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The effect of the magnetic field on the magnetic properties of NiFe/Cu composite wires electroplated under a longitudinal magnetic controlling field is presented. Composite wire samples of 20-μm-diameter Cu electroplated with a layer of Permalloy™ (Ni80Fe20) under the influence of a longitudinal magnetic field of intensities ranging from 0 to 400 Oe were produced, and the microstructure and magnetic properties were measured. The results showed that the longitudinal magnetic field in the composite wire plating makes the packing of the crystals in the plated layer more orderly, and thus increases the uniformity and magnetic softness of the plated material. It also shifts the magnetic anisotropy of the plated layer from circumferential to longitudinal, and increases the critical frequency of the plated composite wire in magnetoimpedance effect testing, at which the magnetoimpedance ratio reaches the maximum.

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.

In this study, for developing microsensors for weak magnetic field, methods for developing high permeability nanocrystalline permalloy by electrodeposition and the relationship between the grain size and magnetic properties of the nanocrystalline permalloy are investigated. By dc plating with and without saccharin added and pulse plating with saccharin added, permalloy samples of grain sizes from 52 nm to 11 nm are obtained. The coercivity and magnetoimpedance (MI) ratio of the samples are tested against the grain size variation. Results show that the coercivity decreases rapidly and MI ratio increases greatly with grain size decrease from 52 nm to 11 nm.