Giant Magneto-impedance Effect Of Microwire And Its Wire-connecting And Temperature Characteristic

Giant Magneto-impedance (GMI）effect is the phenomenon of impedance ofsensitive material driving by the alternating current at relatively high frequency,which is largely changed by external magnetic field. Co-based amorphousmicrowires, as new-type sensitive material, has the advantages of quick-responseand high-resolution, it is more suitable to apply in high-performance GMI sensor. Inthis present dissertation, the basic issues of GMI effect of microwires for magneticsensor application and its wire-connecting by electroplating and temperaturecharacteristic were systematically investigated. As well as the analyses wereconducted, including influence and action mechanism of combinedcurrent-modulation annealing (CCA) on GMI effect of melt-extracted amorphousmicrowire, and effect of geometric and measuring parameters on GMI effect bothfrom numeriacal calculation and experimental points of view according to GMItheoretical model of microwire. The technical path of electro-platingwire-connection to effectively improve GMI output stability, and differential-typetwin-detector sensor (TDS) based on GMI effect and temperature characteristic ofmicrowires were also presented and validated. Finally, the engineering integratedapplication goal of functional materials and advanced structure was achieved, andobtained the following results:Characteristic and relationship of transient temperature rise of amorphousmicrowire during different-type current annealing was described by means oflumped thermal capacity of transient heat conduction. Experimental resultsindicated that the temperature values dramatically increase as the increment ofcurrent amplitude, and reach their maximum then tend to keep constant at differentcurrent annealing, especially for pulse current (PC) annealing, the temperaturestakes on periodic fluctuation variation with stable amplitudes, and the sticking timesextend with an increase of amplitudes, the inner average temperature of microwirestakes on rising tendency. As the caculated average temperature is close to the Curietemperature and crystallization temperature, the magnetic properties become muchworse than previous state. So this numerical calculation could be used as thesignificant methed to choose the reasonable technique parameters of currentannealing. At10MHz, the maximum GMI ratio [ΔZ/Z0]maxand field responsesensitivity ξmaxof CCA wire increase to368.7%and623.5%/Oe, respectively, andthe equivalent magnetic anisotropic field Hkis around2.0Oe. In comparison withtraditional current annealing, the CCA is more effective and pronounced inenhancing the GMI property of as-cast microwire. Meanwhile, the improvement of GMI effect for CCA wires was also tentatively discussed based on the actingmechanism of different-type current annealing. In addition, the magnetic-momentevolution can be illustrated by the variation of turn angle θiunder different appliedmagnetic fields, further to finely describe magnetization mechanism and peakbehavior of GMI profiles.The influence of geometric and measuring parameters on GMI effect wasanalysed by means of free energy minimum principle based on “core-shell”magnetic domain model, the numeriacal calculations are consistent withexperimental results. At f=10MHz, the GMI ratio shows an increase at first and thendecrease with increasing diameter of wire. As diameter is relatively smaller, theGMI property of microwire is unnoticeable, resulting from the inhomogenousdistribution of inner stress anisotropy and impediment of spontaneous magnetizationvector caused by magnetoelastic energy; as diameter is relatively larger, thelong-range order formation of atomic arrangement in some local regions can resultin the generation of magnetocrystalline anisotropy, even can worsen GMI effect.While the GMI property of CCA microwire was improved with an increment ofsample length (3mm~30mm),[ΔZ/Z0]maxand Hkincrease with increasing length at arelatively high frequency. The increased length can effectively reduce the influenceof the intensity of demagnetization field and closure domain at wire-terminal, and italso can be prone to achieve the inner uniform magnetization. Therefore, it can beconcluded that the diameter and length of amorphous microwires for GMI sensorshould be controlled at the ranges of25μm~35μm and18mm~24mm, respectively.The investigation of GMI output stability of microwires influenced byalternating current amplitude (ACA) indicates that its stability is significantlyenhanced after the wire is applied by increasing ACA. At a higher amplitude(Iac≥15mA), the driving force reached the critical threshold value for a completerotation magnetization process, and the whole volume of a wire is stably magnetized;while the ACA below1mA, the“spike” phenomenon is related to the inhomogenousdistribution of the locally critical field. With an increase of alternating currentfrequency, the skin effect becomes stronger, the motion of domain wall is stronglydamped by eddy currents and the magnetic moment rotation gradually dominates theprocess of magnetization until the relative higher frequency. An applied ACA of15mA and a working frequency range of10MHz~12MHz are thereforerecommended for the circuit design of a high-resolution magnetic-field sensor.Generally, microstructural morphology of electro-plated layer at wire-terminalused for wire-connection is influenced by many factors, i.e. cathode current density,duration time and temperature of electrolyte, etc. At the beginning of electroplatingwith relatively low cathode current density, it can generate the inhomogeneousgathering region and disproportionating reaction. At high cathode current density with long duration, the surface of electro-plated layer becomes coarse withcrassitude grain and multiaperture, including the tendency of hydrogen brittlenessand cracks. According to the mathing criterion of coefficient of thermal expansion(CTE), the optimized technique parameter of Ni/Cu combined layer electro-platingwas attained. The Ni/Cu layer possesses well surface wettability, which is favorablefor the final application of wire-connection and encapsulation for GMI sensor. Theaction mechanism of improved GMI output stability by micro-wire-connection ofelectro-plated layer was reasonablely interpreted based on Ohm contact theory.Study on the temperature characteristic of amorphous indicated that the GMIeffect and its output stability change obviously with increasing ambient temperatureand the range of appropriate working temperature for GMI sensor is0℃~80℃.Atrelatively low temperature, the GMI ratio exhibits relatively stable and smoothvariation, but at high ambient temperature it takes on abnormal flectional variationin partial magnifications, the shape of GMI profiles from “drum” to “fan”, whichmeans the worse output stability. It is the reason that the inner structure relaxationwas rapidly adjusted by the action of thermal oscillation with increasing temperature,and the resistivity and magnetic anisotropy increase. Therefore, the arrangement ofmicro-dipole moments becomes seriously disordered, and the vector of magneticmoment generates a non-reversible rotation, further intensity of magneticpolarization and circumferential permeability sharply decrease.The GMI magnetic sensor based on above mentioned temperature characteristicis composed of high-frequency signal generator, differential-type twin-detector,temperature compensating circuit and signal processing module (including peakdetection, low-pass filter circuit, differential amplifier, and degenerative feedback orV/I conversion circuit). Experimental results indicates that TDS sensor by thedifferential method is expected to effectively overcome the GMI fluctuation orimpedance drift induced by ambient temperature, and enhance the precison andstability of GMI sensor. Conclusively, experimental validation of differential-typeGMI sensor with TDS detector shows that it can work normally at relatively highambient temperature, which can subsequently be used for detecting weak magneticfield at relatively wider working range (±1.0Oe). Moreover, the TDS sensor alsoexhibites higher sensitivity (reach to2668.45mV/Oe), more excellent outputlinearity and better repeatability, namely it can achieve the final goal ofhigh-resolution and quick-response GMI sensor application based on GMI effect ofamorphous microwire.