Evidence of ferromagnetic spin waves in the spin-glass state of a re-entrant Ni-Mn alloy

Inelastic neutron scattering measurements have been performed on a Ni 0.784 Mn 0.216 disordered alloy which is ferromagnetic below 330K. At temperatures lower than 40K the AC susceptibility measurements suggest the onset of a frustrated phase. Spin waves have been unambiguously observed at all temperatures below the Curie temperature 330K, and in particular below 40K. The inelastic scattering reveals a marked anomaly of the spin dynamics at about 100K below which a quasi-elastic line appears and grows in intensity as commonly observed in similar re-entrant systems. This anomaly is characterised by a large enhancement of the spin-wave damping, a marked change of the transverse static susceptibility which is higher in the 'spin-glass phase' than in the ferromagnetic phase and, at lower temperatures (below 70K), an increase of the stiffness constant. These experimental results are in contradiction with models suggesting a simple break-up of the infinite ferromagnetic network established at T C .