Giant Magnetoimpedance Effect Of Electroplated Magnetic Composite Wires

Giant magnetoimpedance (GMI) effect is a phenomenon which displays a giant change of the ferromagnetic material’s impedance when it is applied to an external magnetic field. Since the observation of GMI effect in amorphous wires in 1992, GMI effect has been quickly obtained in different soft magnetic materials, such as amorphous ribbons and thin films. Originally, researchers studied the single material system, such as amorphous wires and ribbons. In recent years, GMI effect materials are developed into composite structure system, for example, the sandwich film which is combined with two ferromagnetic layers on the shell and a conductive non-magnetic in the middle. For the GMI effect of wire materials, it is evolved from homogeneous wires to novel composites wires, such as the composite wires that the FeNi, CoNiFe, CoP soft magnetic films coated on the Cu wire by electroplating method or magnetron sputtering method. The composite wires have a significant enhancement of the GMI effect as compared to the crystalline alloy films, because of the interaction between the non-magnetic layer and the magnetic layer. In this work, some soft magnetic alloy films were electroplated on the Cu or NiCu wires, forming the composite wires. We studied and analysed the influence of the saturation magnetostriction constant λs, applied tensile stress σ and dc current annealing on the morphology, magnetic properties and GMI effect. Several studies have been done as follows:1. We successfully fabricate NixCo1-x/Cu composite wires with different contents by electroplating method. For the NiCo alloy film, different contents generate different value of the λs. We studied the influence of the tensile stress on the GMI effect for NixCo1-x/Cu composite wire with different contents. The results display that the stretched Ni/NiCu composite wire shows the best GMI effect, owing to its high absolute value of the λs. In order to prove the influence of the tensile stress on GMI effect, the stretched composite wires were annealed in a vacuum furnace. For the stretched sample after heat-treatment, the maximum GMI ratios return to the original GMI ratios of the as-cast samples, which are resulted from stress relaxation during the annealed process. It is certified that the improvement of the GMI effect can be realized by the tensile stress.2. The influence of different tensile stress on GMI effect for Ni/NiCu and FeNi/NiCu composite wires, which were produced by the electrodeposition method. The GMI ratios of the two composite wires present a rise tendency with increasing tensile stress values. Under the same tensile stress value, the GMI ratio of Ni/NiCu composite wire is higher than that of FeNi/NiCu composite wire, because the GMI effect is more sensitive to the value of λs.3. We prepared FeNi/Ni/NiCu composite wire by electroplating method, taking advantage of the high λs absolute value of pure Ni film and good magnetic properties of FeNi film. When the value of the tensile stress is lower than 150 MPa, the GMI ratio of FeNi/Ni/NiCu composite wire is higher than that of Ni/NiCu composite wire, which is attributed to external FeNi layer with better soft magnetic properties. When the value of the tensile stress is higher than 150 MPa, the GMI ratio of FeNi/Ni/NiCu composite wire is less than that of Ni/NiCu composite wire, which may be that the FeNi layer restrains the shape change of the Ni layer.4. The influence of the diameter of the non-magnetic conductive NiCu wire on the GMI effect was studied. We compared the Ni/NiCu composite wires with the core diameter of 200 μm and 400 μm, respectively. The results display that the composite wires with larger core exhibit a better GMI effect.5. This research is based on the fundamental theory that circular permeability plays an important role on the GMI effect, a simple techniquet applying a circular magnetic field to regulate the magnetic moments. We contect the Ni/NiCu and FeNi/NiCu composites wires to the electric circuit and a current through the composite wire. The GMI ratios of the two composite wires increase with increasing current values.