The appearance of critical behaviour at the onset of ferromagnetism in AuFe alloys

Abstract

Reports measurements of the dynamic response, chi '( omega ), for AuFe alloys. It is shown that this experiment can indicate the infinite percolation cluster concentration C_p. The thermal critical behaviour of the zero field chi '( omega ) in alloys containing an infinite cluster is discussed in terms of the cross-over from isotropic short-range exchange-dominated to dipolar-dominated behaviour, although renormalisation of the critical exponents is necessary. Coexistence of finite clusters with the infinite cluster is also confirmed; the freezing temperature of these finite clusters well below T_c shows up very clearly in the simple magnetic response experiments. (These effects should not be associated with a phase transition from a ferromagnetic to a spin-glass regime.) For alloys below the percolation threshold the fluctuations within very large clusters of highly correlated spins are essentially quasi-critical and very similar to those in alloys containing an infinite cluster. The critical temperature T_c of the percolation cluster, however, was found to be finite and non-zero, suggesting a possible modification of the pure percolation processes by the long-range RKKY interactions between clusters.