| Giant magneto-impedance (GMI) effect has a broad prospect for application in highly sensitive magnetic sensors and magnetic heads due to its characteristics of high sensitivity, quick response and non-contact. In addition, GMI can also be used as a research tool to investigate the magnetization process of ferromagnetic material and evaluate many magnetic parameters related to GMI. Recently, a study of GMI effect in CoSiB/SiO2/Cu/SiO2/CoSiB films showed that the GMI ratios of sandwich film can be improved by adding an insulator layer of silicon oxide between the conductive layer and ferromagnetic layer. Because the structure of composite wire is similar to that of the sandwich film, the composite wire with insulator is expected to show excellent GMI effect and obtain an obvious GMI effect at low frequency. Howerver, the study of GMI effect in composite wire with insulator is still at an early stage and more experiments and theories are needed. The GMI effect is related to not only magnetic properties of ferromagnetic material and composite structure but also measurement circuit. Therefore, it attracts much attention to study the resonance enhancement and frequency dependence of GMI after introducing capacitance into the measurement system to constitute LC resonant circuit.In this dissertation, we first deduce the expression of impedance of the composite wire with insulator in theory, and then investigate experimentally the GMI effect in detail. Secondly, the magnetization property of the specimen is analyzed in terms of equivalent circuits, complex impedance spectroscopy and the effective magnetic permeability calculated using the expression. Finally, a composite wire LC-resonator is constituted by changing the connecting method of composite wire during measuring. One terminal of impedance analyzer is connected to CuBe core and the other to ferromagnetic layer. As a result, a capacitance is formed between them and the insulator layer works as a dielectric layer. The composite wire constructs a serial LC resonant element by itself and the GMI effect in it can be enhanced by LC resonance. The influence of physical dimension and magnetic property of composite wire LC-resonator is systematically discussed on resonance frequency, GMI effect and high-frequency giant magneto-reactance effect. The main contents and results are as follows:1. The expression of impedance of the composite wire with insulator is deduced in theory. The GMI effect and the magnetization property are investigated in CuBe/Insulator/NiCoP composite wires prepared by electroless deposition. The main results are as follows:(1) The expression of impedance of the composite wire with insulator is deduced by an energy conversion model. According to the expression, it is predicted that the GMI effect in composite wire with insulator is larger than the wire without insulator and an obvious GMI effect can be observed at much lower frequency.(2) CuBe/Insulator/NiCoP composite wire was prepared by electroless deposition on an insulated CuBe core, in which the influence of conventional annealing and DC Joule annealing on the GMI effect is investigated. The experimental results show that a maximal GMI ratio of 239% is achieved for the specimen annealed by conventional annealing at 600 kHz. For the specimen annealed by DC Joule annealing with an annealing current density of 283 A/mm2, the largest GMI ratio is 1128.9% at 220 kHz, and the maximal field sensitivity is 74%/Oe. Furthermore, a more obvious GMI effect is observed in it at lower frequency. For example,ΔZ/Z is 180.3% though it is only 7.5% for the specimen annealed by conventional annealing at 10 kHz. The experimental results agree well with theoretical prediction.(3) The magnetization property of the specimen is analyzed in terms of equivalent circuits and the effective magnetic permeability calculated using the expression. The magnetic domain structure of specimen annealed by conventional annealing or DC Joule annealing is changed from circumferential and longitudinal structure to preference circumferential structure. The response of specimen to Hex is enhanced, resulting in the GMI effect increase. Because the sample annealed by DC Joule annealing with the optimal current density (J=283 A/mm2) has a preference circumferential structure,μ′andμ″change obviously with increasing Hex at low frequency, leading to an obvious GMI effect observed at much lower frequency. As the variations ofμ′andμ″' for the specimen annealed by DC Joule annealing with the optimal current density are greater than that for the specimen annealed by conventional annealing, the GMI effect can be drastically enhanced by DC Joule annealing with suitable current density.(4) The effect of driving current on GMI and magnetization property of the composite wire with insulator is investigated. Because the magnetic field induced by driving current increase with increasing driving current, the magnetization process of ferromagnetic material is influenced by magnitude of driving current. As a result, the rotation of magnetic moment is increased and the relaxation process of magnetic moment rotation is prolonged, and thus the GMI effect is influenced.2. The GMI effects enhanced by LC resonance are investigated in CuBe/Insulator/NiCoP and CuBe/Insulator/CoP composite wires prepared by electroless deposition. The results demonstrate that:(1) Because the CuBe core and CoP layer are completely separated by the polyester insulator coating, a capacitance is formed between them and a serial LC resonance occurs. The impedance is the total impedance of whole circuit, which is related to not only magnetic properties of ferromagnetic material but also the capacitance and measurement circuit. GMI ratios change suddenly at near LC resonance frequency f0, and then decrease obviously at frequencies apart from f0, showing distinct selectivity of frequency.(2) Due to LC resonance, the composite wires show excellent GMI effect near to resonance frequency. For example, GMI ratios increase from 229.0% to 1333.7% for the composite wire with a length of 11 cm at 30.5 MHz, which is annealed by DC Joule annealing with current density of 210 A/mm2, about 4.8 times. The maximal field sensitivity is 91%/Oe.(3) The phase and impedance curves were simulated using an equivalent circuit model and the calculated results agreed well with the experimental data. It means that both the design of equivalent circuit and the: choice of parameter are reasonable, close to the practical circuit during measuring. Based on the comprehension to the above model and calculation, a simpler model is proposed in order to analyze directly the influencing factor of resonance frequency and GMI effect.(4) The influences of length of the composite: wire, diameter of CuBe core and thickness of ferromagnetic layer are investigated on resonance frequency. The LC resonance frequency f0 decreases with the increasing length of the composite wire and the curve of f～1/l shows nearly a linear relation. Meanwhile, f0 first increases with increasing diameter of CuBe core and then decreases, but it decreases monotonously with increasing thickness of ferromagnetic layer.(5) The magnetic domain structure of sample is changed by DC Joule annealing, leading to the variation of magnetic permeability. As a result, the inductance coefficient and resonance frequency are changed. Because the internal stress in as-cast sample is released and the soft magnetic property is improved by suitable DC Joule annealing, the variation magnitude of magnetic permeability with external DC field is raised and the GMI effect is drastically enhanced.(6) Near to resonance frequency, a sudden change of giant magnetoreactance ratio occurs and very large high-frequency magnetoreactance effect is observed. The maximal magnetoreactance ratio for the composite wire with length of 5 cm is 1.08×107% at 58924500Hz. The practical working frequency, at which the very large high-frequency magnetoreactance effect is observed, can be controlled through modulating the LC resonance frequency.
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