Temperature dependence of the off-diagonal magnetoimpedance in sensor configuration utilizing Co-rich amorphous wires

摘要

For sensor operation, the temperature stability is of primary importance. In this work, we have investigated the temperature dependence of the magnetoimpedance (MI) in glass-coated Co-based amorphous wires which are widely used as MI sensing elements. The magnetic alloy has high Curie and crystallization temperatures, yet, the MI change at moderate temperatures of 20-90 ℃ and low-magnetic fields can be about 100%. This is explained by high-temperature sensitivity of the residual stress due to fast solidification and the difference in thermal expansion coefficients of glass and metal. To reduce the temperature effect, annealing treatment was proposed and applied to the whole sensing element in off-diagonal configuration comprising the MI wire and detection coil. Annealing treatment at a temperature of 160 ℃ during 2-3 rnin makes it possible to significantly improve both the magnetic field sensitivity and temperature stability. After annealing, the output voltage sensitivity in the linear region (±0.7 Oe) increases more than 2.5 times and shows little variations with temperature up to 90 ℃. For higher magnetic fields, the MI profile still shows noticeable change with temperature: the maximum of the output voltage decreases at a rate of about 0.15%/℃ as the temperature is increased. After optimum annealing, the MI curves become almost symmetrical suggesting that a well-defined circumferential anisotropy is established.