Researchs Of Giant Magnetoimpedance Effect In Composite Wires

The giant magnetoimpedance (GMI) effect is a phenomenon that there will be a giant change of the impedance of a ferromagnetic material when it is subjected to an external magnetic field. The phenomenon has attracted a great deal of interest because of its prospective applications in magnetic recording heads and sensor elements. For uniform ferromagnetic film, ribbon or wire, the GMI effect may be explained well using skin effect. More active researches recently have been done on the GMI effect in sandwich films and composite wires, consisting of an outer ferromagnetic coating and a nonmagnetic inner core with high conductivity. Compared with the homogenous ferromagnetic materials, the GMI effect of such composite structure materials characterizes two evident aspects. One is the great enhancement of the GMI ratio, the other is that GMI effect can be observed at much lower frequency. Thus, some researchers pointed out that the skin effect is weak and would not be essential for GMI effect in the composite structure materials. However, up to now, this problem still remains not sufficiently understood although great achievements have been made on the GMI effect of composite structures in experimental.In this dissertation, the GMI effect in composite wires has been investigated both in theoretically and in experimentally, especially, the electromagnetic interactions and skin effect, which tightly related with the GMI effect in composite wires were discussed in detail. The dissertation consists of several parts as follows.1. Using the Maxwell's equations, the GMI effect model of composite wires was set up. In the Foundation of the model, the permeability in the magnetic coating with helical magnetic anisotropy of composite wires has been calculated. In the calculation, both the contributions from domain wall displacement and magnetization rotation have been considered, which makes the model be more reasonable for the real materials. Simulated results agreed well with the experimental results have been reported in many previous literatures, demonstrating the valid of this GMI model.2. Through the simulations of current density distributions together with the GMI effect in Cu/FeCoNi composite wires and FeCoNi homogeneous magnetic wires, it can be confirmed that the current distributes much closer to the surface for the composite wires than that of homogenous ferromagnetic wires at the same frequency under same geometric size and magnetic structure. A strong skin effect...