Giant Magneto-impedance Effect And Optimization For Fe-based Nanocrystalline Microwires

Giant Magnetoimpedance(GMI) effect consists of significant changes of AC impedance of soft magnetic materials under the external magnetic field. Because of its small volume, high sensitivity, low power consumption and other characteristics, which makes it have great potential for the field of magnetic sensors. The glass coated wire can be used as miniaturized GMI sensor device which has small size, good soft magnetic properties and corrosion resistance. The thesis first introduces the GMI effect, its magnetic structure and soft magnetic properties. Then, the preparation process and development of glass coated wire are described in detail. Finally, the results focus on the GMI effect of Fe-based glass coated wires with different annealing treatment and dimensions, in order to find the optimal size and annealing conditions. The main contents are as follows:1.Current annealing(Current density:2.1 x 107A/m2-7.0 x 107A/m2)For current annealing with longitudinal magnetic structure of FeCuNbSiB amorphous glass coated wire, it can be found that afler the appropriate current annealing, glass coated wire grow nanocrystalline, and effectively reduce the magnetostrictive coefficient. In the same time, the current annealing produce magnetic field can change the magnetic moment rearrangement, and began to appear in the circuit of magnetic structure, effectively improve the GMI effect But when the current density is too large, it will appear hard magnetic phase in the wire, resulting in a sample of crystallization and the GMI effect weakened sharply. Therefore, the optimum annealing current density is 4.2 x 107 A/m2.2.Dimension(Length:0.5-3.0cm,total diameter:13.5-38.8μm)(1) The thesis studied the effect of lengths on GMI effect of nanocrystalline glass coated wire. It is found that the maximum impedance ratio decreases with the decrease of the length of the Fe-based glass coated wire, the corresponding characteristic frequency increases, and the length decreases with the increase of the anisotropy field. Finally, it is concluded that the glass coated wire with a length of lcm is better and more suitable for the sensitive component.(2) Then studied the effect of diameters on GMI effect of nanocrystalline glass coated wire. It was found that with the increase of the diameter ratio of the wires, the maximum impedance ratio first increases and then decreases, the corresponding characteristic frequency decreases, and the anisotropy field increases at first and then decreases. Finally, it is concluded that the diameter of the metal core is 18.2μm, the thickness of glass-coated is 1.65μm.3. Stress-current annealing(density:4.2 x 107A/m2,applied stress:10-70MPa)(1)For FeCuNbSiB nanocrystalline glass coated wire can be found that through appropriate applied stress, can give birth to the circuit anisotropy field in glass coated wire, and the more magnetic moment will transferred to the circuit to improve the circumferential permeability and improve the GMI effect. But the external stress is too large, will make the GMI effect weakened. Therefore, the best applied stress is 40MPa.(2) After the stress current annealing, the wire has high sensitivity at lower frequency, and reaches 161.0%/Oe at 3MHz, which improves about 79% comparing with the sensitivity of the best current annealing glass-coated wire is 90%/Oe.