NANOCRYSTALLIZATION OF FERROMAGNETIC CO-RICH AMORPHOUS-ALLOYS AND MAGNETIC SOFTENING

Ultra-fine grain structures were obtained in soft Co-based amorphous a lloys subjected to suitable heat treatments within a restricted range of temperatures which begins well below the conventional crystallizati on temperature, T-cr, of the alloy. These structures lead to superior soft magnetic properties and consist of magnetic nanocrystallites with an average grain size in the range 10-12 nm embedded within a residua l ferromagnetic amorphous matrix. The results refer to amorphous ribbo ns of Vitrovac 6025, which were obtained by the single-roller quenchin g technique and had the nominal composition Co65Fe4Mo2B11.5Si16.5. Wit h regard to the coercive field and the initial reversible permeability , the best softening nanocrystallization effects occurred in toroidal samples heat treated for about half an hour at around 500 degrees C. F or the amorphous alloy, the nominal values of Te (the Curie temperatur e) and T-cr are 210 and 540 degrees C, respectively. The results are i nterpreted on the basis of the random anisotropy model generalized to two-magnetic-phase systems.