Magnetic Properties And Theory Research Of Composite Wires

Giant magneto-impedance (GMI) effect has many advantages, such as high sensitivity, quick response, etc. It has attracted much attention for its potential application on target detection, nondestructive test, weak field measurement, and so on. A great deal of GMI research on Fe-based, Co-based, Ni-Fe homogeneous alloy have been first reported, then better GMI effect was found in sandwich films and composite wires. Further enhancement in composite wires with insulator layer has been observed. GMI materials could be produced by a variety of techniques, including magnetron sputtering and electronic/electroless plating. After that, the improvement of as-cast materials could be done by annealing. In this thesis, magnetron sputtering and chemical plating method would be used to preparing composite wires; A DC current would be simultaneous subjected on subtract layer during the preparation to induce circumferential domain structure. For the composite wires with helical magnetic anisotropy, susceptibility tensor could be obtained by solving linear Landau-Lifshitz-Gilbert (LLG) equation, then the model of composite wires based on Maxwell equations could be set up, numeric calculation was done and the GMI effect was obtained. The dissertation consists of several parts as follow:1. Through conventional chemical plating research, the composite wires with circumferential anisotropy and relatively high GMI effect was obtained when the concentration of NaH2PO2·H2OO is 0.30 ml/L, pH value is 8.4 and plating time is 1 hour. On the basic of this, the Ni80Fe20/CuBe composite wires were prepared under DC current, namely CPUDC. The circumferential domain structure would be induced by the magnetic field. The free iron would be continuously transmitted onto the surface of the plating layer because of the magneto hydrodynamic effect. Hence, the reaction rate was increase. It was found that GMI ratio reached 870% when the DC current is 150 mA.2. A series of Ni80Fe20/Cu composite wires were sputtered under different current amplitude while the Ni80Fe20 coating thickness was fixed at 520 nm. With the current imposed, the temperature of coating layer increases, the diffusion capacity of atom was enhanced, which result in uniformly dense membrane. Magnetic anisotropy governed by the spontaneous magnetization, is usually decreasing with the temperature. A maximum GMI of 85% can be reached when the current is 100 mA. 3. A series of Ni80Fe20/Cu composite wires were sputtered with different thickness of Ni80Fe20 layer while the current was fixed at 100 mA. With the increasing of coating thickness, magnetic effect on the coating will be weakened, the thermal effect of the current continues to rise and reduces the magnetic anisotropy. A maximum of 194.8% can be reached when the coating thickness is 780 nm.4. Using the model set up by Maxwell and LLG equation, the influence of several factors on the GMI of composite wires were carried on, including magnetic anisotropy, the angle of magnetic anisotropy, ferromagnetic layer thickness, and the radius of Cu core.5. A series of Ni80Fe20/SiO2/Cu composite wires were sputtered with different thickness of insulator (SiO2) layer. With the increasing of insulator layer thickness, the GMI ratio first increase then decline and reach the maximum when the insulator layer thickness is 5.25 μm. With increasing of driving current amplitude, Ni80Fe20/SiO2/Cu composite wires show great difference among GMI curves, and reach maximum GMI when the driving current amplitude is 20 mA.6. To understand the enhancement of inserting insulator layer into composite wire, a conventional electromagnetic model and a modified model considering the magnetic anisotropy have been employed.The results show that the enhancement is against by the conventional electromagnetic model, but verified by the modified model. Thus, we can conclude that the thickness of insulator layer affect the growth of ferromagnetism layer and the domain structure.7. Solve LLG equation by numeric method and study the effect of AC driving field on magnetic moment precession. It can be seen that higher harmonic appear when higher amplitude Ac driving field was applied. It indicates that conventional electromagnetic model is suitable only for small driving current condition. Some physics problem, such as different driving current or different thickness of insulator layer, the effect of driving field should be considered.