Effect of annealing on the magnetic properties and performance of cold-drawn micro Ni₈₀Fe₂₀/Cu wires in magnetic sensors

To enhance the performance of weak magnetic field sensors such as magneto-impedance (MI) effect sensors or orthogonal fluxgate sensors, the magnetic permeability of the ferromagnetic sensing elements has to be increased. In this study, the effects of annealing on the magnetic properties and sensing performance of cold-drawn Ni₈₀Fe₂₀/Cu wire sensing element have been investigated. The Ni₈₀Fe₂₀/Cu wire specimens were cold-drawn to a diameter of 34 μm and were annealed at different temperatures. The effects of final annealing temperature on the anisotropy, coercivity and MI effect of the composite wires were investigated. The results showed that the permeability increased with increasing annealing temperature until a critical value, and then decreased drastically with further increase in the annealing temperature. The decline in permeability and MI effect with further increasing annealing temperature were attributed to grain growth in the NiFe and diffusion between the NiFe and Cu. Sensitivity of the sensors with the annealed cold-drawn Ni₈₀Fe₂₀/Cu wires showed the same trend as that of permeability.