Relating Current Annealing And Stress To Magnetic Domain Structure And GMI Effect In Amorphous Microwires

In this paper, three type of amorphous wires with different magnetostrictivecoefficient, which were Co-based，Fe-based and CoFe-based, were prepared by meltextraction technique. The effect of modifying in different current and differentdeformation methods, including surface electroplating, current annealing in lowtemperature medium, tensile and torsion stress, on the structure of magnetic domainof these three type were systematically studied by MFM. Meanwhile, we analyzedthe regularity of the GMI of the wires in different treatment and attempted to revealthe relationship between treatment and the GMI effect.The GMI effect of the as-prepared amorphous wires was dependent on themagnetostrictive coefficient and distribution of the structure of magnetic domain.Compare to near-zero magnetostrictive amorphous wires, width of magnetic domainof positive and negetive magnetostrictive amorphous wires was narrower. And thedomain distributes uniformly with high volume of circumferential domain, resultedin high GMI effect, and theΔZ/Z was320.18%, magnetic activated sensitivity wasup to112.59%/Oe.Observation on the structure of magnetic domain of the electroplating layersurface of amorphous wires showed that the structure changed a lot afterelectroplating. Electroplating of Ni layer made the structure change from the earlystripe into maze domain, while electroplating of Cu layer made the surface structuredisappear. Test on GMI showed that movement of maze domain resulted from the Nilayer was resisted in the external field, and the GMI was lower than as-prepared one.And the GMI effect disappeared after electroplating of Cu layer. As a result, itshould be avoided that the electroplating layer of wire edge is too long in process ofproduction of micro-magnetic sensor and the eletroplating layer being in the worksegment of the wire is also prohibited.The current annealing in low temperature showed that it led to significantimprovement of the structure of magnetic domain and the GMI. With low current,the structure of magnetic domain didn’t change evidently, neither did the GMI.However, with high current which exceeded200mA, the GMI of amorphous wireswith positive and negetive magnetostrictive coefficient changed but not toosignificant while the GMI of amorphous wires with near-zero magnetostrictivecoefficient changed evidently. And magnetic sensitivity increased from112.59%/Oeto526.09%/Oe rapidly. It can be explained by the significant increase of equivalentanisotropy after current modifying and gives rise to double-peak evolution of theGMI curve. According to the simulation of the design and production of micro-magneticsensor, it shows that when the tensile stress was loaded on the amorphous wires, themagnetic domain width of wires with positive and negetive magnetostrictivecoefficient increased firstly and then decreased, while wires with near-zeromagnetostrictiv coefficient decreased all over. When the tensile stress reached30.8MPa, streamline of circumferential magnetic domain of wires with near-zeromagnetostrictive coefficient became fine and close, and distributed uniformly. Themagnetic domain wall was clear and the GMI was improved evidently. When thetorsion stress was loaded, the streamline of the magnetic domain of the three kind ofamorphous wires tilted, and angle of deflection of magnetic domain increased. Thedecrease of the circumferential magnetic anisotropy and circumferential magneticpermeability was found, as well as the decrease of the rate of change in impedanceand sensitivity and the GMI. As a result, certain present tensile stress should beloaded on microwire in production and application, in case of torsion stressappearing.