SOME STRUCTURAL ASPECTS OF MAGNETIC PROPERTY EVOLUTION IN FINEMET-TYPE SENSOR MATERIAL DURING AMORPHOUS-NANOCRYSTALLINE TRANSFORMATIONS

Devitrification of glassy Fe73.5Si13.5B9Nb3Cu1 alloy used as potential sensor material was followed by magnetic measurements and X-ray diffraction, and transmission electron microscopy. It was found, that the permeability, and the coercive field of the alloy changed well below the crystallisation onset, showing a remarkable increase, which was attributed to the structural relaxation and a heterogen nucleation of iron rich phase on Cu clusters. On the basis of lattice parameter measurements the iron rich clusters were depleted from Si atoms during the nucleation process and the Si atoms built in them only during annealing at higher temperatures as 500 \circC and 540 \circC forming a DO3 structure. From the structural study of samples annealed near to the optimal transformation stage it was concluded, that size distribution of the bcc-Fe(Si) nanocrystallites (not exclusive factor) was only one of the most important factors to achieve the optimal soft magnetic properties, but at the same time, their volume fraction, as well as the degree of ordering in DO3 structure did also contribute to the property optimization. The average grain size produced by laser pulse heat treatment was nearly the same, as that obtained during the traditional heat treatments