Resonance Enhancement Of Giant Magnetoimpedance Effect And Its Applications

The Giant Magneto-Impedance (GMI) effect consists of significant changes of the complex impedance value of soft magnetic materials upon the application of an external magnetic field. The phenomenon has attracted much attention over the past years because of its potential applications in magnetic recording heads and sensors, which are characterized by high sensitivity and quick response, etc. Nowadays, high sensitivity and miniaturization are the trend of development of magnetic sensors. In this dissertation, in order to enhance the MI ratio and improve the performance of MI sensors, enhancement of GMI effect utilizing LC and magnetoelastic resonance were investigated, and the enhancement physical mechanism and choices of optimum condition were discussed. Meanwhile, three types of magnetic sensors were developed: LC-resonance-type MI sensor, magnetoelastic-resonance-type MI sensor and frequency-modulation-type MI sensor. The main contents and results are as follows:1. Composite wire specimens were produced by sputtering a copper layer on the outside of Fe-based glass-coated microwires. The composite wire itself constructed a LC resonance element, in which a capcitance was formed between the ferromagnetic core and the outer conductive layer and the glass insulator worked as a dielectric layer. Experimental results demonstrated that:(1) LC resonance frequency of composite wire and enhancement effect of GMI can be modulated by changing the length or the thickness of copper layer. Enhancement amplitude of MI ratio is related to resonance frequency and line width. When the optimum frequency of GMI effect is close to the maximum slope position of the resonance curve, the MI ratio has a large enhancement. (2) For the composite wire, connecting one terminal between the inner core and the outer layer can enlarge effective capacitor and reduce resonance frequency. Therefore, it is easier to obtain resonance enhancement of GMI effect in miniature element. The experimental data were simulated using the distributedparameter method and the calculated results agreed well with the experimental data. This method will serve as guidelines to design miniature element. (3) Based on the LC resonance mode, a LC-resonance-type MI current sensor was developed using Fe-based glass-coated wire. It was found that the sensitivity and linear range were related to the driving frequency and capacitor. In our experiment, the linear range of current sensor is 0³0 A, and the linear correlation coefficient reaches up to 0.9998, whereas the radius of MI element is only 2.5 mm.2. Adopting longitudinal driven mode, giant magnetoimpedance effect in Fe-based amorphous and nanocrystalline ribbons was measured, and the enhancement effect of GMI utilizing magnetoelastic resonance was investigated. The results demonstrated that:(1) In order to achieve large enhancement effect of GMI, the magnetic material should be characteristic of high magnetoelastic coupling coefficient K33 and quality factor Q, i.e. of high magnetostrictive coefficient and permeability.(2) The Fe-based nanocrystalline ribbons have high magnetostrictive coefficient and permeability after temperature annealing, and magnetoelastic resonance occurs under a certain driving frequency. As a result, the MI ratio was enhanced drastically. A maximum MI ratio of 15462% was achieved at 133 kHz for the ribbon with the length of 1.5 cm annealed under 480 °C, which was twelve times as much as the original MI ratio. The maximum sensitivity reached up to 7300 %/Oe, which was almost twenty-nine times higher than that without magnetoelastic resonance.(3) Based on the magnetoelastic resonance mode, a MI sensor with sensitivity of 10^-7 T was developed.3. Output characteristics of frequency-modulation-type MI sensors with Fe-based ribbons annealed under different tensile-stress were investigated. The experimental results show that the magnetic moment rotation in magnetization processes is helpful to improve the linearity of sensors. A sensor with a sensitivity of 15 %/Oe and a linear range of ± 2 Oe has been developed using the Fe-based ribbonannealed at 59.8 MPa.