Magnetization And Giant Magneto Impedance Effect Study Of Melt Extracted Co-Rich Amorphous Wires

The sensitive change of impedance with direct current magnetic field is called giant magneto impedance (GMI) effect, which has great potential applications in magnetic sensors and magnetic storages for its high sensitivity and fast response. The amorphous wires are excellent soft magnetic materials to date and can be used for MI sensor design. We measured the soft magnetic properties and magnetization of Co68Fe4.5Si15B12.5 melt extracted wires and investigated magnetization and GMI effect under different driving current, its geometry effect on GMI effect, and the influence of magnetic interaction, anneal treatments and tensile stress on GMI effect.The influence of AC driving on the magnetization and GMI effect of these Co-based amorphous wires were investigated using transverse Kerr effect. The eddy current and skin effect of AC frequencies were the main functions to influence the magnetization and GMI effect. Domain wall motion was the key magnetization process at lower frequencies and magnetic moment dominated the magnetization process with increasing frequency, gradually. Then circumferential magnetization of the circular domains in the out shell was improved by skin effect at higher frequencies which gave rise to a higher circumferential permeability and hence stronger GMI effect. What's more, the eddy current damping also resulted in an increase of coercivity and the effective anisotropy field Hm. The circumferential magnetization induced by DC magnetic field was reduced when AC current was increased leading to a decrease of GMI effect at lower frequencies. GMI effect could be optimized by selecting an adequate value of driving current Ip at frequency ranging from 0.2MHz to 1MHz and Ip increased with increasing frequency. When frequency increased, the influence of AC current on magnetization and GMI effect became weak for the strong skin effect. When f > 15MHz, the influence of AC current on circular magnetization in DC magnetic field and GMI effect turned out insignificant and skin effect became the key role to dominate the magnetization.The influence of wire length on GMI effect showed that the increase of demagnetizing field as sample length decreasing reduced the effective field and GMI effect at lower frequencies. For example,ΔZ/Z dropped to 1.03% from 8.07 when sample length decreased from 20mm to 4mm at 1MHz. When frequency was over 2MHz, the decrease of sample length resulted in both an increase of border effect to damp the magnetic moment which was adverse to higher circumferential permeability and a decrease of resistance which was favorable to GMI effect on the other. This two-edge function induced an optimal sample length of 7mm to get better GMI effect of this Co-based amorphous wire with a diameter of 34μm and the impedance ratioΔZ/Z reached 86.1% at 8MHz.The microstructure and soft magnetic properties of wires with different diameters were studied using (HRTEM), their magnetic properties and GMI effect were also analyzed. The decrease of solidification cooling rate as diameter increasing generated the growth of short ranged orders into long ranged orders or crystallization phases which increased the local magnetic anisotropy and decreased the soft magnetic properties. Consequently, GMI effect was not good in wires with larger diameters. And GMI effect was not good either when the diameter was too smaller attributed to the inhomogeneity of the initial stress for larger cooling rate. Wires with a diameter of 35μm or so possessed better GMI effect with amorphous matrix and homogeneous initial stress.There was magnetic interaction between two paralleled Co-based amorphous wires when their distance was less than 8mm. Two Barkhausen jumps in the longitudinal Kerr effect loops and two peaks in GMI profiles were observed for the asynchronous magnetization in wires. This interaction had shielding effect on the effective magnetic field applied on each wire and also induced a closure magnetization in multiple wires. As a result, the circumferential permeability and GMI effect were both enhanced when the distance was not over 8mm at which the two-wire system had best GMI effect. For example, impedance ratiosΔZ/Z of single wire and the two wires system were 74.7% and 172.4% at 10MHz, respectively. When the distance further increased to 12mm, the synchronous magnetization decreased the GMI effect. Adding wires increased this interaction and hence GMI effect at lower frequencies whilst it also enhanced the shielding effect at higher frequencies. It was found that two wires were better to get strong GMI effect when f>2MHz. And the field sensitivity was increased effectively when there were three parallel wires for symmetrical interactions applied on the middle wire inducing weak multi-peak GMI profiles. The largest field sensitivity was as high as 0.81%/(A/m) at 0.2MHz.We also found the magnetic field and stress field were beneficial to the improvement of circular domains and GMI effect. The GMI effect and field sensitivity of these wires were observed significantly improved upon application of an external small tensile stress (<100MPa). The circular domains and anisotropy were increased under a circular magnetic field induced by applying tensile stresses and as a consequence, GMI effect was increased. Better lowe frequency GMI effect was obtained when a tensile stress of 84.5 MPa applied and the field sensitivity was up to 4.2%/(A/m) at 0.6MHz. When frequency was over 1MHz, a stress of 18.4MPa was sufficient to yield a better GMI effect because the increase of effective anisotropy field Hm was disadvantageous to high field sensitivity in spite of the increase ofΔZ/Z and the sensitivity was up to 6.7%/(A/m) at 15MHz. Further more, the impedance of these Co-based melt extracted amorphous wires was sensitive to tensile stresses, showing giant stress impedance (GSI) effect and GSI was -48.2% at 10MHz, indicating that these wires were suitable for both stress and magnetic field sensors design.