Research of super permeability NIFE/SIO2/CU composite wires for micro magnetic sensors

Abstract

Extremely high permeability magnetic materials play significant role as sensing elements in the application of ultra-weak magnetic field detection sensors. In order to achieve the super permeability, a variety of magnetic materials and an extensive range of structures of sensing elements have been developed in the past decade. In this thesis, the main objective will concentrate on the study of a novel magnetic sensing element, NiFe/SiO2/Cu composite wire to further enhance the permeability of magnetic sensing elements.

Three main aspects of research have been carried out in this study: the investigation of the GMI effect in the NiFe/SiO2/Cu composite wire in relation to the insulation layer SiO2, the optimization of the insulation layer SiO2 to achieve the improved performance of NiFe/SiO2/Cu composite wire, and the study of NiFe layer for further enhancement in the permeability of NiFe/SiO2/Cu composite wire.

First of all, it was concluded that the addition of the insulation layer is capable of enhancing the GMI effect in the NiFe/SiO2/Cu composite wire by increasing the eddy current and the impedance of NiFe layer as well as improving its magnetic properties, such as the softness and anisotropy. At the same time, the existence of the insulation layer also could influence the frequency dependence of the MI curve and a superior frequency range (2 MHz and 10 MHz) was obtained in the Ni80Fe20/SiO2/Cu composite wire for micro magnetic sensor applications.

Furthermore, the investigation of optimum parameters for the insulation layer SiO2 in the NiFe/SiO2/Cu composite wire was conducted with various thicknesses of insulation layers. It was found that the thickness of the insulation layer at the magnitude of micrometers displayed the larger GMI effect compared to the wire with the insulation thickness at the magnitude of nanometers with an optimum thickness of 5 ?m. This might be due to the enhancement of the interaction between the ferromagnetic shell and the copper core by the thick insulation layer and the increase in the thickness of insulation layer could improve the circumferential permeability of the Ni80Fe20/SiO2/Cu composite wire. Moreover, it was observed that the thicker insulation layer could reduce the frequency of the maximum MI ratio in the Ni80Fe20/SiO2/Cu composite wire by enhancing the skin effect.

In addition, the investigation of the optimum thickness proportion of NiFe and SiO2 layers and the improved current density for electroplating NiFe layer were carried out. An optimum thickness proportion of the SiO2 and Ni80Fe20 layers, 1.2, was found, where the thickness of the insulation layer and the ferromagnetic layer were 5 and 6 ?m, respectively. The result can be explained by the competition between the improvement in the magnetic properties of NiFe layer and the influence of skin effect as the increase in the thickness of NiFe layer. An optimized plating current density, 4 A/dm2, was also found as a result of the competition between the enhancement of circumferential permeability by the induced circumferential magnetic field and the reduction in the permeability caused by stress induction in the NiFe layer.