Magnetic field driving gradient effects on the microstructure in amorphous-nanocrystalline cobalt alloy ribbons

Field effects on the early crystallization of Co-rich amorphous ribbons (Co _68.15Fe _4.35Si _12.5B ₁₅, atomic%) performed at 450 °C for 30 minutes in an applied 10 Oe longitudinal or transverse external magnetic field are investigated by comparing with the as-spun and annealed samples in zero applied magnetic field. Results indicate that the crystallization on the surface skin of the ribbon differs from that of the middle section due to the combined effects from the field direction and the defects or stress characteristics across the ribbons. In particular, the ribbons annealed under a transverse field exhibit an extremely distinct graded microstructure across the ribbon, which features amorphous-phased skin layers (top and bottom) of about 4-5 μm thickness and a middle amorphous- nanocrystalline composite layer about 10-12 μm thick, with the nanograins of 434 ± 99 nm dispersing in the amorphous matrix.