Magnetic and electric property evolution of amorphous cobalt-rich alloys driven by field annealing

Abstract

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.