Diffusion in annealing of nanocrystalline Ni₈₀Fe₂₀/Cu composite wires

Weak magnetic field sensors have always been an area of great interest due to their immense potential applications in a wide array of fields such as magnetic anomaly detectors (MAD) in defence-related industry or bio-magnetic field sensors in biomedical industry. The performance of such sensors greatly depends on the magnetic softness of the sensing elements. Thus, optimisation of the post-heat treatment process is crucial as it serves to release stresses induced during the deposition process. In this study, furnace annealing was carried out on Ni₈₀Fe₂₀/Cu composite wires at a range of annealing temperatures from 210°C-1,050°C. Inter-diffusion of Ni and Cu was observed at annealing temperature of 350°C, with the diffusion effect more pronounced at elevated annealing temperatures. This diffusion effect, coupled with increases in the surface roughness and average grain sizes as the annealing temperature was increased, resulted in the deterioration of magnetic properties and sensing performance of the composite wires, as indicated by the increase in coercivity, decrease in magneto-impedance (MI) effect and decrease in sensitivity.