Mössbauer spectroscopy of amorphous Fe–Si–B alloys with different free volume content

The hyperfine field distributions of as-cast and annealed Fe81.5Si4B14.5 amorphous alloys prepared with different quenching rates have been measured at room temperature by means of Mössbauer spectroscopy. No significant differences have been found between the spectra of as-cast materials produced at different quenching rates, in contrast with the results of measurements of the electrical resistivity and the magnetic permeability aftereffect. This result indicates that the variation of the distribution of pressure–density fluctuations in differently quenched systems does not modify the value of the hyperfine field, which is rather dominated by the effect of the local magnetic disorder, and not substantially influenced by small variations of the quenching rate. The hyperfine field distributions of annealed ribbons are coherently shifted towards higher values with respect to the ones of as-cast alloys. This result is related the increase of the Curie temperature, observed in these alloys after annealing, and is interpreted as the effect of the irreversible reduction of the local magnetic disorder induced by structural relaxation.