Magnetic field dependence of the spin dynamics in Fe₂₈Cr ₇₂

The field dependence of the magnetic excitations in the re-entrant spin glass (RSG) Fe₂₈Cr₇₂ is investigated over a wide range of temperatures. The zero-field spin dynamics are anomalous not only in the spin-glass phase but also in the paramagnetic phase. In an applied field (H=8.8 kOe) spin waves are induced at low temperatures with an energy gap g mu _BH approximately=0.1 meV. This observation is in marked contrast with results from Fe_1-xAl_x, where no gap is induced. The results suggest that the ground state of RSG is not unique.