Magnetic Domain Modulation And Its Correlation For Giant Magneto-Impedance Effect Of Melt-extracted Amorphous Microwires

In this thesis, magnetic domain structure of amorphous microwire has been modified by using the liquid medium joule annealing, the stress annealing, dc joule heat treatment or combined with electrolytic polishing treatment, stepped annealing,and electroplating. The correlation between the magnetic domain structure modulation and giant magneto-impedance(GMI) effect has been systematically studied.Tensile stress can significantly improve the microwire circumferential domain structures and increase the circumferential anisotropy field of amorphous microwire.For the sample treated with 203.7MPa tensile stress, the [(35)Z/Z]max% ratio is 223.6%,22.7% higher than the ratio of the as-cast sample. The torque induced by the torsional stress can result in the helical anisotropy. The circumferential component of helical anisotropy is beneficial for the circumferential anisotropy, improving the domain structure, enhancing the circumferential permeability, which facilitates the improvement of GMI effect. The [(35) Z/Z]max% can reach up to 194.4% when the torsional strain ? is 20.4(2?rad/m) and the frequency f is 15 MHz. Magnetic domain with an average size of 1.1?m has changed to zigzag shape.Electroplating can modify the magnetic anisotropy field of the amorphous microwire surface and then exert a crucial role on the GMI performance. The(35)Z/Zmax(%) can reach up to 251.1% after Ni electroplating of spacing ring with three pieces(coating width: 2mm), 40.4% larger than the as-prepared state; After electroplating, the domain wall of circumferential domain average domain size of0.73?m in the un-electroplated area of amorphous microwire become clearer. The thickness of Ni electroplating is about 3μm, and the maze-shaped domains exhibit circumferential distribution. For the case of helical Ni electroplating, when the screw spacing ranges 50~200μm, the(35)Z/Zmax(%) of amorphous microwire is larger than the as-prepared state. The domain of Ni layer shows the maze-like shape with unclear domain wall. The surface of amorphous microwire exhibits clear circumferential domain with an average domain size of 0.83?m.After stepped joule annealing(SJA), the(35) Z/Zmax(%) of the amorphous microwire can reach up to 469.6% at as-prepared state. At f=7.4MHz, the(35)Z/Zmax(%)is 654.1% for H>0Oe, and 650.2% for H<0Oe respectively) after 80 m A SJA. The(35)Z/Zmax(%) is 631.9% for H>0Oe and 624.6% for H<0Oe after 100 m A SJA with large response range of-1.5~0Oe and 0~1.5Oe respectively, which corresponds to a response sensitivity(?) of 401.0%/Oe and 397.5%/Oe. Surface domain structure ofamorphous microwire can be dramatically improved after annealing. After 100 m A stepped current annealing, the amorphous microwires show more circumferential domains and some intersected domains, with an average domain width of 0.98μm.The GMI performance usually results from the skin effect. A structure with the crystalline phase in the "core" and amorphous phase in the shell can be obtained after 300 m A annealing in liquid nitrogen. Such unique structure is due to the low temperature of the liquid nitrogen which can suppress the crystallizaition of the amorphous shell with ~100nm in thickness. At f=8.1MHz, GMI ratio can reach425%, with a response field of 2.5~6.5Oe and a response sensitivity of 99.4%/Oe;After 200 m A current amplitude annealing with f=4~12MHz, GMI curve presents a monotone increasing response at the range of 0~6Oe, and a linear response at the range of 10~80Oe; The linear response characteristics can be used to detect biological leakage magnetic field of magnetic sensor and physical therapy products,which confirm that the GMI effect of the amorphous microwire depends on the surface performance. After 300 m A current amplitude annealing, domain obtained by magnetic force microscopy(MFM) show a composite structure of weak circumferential domain and maze-shaped domain. The circumferential domain has the higher ordered degree and an average width of 0.76?m.The structure and formation mechanism of magnetic domain of the melt-extracted(Co-Fe)-based amorphous microwire has been studied. The ideal magnetic domain structure of melt-extracted(Co-Fe)-based amorphous microwire with high GMI and response sensitivity should possess three features:(a) single circumferential and regularly distributed domain without stray domain or "burr",(b)smooth circumferential domain with uniform width of ~1 ? m, and(c) clear 180°Bloch domain wall 180° with full circumference and ~10nm in thickness and without without domain wall pinning phenomenon.