Abstract
The influence of dc Joule annealing of electroplated Ni80Fe20/Cu composite wires on their giant magnetoimpedance (GMI) effect is presented in this article. Successive thermal treatments by dc Joule annealing with the current density ranging from 2.4 to 9.6×108 A/m2 were applied to the composite wires for a fixed annealing time of 1 min to release the residual stresses in the plated NiFe layer. The results showed that the magnetoimpedance (MI) effect of the composite wires can be greatly enhanced by dc Joule annealing. For electroplated 1.5 μm of the Ni80Fe20 layer on a copper wire of 20 μm diameter, the optimum annealing current density was found to be 7.2×108 A/m2 and from such annealing the MI ratio was increased from 637% to 1110%, a 74% increment. The optimum ac driving frequency for the GMI effect of the annealed wires under the influence of the annealing conditions in relation to the skin effect and permeability variation in the wires was investigated and discussed. The optimum procedure of dc Joule annealing and the cooling rate against the MI ratio of the annealed wire were also investigated. The results showed that stepped annealing is better than single step annealing and the lower the cooling rate the higher the MI ratio.
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