Influence of isothermal annealing on the critical behaviour of amorphous Fe_90+xZr_10-x alloys

Extensive ferromagnetic resonance (FMR) measurements have been performed in the critical region on amorphous Fe₉₀Zr₁₀ and Fe₉₁Zr₉ alloys before and after they have undergone isothermal annealing at 400 K for durations of time ranging from 10 to 240 min. Saturation magnetization M_S and resonance field H_res as functions of temperature in the critical region are accurately determined by a detailed lineshape analysis of the observed FMR spectra. The 'range-of-fit' scaling-equation-of-state analysis is then employed to arrive at the asymptotic values of the spontaneous magnetization and initial susceptibility critical exponents beta and gamma from the M_S(H_res,T) data so obtained. Isothermal annealing causes a significant enhancement in the Curie temperature T_C, but leaves the values of the exponents beta and gamma , which are fairly close to those predicted by the three-dimensional Heisenberg model, unaltered. The FMR linewidth versus temperature curves exhibit an abrupt slope change at T_C, indicating a well-defined phase transition from the ferromagnetic to the paramagnetic state.