Temperature effects on magnetization processes and magnetoimpedance in low magnetostrictive amorphous microwires

This paper discusses two different mechanisms of temperature effects on magnetoimpedance (MI), namely, the internal stress relaxation upon heating and critical behavior of magnetic parameters near the Curie temperature Tc. When a microwire is heated, the residual stress is partially released changing the effective anisotropy, which inevitably leads to the temperature dependence of hysteresis and MI. A very large change in impedance up to 200% with moderate temperature increase (50-70 °C) occurs in microwires of Co66.94Fe3.83Ni1.44 B11.51Si14.59Mo1.69 compositions with high Tc≈340°C where the magnetostriction changes a sign from negative to positive due to stress relaxation. Proper annealing is demonstrated to stabilize such temperature variations in MI. Near the Curie temperature, the impedance undergoes large variations due to a corresponding drop in the magnetization saturation and magnetic anisotropy. The theoretical modelling reveals an increase in the dc initial permeability when approaching Tc, which results in different temperature behaviours of MI at low and high frequencies. This was confirmed experimentally for Co23.67Fe7.14Ni43.08B13.85Si12.26 amorphous microwires (Tc = 62 °C): at a frequency of 10 MHz the impedance increases with temperature until very close proximity of Tc, whilst at a frequency of 300 MHz it continuously decreases with increasing temperature. Controlling the value of Tc the impedance temperature dependence can be realized in the industrial temperature range of 20-100 °C. This has potential applications for developing embedded miniature temperature sensors.