FMR study of changes in the magnetic behaviour of amorphous Fe_83-xW_xB₁₇ (x = 5,9) alloys due to isothermal annealing. II. In the critical region

Abstract

Extensive ferromagnetic resonance (FMR) measurements have been performed in the critical region, -0.15 ≤ ε=(T-T_C)/T_C≤0.15, on amorphous Fe₇₈W₅B₁₇ and Fe₇₄W₉B₁₇ alloys, before and after they had undergone isothermal annealing treatment at a temperature T_A = 400 K for durations of time t_A = 1 and 2 h. The values of saturation magnetisation, M_s, and resonance field, H_res, at different temperatures are accurately determined through an elaborate lineshape analysis of the observed FMR spectra. From the M_s(H_res, T) data so obtained, the asymptotic values of the critical exponents β and γ for spontaneous magnetisation and initial susceptibility are estimated using the 'range-of-fit' scaling-equation-of-state analysis. Consistent with the predictions of the infinite three-dimensional (3D) ferromagnetic (FM) matrix plus finite spin clusters model and with the well-known Harris criterion, isothermal annealing, which enhances the value of the Curie temperature, T_C, considerably, leaves the values of the exponents unaltered from their 'as-quenched' values, which are close to those predicted by 3D nearest-neighbour Heisenberg model. In the critical region, the Gilbert damping parameter and the Lande splitting factor are temperature-independent and structural relaxation has no effect on them.