EFFECT OF ANNEALING ON OFF−DIAGONAL MAGNETOIMPEDANCE IN AMORPHOUS MICROWIRES

Magnetoimpedance (MI) effect in amorphous ferromagnetic microwires represents is the ideal base for sensing technology and is currently used to develop high sensitive sensors of weak  magnetic fields with a resolution up to few micro−Oersteds. The effect  of heat treatment on off−diagonal MI in glass coated ferromagnetic amorphous microwires has been studied in order to improve MI sensitivity and temperature stability. We have shown the dependence of sensor signal on temperature. The wires had Co−based composition and internal stress induced helical or circumferential anisotropy. We have demonstrated that annealing of the entire sensing element including the electric contacts and the detection coil may improve the sensitivity of the output signal  to an external magnetic field by about 25% and  decrease its temperature sensitivity almost twofold in the −30…+80 °C range. These improvements require strict control  of the annealing parameters. The best results are obtained for annealing at 160 °C for 2−3 minutes. The experimentally observed changes  are  related with stress relaxation during  annealing; in particular; relaxation of the  stresses occurring during  solidification  due  to the  difference in the  thermal expansion coefficients of the metal core  and the glass sheet.

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