Spin waves in the Laves compound Ho.88Tb.12Fe2

Abstract

The energies of spin waves in the ferrimagnetic cubic Laves compounds Ho.88Tb.12Fe2 (TC=610°K) have been investigated by neutron spectrometry. The measurements were carried out at room temperature for wave vectors q↘ in the [100], [110], and [111] principal symmetry directions. At this temperature the alloy studied exhibits small magnetic anisotropy. Thus the acoustic branch of the dispersion relation is characteristic of that expected for a cubic ferrimagnet with only isotropic Heisenberg exchange interactions, i.e. it possesses a negligible energy gap (<0.2 meV) and for small q it is isotropic and shows nearly quadratic dispersion, h/ω?Dq2 with D=84 meV A−2. The lowest energy optic branch at about 7.5 meV shows very little dispersion and is nearly degenerate with the acoustic branch at the zone boundaries. Qualitative considerations of the scattering structure factors suggest that this optic branch is primarily associated with the rare‐earth sublattice, while the remaining four optic branches, all above 15 meV, are primarily associated with the iron sublattice. One of these optic branches is particularly interesting in that it shows extremely strong dispersion, similar to that exhibited by the spin waves in pure iron. Preliminary calculations based on a simple near‐neighbor exchange model indicate that such a model is inadequate to describe the experimental results.