Co-Based Amorphous Material for Giant Magnetoimpedance and Fluxgate Sensing Cores

Alloy composition (Co 94 Fe 6 ) 72.75 Si 12.25 B 13.25 Cr 1.75 was designed to develop nearly zero-magnetostrictive materials in the form of ribbons and wires to obtain suitable magnetic properties for use as giant magnetoimpedance (GM!) and low-noise fluxgate sensor cores. During the development of magnetic sensing core using amorphous wires of diameter 80 μm for GM! sensing, we observed a maximum impedance change (GMI ratio) of 430% with as-cast wires, which further enhanced to 650% when annealed at 300°C for 5 min. The effect of annealing temperature and stress on the magnetic properties of fluxgate sensing core materials has been studied. Magnetic hysteresis loops were measured with 3-mm-wide and 45-μm-thick amorphous ribbons on a toroidal ring of 50 mm diameter for fluxgate sensor core. A coercivity of 10.57 A/m and saturation field of ~150 A/m were observed with as-quenched ribbon, which improved to a coercivity of 2.16 A/m and saturation field ~100 A/m when annealed without stress just above its Curie temperature. An induced anisotropy transverse to excitation direction was achieved by applying temperature with stress and time (300°C/ 100 MPa/20 min) to use it for fluxgate sensor core.