Magnetic relaxation in amorphous ribbons prepared with different quenching rates

The relaxation of magnetic permeability has been followed over a wide temperature range on amorphous magnetic ribbons (composition Fe81.5B14.5Si4), prepared with different quenching rates, and thus characterized by thickness varying between 21 and 61μm. The measurements were performed on as quenched and annealed (2 h at 330°C in vacuum) samples, by taking the Fourier transform of permeability pulse envelopes, having a repetition time τ = 25 s. The magnitude F(ω, T) at an appropriate frequency at each temperature T is proportional to the decay of permeability after a time τ. By plotting this quantity vs. temperature (between 4 and 600 K), significant differences are found in the relaxation behavior of the examined samples, the relaxation effects being much more pronounced on thinner specimens. The room temperature relaxation is actually found to be closely proportional to the inverse of thickness, that is to quenching rare. These differences are somewhat, but not completely, eliminated on the annealed samples. These results thus provide evidence of structural differences in amorphous alloys due to different preparation procedures. The presence of maxima at higher temperatures on the curve of the permeability relaxation vs. T also provides evidence of differences in structural relaxation due to annealing in the amorphous state of the various samples.