particular at $Q_0$ there is a broad distribution of scattering as a function of energy with a maximum at $E\sim 12$ meV. Time-of-flight results are also presented for a single crystal and a powdered crystalline material in order to gain a better understanding of the corrections needed to obtain accurate data, and to establish that the instrument performs properly. The advantages as well as the limitations of this polarized-beam technique are discussed.