MAGNETIC PHASE-TRANSITIONS IN FENIBSI ALLOYS

The magnetic behavior of amorphous Fe4.9Ni76.0B12.5Si6.6 and Fe1.1Ni76.5B18.4Si4.0 is investigated by measuring the magnetic moment in the critical regime and in the paramagnetic regime for applied fields from 10 mT to 1.5 T. The Fe4.9Ni76.0B12.5Si6.6 alloy shows a ferromagnetic phase transition, and the critical exponents beta, gamma', gamma and delta are determined by modified Arrott plots, scaling plots, ln J vs. ln(mu-0H) plots and the method of Kouvel and Fisher. The Kouvel-Fisher exponent gamma-(T) exhibits the nonmonotonic temperature dependence characteristic for many disordered ferromagnets. In contrast, Fe1.1Ni76.5B18.4Si4.0 is characterized by a spin-glass-like state with vanishing spontaneous polarization. In order to demonstrate this we have applied all the aforementioned methods for the investigation of the ferromagnetic transition and show that they fail. Surprisingly, in large fields a good scaling behavior is observed for the polarization, with unusual exponent values.