Magnetic phase transitions in FeNiBSi alloys

The magnetic behavior of amorphous Fe 4.9Ni 76.0B 12.5Si 6.6 and Fe 1.1Ni 76.5B 18.4Si 4.0 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 Fe 4.9Ni 76.0B 12.5Si 6.6 alloy shows a ferromagnetic phase transition, and the critical exponents β, γ′, γ and δ are determined by modified Arrott plots, scaling plots, 1n J vs. 1n(μ 0 H) plots and the method of Kouvel and Fischer. The Kouvel-Fischer exponent γ( T) exhibits the nonmonotonic temperature dependence characteristic for many disordered ferromagnets. In contrast, Fe 1.1Ni 76.5B 18.4Si 4.0 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.