Nanocrystallization of ferromagnetic Co-rich amorphous alloys and magnetic softening

Ultra-fine grain structures were obtained in soft Co-based amorphous alloys subjected to suitable heat treatments within a restricted range of temperatures which begins well below the conventional crystallization 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 residual ferromagnetic amorphous matrix. The results refer to amorphous ribbons of Vitrovac 6025, which were obtained by the single-roller quenching technique and had the nominal composition Co65Fe4Mo2B11.5Si16.5. With 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. For the amorphous alloy, the nominal values of Te (the Curie temperature) and T-cr are 210 and 540 degrees C, respectively. The results are interpreted on the basis of the random anisotropy model generalized to two-magnetic-phase systems.