Field effects on the early crystallization of Co-rich amorphous ribbons (Co(68.15)Fe(4.35)Si(12.5)B(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.

Microstructure changes of the cross-section of Co-rich amorphous ribbons (Co68.15Fe4.35Si12.5B15, atomic%) annealed at 450 °C for 30 min with a zero field, a 10 Oe longitudinal external magnetic field or a 10 Oe transverse external magnetic field are investigated by comparing with as-spun ribbons. The ribbons annealed without a field, and under the transverse or longitudinal field exhibit different gradient microstructures along the cross-section of the ribbons, forming gradient amorphous–nanocrystalline (AN) composites different from the as-spun amorphous species. These kinds of AN composites preserve unique magnetic properties and enhanced conductivities when compared with the as-spun ribbons, leading to excellent giant magneto-impedance (GMI) effects. In particular, the AN composites prepared from the ribbons annealed under the longitudinal field lose the dual-peak feature shown by the as-spun ribbons but a mono-GMI peak appears, preserving a maximum MI ratio as high as 520% and a low magnetic field (0–1 Oe) sensitivity of 80% Oe−1 at 15 MHz and a maximum field of 40 Oe. The distinctly graded AN composites prepared from the ribbons annealed under the transverse field still preserve the dual-peak feature as the as-spun ribbons, having a maximum MI ratio of 320% and a significantly enhanced low magnetic field (0–1 Oe) sensitivity of 200% Oe−1 at 10 MHz and a maximum field of 40 Oe.