Effect of isothermal annealing on the magnetic behavior of amorphous ferromagnets with composition near the percolation threshold. II. In the critical region

Abstract

Extensive ferromagnetic resonance (bulk magnetization) measurements have been performed on amorphous Fe90+XZr10−X alloys with X=0 and 1 (X=0, 1, and 2) in the critical region, −0.1≲(T−TC)/TC≲0.1, before and after they had been subjected to isothermal annealing treatment at 400 K for durations of time, tA, ranging from 10 to 240 min (in the ‘‘as-quenched’’ condition) with a view to study the alterations in the critical behavior as the tricritical point, Xc≂3, is approached along the ferromagnetic (FM)-paramagnetic (PM) transition line in the magnetic phase diagram and as the topological (TSRO) as well as chemical (CSRO) short-range order undergo changes on account of structural relaxation. Asymptotic values of the critical exponents β and γ and critical amplitudes m0 and (m0/h0) for spontaneous magnetization and initial susceptibility have been determined for the as-quenched as well as annealed samples. Consistent with the predictions of the infinite FM matrix plus finite spin clusters model and with the well-known Harris criterion, change in CSRO and TSRO caused by altering the alloy composition and/or by isothermal annealing leaves the values of the critical exponents β and γ, and the universal ratio m0/MS(0) unaltered from those predicted by the renormalization group theory for an ordered spin system with spatial as well as spin dimensionality of three. The presently determined values of the critical amplitudes reveal that the fraction of spins that actually participates in the FM-PM transition is as low as ≊10% and reduces drastically as X→Xc but for a given composition, it declines slowly with increasing tA.