Magnetic field dependence of the spin dynamics in the reentrant spin glass Fe70.4Al29.6

The field dependence of the inelastic neutron scattering from the reentrant spin glass (RSG) Fe70.4Al29.6 is investigated below TC. In the intermediate paramagnetic phase 100 K<T<Tcinv 200 K spin waves are induced in an applied field. The peaks broaden rapidly with increasing q and decreasing temperature. Remarkably these excitations exhibit a dispersion curve with a negative slope. In a field of 10 kOe the energy gap corresponds to a Zeeman splitting g BH 0.11 meV. In the spin-glass phase at 10 K no inelastic scattering is observed in a field, although the sample is close to saturation. The results are interpreted in terms of a field-induced dispersion, which has been observed recently in a diluted random antiferromagnet.