Giant magnetoimpedance effect in Ag/NiFe plated wire

In the present work, the giant magnetoimpedance (GMI) effect of Ag/NiFe plated wire was investigated at room temperature. The value of the magnetoimpedance (Z(H)−Z(0))/Z(0) under H=90 Oe can reach about 40% at 50 kHz. With the increase of the field the magnetoimpedance (Z(H)−Z(0))/Z(0) reaches a peak. The peak-field HP increases firstly with frequency f at low frequencies, and finally approaches its saturation value at frequencies f≥1 MHz. The maximum positive-magnetoimpedance (Z(HP)−Z(0))/Z(0) also shows a peak, which is mainly due to the reactance change. The negative- magnetoreactance decreases with increasing frequency, resulting from the reduction of permeability. In contrast, with an increase of frequency, the negative-magnetoresistance undergoes a maximum, and then drops again. The increase of magnetoresistance can be attributed to the enhancement of skin effect with frequency. The occurrence of a maximum magnetoimpedance with increasing frequency is due to the competition between the enhancement of skin effect and the reduction of permeability. There exist two intersecting points of the frequency dependence of magnetoimpedance, magnetoresistance and magnetoreactance. Both intersecting frequencies increase with applied field. The giant magnetoimpedance effect in plated wires can be very large at a relative low frequency such as 50 kHz. This is correlated with the existence of the highly conductive Ag inner-core, which increases not only the effective conductivity but also the effective distance of the magnetic outer-layer NiFe from the axis of wire, reducing the effective critical frequency of skin effect.