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

Abstract

Results of detailed ferromagnetic resonance measurements taken on amorphous (a-) Fe_90+XZr_10-X alloys with X=0 and 1 in the horizontal-parallel and vertical-parallel sample configurations at a fixed microwave frequency of =9.23 GHz in the temperature range 77 to 300 K before and after these alloys have undergone isothermal annealing treatment at 400 K for durations of time, tA, ranging from 10 to 240 min are presented and discussed. While only a single (primary) resonance is observed for T≤ TC (the Curie temperature), an additional (secondary) resonance first appears at T= T_C and then gets fully resolved for T≥ (T_C+10 K). For the primary resonance (i) the "in-plane" uniaxial anisotropy field, Hk, and the Gilbert damping parameter, λ, both scale with the saturation magnetization, MS, in the temperature ranges 77 K≤ T≤ T_C and 0.5T_C≤ T≤ 0.8T_C, respectively, at all tA including tA=0; (ii) isothermal annealing has essentially no effect on MS(T), the "peak-to-peak" linewidth, Δ Hpp(T), Δ H0 (the frequency- and temperature-independent contribution to Δ Hpp) and λ (T); (iii) with increasing Fe concentration, the value corresponding to the peak in λ (T) decreases, λ (0.5TC) increases while λ (300 K) stays constant at 1×10⁸ s^-1, and (iv) isothermal annealing up to a time duration of 30 min results in an enhancement of about 1.5% in TC and a reduction of nearly 40% in the characteristic temperature T0 (which is a measure of the re-entrant transition temperature) whereas no further change occur- s in both TC and T0 at higher annealing times. By contrast, for the secondary resonance, (a) Hk=0 at all temperatures T≥T_C and values of tA covered in the present experiments, (b) isothermal annealing leaves the resonance field unaltered, and (c) a steep decline in the value of Δ Hpp' occurs for tA≤ 10 min but at higher values of tA, Δ Hpp' remains constant for a-Fe₉₀Zr₁₀ whereas it increases at first and then saturates in the case of a-Fe₉₁Zr₉. The effect of isothermal annealing on the quantities of interest, i.e., T_C, T0, Δ Hpp, etc., can be qualitatively understood in terms of a finite spin clusters plus an infinite ferromagnetic matrix model.