Dynamics of spin freezing in the re-entrant spin glass FeNiMn

For a polycrystalline sample of (Fe 0.65 Ni 0.35 ) 1− x Mn x with x = 0.113 the intermediate scattering function s ( Q , t ) was observed by paramagnetic neutron spin echo techniques at scattering vectors 0.26 nm −1 Q −1 and at temperatures ranging from 120 to 40 K, thus covering both the ferromagnetic phase and the RSG phase of the alloy. The normalized intermediate scattering function S ( Q , t ) = s ( Q , t )/ s ( Q ) strongly depended on Q in the FM phase as would be expected. However, even in the mixed RSG phase the Q -dependence was appreciable, and thus it differed from the behaviour observed in classical spin glasses. It deviated from an exponential decay and was represented by a stretched exponential (with exponent n = 0.4–0.9 typically). The spin freezing, as observed by the increase of the inverse time constant, changed by more than two orders of magnitude between the onset of the RSG phase at 90 K and the strongly irreversible RSG phase at low temperature (⩽ 50 K).