Correlation of soft magnetic properties with free volume and medium range ordering in metallic glasses probed by fluctuation microscopy and positron annihilation technique

Amorphous ribbons of different thicknesses of Co64.5Fe3.5Si16B14Ni2 alloy were synthesized using the melt spinning technique by varying wheel speed. The effect of cooling rate on the ribbon thickness and their soft magnetic properties have been studied. The amorphous structure has been characterized in terms of structural free volume and medium range order (MRO) by positron annihilation spectroscopy and fluctuation electron microscopy techniques. Positron lifetime spectra of amorphous samples showed two lifetime components. The first component was found to be correlated with MRO whereas, the second lifetime component was found to be associated with nanovoid type of defects, and the second component was strongly dependent on processing conditions. It could be established that the coercivity of the amorphous samples produced by the rapid solidification technique mainly depends on the defects formed during processing rather than change induced in MRO.

► In Co64.5Fe3.5Si16B14Ni2 alloy, the ribbon's width “d” varies with wheel speed (v) as d=360v−0.58. ► FEM and PLS studies suggest that the first component τ1 is related to MRO in the amorphous structure. ► The second lifetime component τ2 is due to annihilation in nanovoids. ► The distribution (size and concentration) of nanovoids is found to be dependent on cooling rate. ► Hc is mainly controlled by defects formed during processing rather than the small change in MRO.