Anomalous critical spin dynamics of ferromagnetic Au-17 at.% Fe and Au-19at. %Fe alloys: Neutron-spin-echo studies

Abstract

Measurements are presented of the temperature and wave-vector dependence of the critical spin dynamics of the ferromagnetic Au-17 at. %Fe and Au-19at.%Fe alloys above T _c, using the neutron-spin-echo-scattering technique. The relaxation function in both alloys was found to exhibit a slow non-exponential time decay behaviour, quite unlike that observed in usual ferromagnetic systems. This anomalous behaviour of the relaxation function is discussed in terms of the combined effects introduced by the disordered fractal nature of the infinite cluster and the presence of the finite clusters that give rise to localization phenomena. An analysis of the data in terms of the dynamic scaling hypothesis suggests that the above-mentioned effects wash out the true critical region but do not cause it to revert to the hydrodynamic nature of the spin dynamics.