Determination of asymptotic critical exponents and amplitudes for amorphous 3D ferromagnets from bulk magnetization measurements

Abstract

High-resolution bulk magnetization data have been taken in the critical region (−0.05 ≤ ϵ = ( T − T C)/ T C ≤ 0.05, where T C is the Curie temperature) on amorphous Fe x Ni 80- x P 14B 6 alloys with x = 20, 30 and 40. An elaborate data analysis, besides yielding accurate values for the spontaneous magnetization and initial susceptibility asymptotic (leading ‘correction-to-scaling’) critical exponents and amplitudes (amplitudes), reveals that the values of the asymptotic critical exponents and universal amplitude ratios m o/ M S( O) and Dm o δ / h o are composition-independent and exactly equal to those for the pure (ordered) spin system with space as well as spin dimensionality of three. However, the main difference between crystalline (ordered) and amorphous (quench-disordered) three-dimensional Heisenberg ferromagnets is that nearly all the spins participate in the ferromagnetic (FM)-paramagnetic (PM) phase transition in the former case while only a small fraction of spins is actually involved in such a transition in the latter case. This fraction reduces drastically as the percolation threshold for the long-range FM order is approached along the FMPM phase transition line in the magnetic phase diagram. Asymptotic critical exponents β, γ and δ obey Widom scaling relation, βδ = β + γ, and the magnetization data satisfy the scaling equation of state for second-order phase transition in the asymptotic critical region.