Temperature Effects on the Magnetoimpedance in Glass-Coated Amorphous Wires

The temperature dependence of magnetoimpedance (MI) in glass-coated amorphous microwires of two compositions Co_66.94Fe_3.83Ni_1.44B_11.51Si_14.59Mo_1.69 and Co_23.67Fe_7.14Ni_43.08B_13.85Si_12.26 having high (613 K) and low (335 K) Curie temperatures, respectively, was investigated. The wires of the first type have a small negative magnetostriction and are widely used as MI sensing elements. For sensor operation, the temperature stability is of primary importance, however, it was found that the MI change at moderate temperatures of 20°C-90°C and low magnetic fields can be nearly 200%. This is explained by high temperature sensitivity of the residual stress and stress-dependent magnetostriction, which resulted in abrupt change in the easy anisotropy direction. The wires of the second type with low Curie temperature T_c and positive magnetostriction demonstrate large and sensitive change in impedance only in the vicinity of T_c due to a corresponding drop in the magnetization saturation and magnetostriction, whilst the bistable type of the magnetization reversal remains. The frequency dispersion of the dynamical permeability shifts toward lower frequencies when approaching T_c suppressing the high-frequency properties and MI. Such temperature dependence of MI is well controllable and is proposed for applications in temperature sensors.