Observation of linear spin wave dispersion in the reentrant spin glass Fe0.7Al0.3

Fe0.7Al0.3 is a reentrant spin glass, which undergoes a transition from a paramagnet to a disordered ferromagnet at Tc∼500 K; at a lower temperature the spins progressively freeze and it exhibits a spin-glass-like behavior. In the ferromagnetic phase spin waves with a q2 dispersion are observed at small q, which broaden rapidly and become diffusive beyond a critical wave vector q0. On cooling the spin waves also disappear and a strong elastic central peak develops. For measurements around the (1,1,1) Bragg peak, a new sharp excitation is observed which has a linear dispersion behavior. It disappears above Tc, but persists throughout the spin-glass phase. It is not present in the stoichiometric Fe3Al material.