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

Abstract

Results of detailed ferromagnetic resonance (FMR) measurements performed in the temperature range 77 to 480 K at a fixed microwave frequency of ≅ 9.23 GHz, using horizontal-parallel and vertical-parallel sample geometries, on amorphous (a-) Fe_83-xW_xB₁₇ (x = 5 and 9) alloys before and after these alloy samples had undergone isothermal annealing at T_A = 400 K for durations of time t_A = 1 and 2 h are presented and discussed. The FMR resonance persists to temperatures as high as 480 K, broadens out and shifts to higher fields as the temperature increases. The 'in-plane' uniaxial anisotropy field, H_k, scales with the saturation magnetisation, M_s, in the temperature range 77 K≤ T ≤ T_C (Curie temperature), while the Gilbert damping parameter, λ, is temperature-independent. Isothermal annealing has practically no effect on Ms(T) (and hence on the resonance field, Hres(T)) the 'peak-to-peak' linewidth, ΔH_pp(T), and λ but enhances T_C by nearly 2.5%. With increasing W concentration, the Curie temperature reduces drastically whereas λ increases. All these observations can be qualitatively explained on the basis of the infinite ferromagnetic matrix plus finite spin clusters model.