FERROMAGNETIC CRITICAL CORRELATIONS AND DYNAMICS IN AUFE REENTRANT FERROMAGNETS

We determined the paramagnetic critical scattering of the reentrant fe rromagnets Au1-xFex, with x=0.2, 0.18, and 0.16, i.e., close to the th reshold (x(c) similar to 0.155), below which ferromagnetism disappears and a spin-glass state sets in. Following the Ornstein-Zernicke forma lism, we derived the inverse critical ferromagnetic correlation length (kappa=xi(-1)) and the related magnetic susceptibility. The macroscop ic susceptibility chi(q)=0 Of the samples was also determined in a sep arate experiment with a Faraday balance. The neutron results show the same behavior of critical type with chi(q)=0 for x=0.18 but not for x= 0.16. We interpret this discrepancy as an indication for a non-well-de fined ferromagnetic transition for x=0.16. A further comparison of our results with those for Fe shows that the amplitude of the correlation length increase with increasing disorder so that, at the same reduced temperature tau, the correlations are much larger in the disordered s amples than in Fe. As a consequence of this, the ferromagnetic correla tions in the disordered samples persist well above T-c and lead to a m uch wider critical region than is the case in Fe and in usual ferromag nets. (C) 1996 American Institute of Physics.