Giant Magneto-impedance Of Soft Magnetic Alloy

The giant magneto-impedance( GMI) effect is a newly discoveredphysical effect in amorphous soft magnetic alloys, which is referred to thesensitive changes of impedance with applied external magnetic field.Recently, many researches on giant magneto-impedance effect in magneticmaterials attract great attentions for high sensitivity, small dimension, rapidresponse and less energy consumption for GMI sensors compared with MR,GMR and magnetic flute gate sensors. In this thesis, we choose the Co-basedand Fe-based alloys respectively which have good soft magnetic properties,and get excellent GMI effect through components optimization and heattreatment optimization, at the same time, the magneticproperties ,microstructure and the resulting changes in the correspondingGMI effect were studied .The main results are generalized as follows: Co_72-xFe_xZr₈B₂₀(x=0,2,5,7) amorphous ribbon were successfullyprepared ,which have good soft magnetic properties.The Influence of theadditions of Fe in substitution for element Co on MI effect wassystematically studied. Magnetic properties and MI effect measurementswere measured using a vibrating sample magnetometer (VSM) and animpedance analyzer, found with Fe content increase , saturationmagnetization and initial permeability decline, the soft magnetic propertiesof Co_72-xFe_xZr₈B₂₀(x=0.2.5.7.)decline, the corresponding GMI also show thesame characteristics. Perhaps it is because the increase of Fe content thatincrease the magnetostriction which are not conducive to domain wallmotion, therefore the soft magnetic properties decline. And found thatcoercive force with the increase of Fe content increase at first, then decline ,which shows that coercive force is not the determining factor, GMI is thecombined result of the factors which affects the soft properties.Based on the result from giant magneto-impedance effect inCo72-xFexZr8B20(x=0.2.5.7.)amorphous ribbon, we choose Co70Zr8B20amorphous ribbon which show the best GMI effect, The heat treatmentimpose on GMI of Co70Zr8B20 amorphous alloy have been observed . XRDand resistivity measurements shows;amorphous ribbon become crystallineafter anneal and with the increase of temperature,the crystallinizationperscent increase .VSM results indicate that with anneal temperature increase,coercive force increase , GMI decline, the shape of hysteresis loop changesfrom line at 4900C to "S" at 5400C ,which means magnetic structure changesfrom vertical to horizontal, GMI measurements of different temperatureanneal samples show with the percent of crystallization increase ,GMI hascontinuously dropped,and because magnetic structure changes from verticalto horizontal, it led to the emergence of double peaks . crystallization lead tothe decline of soft magnetic properties ,so for the Co-base amorphous alloy,crystallization is not conducive to the enhancement of GMIThe crystallization percent of Fe84.5Zr4Nb2B8.5Cu1 alloy ribbonincrease,after anneal at different temperature from 245 0C -665 0C .TheVSM measure show that the sample anneal at 575 0C have the best softmagnetic properties, so the sample show a very high MI effect during theGMI measurement at a relatively low frequecne 600kHz, the maximum valueof GMI effect reaches 775 % .this result is better than theFe84Zr3.5Nb3.5B8Cu1 nano-crystalline alloy which has been reported by He Jet al[94], The maximal magneto-impedance ratio of the Fe84Zr3.5Nb3.5B8Cu1nano-crystalline alloy is about 383% at 2.5MHz.The GMI effect of sampleshas frequency dependent characterization. As can be seen from the change ofGMI along with frequency after the sample annneal at 2450C and 575 0CThe GMI effect is different with different driving current frequency, butexisting the Optimal one.The optimal frequecny of sample after anneal at2450C is 800kHz,the other is 600kHz after anneal at 5750C ,compared withthe sample after anneal at 2450C ,the sample after anneal at 5750C has lowercoercive force ,larger saturation magnetization and initial permeability,whichshow it has the better soft magnetic properties.so we can draw aconclusion :The sample with good soft magnetic properties can show a largeGMI effect at a low frequence. We also found that alpha -Fe can significantlyimprove GMI, as to soft properties and GMI,its crystal grain size haveconsiderable impact. the relatively small crystal grain size attribute toimprove GMI.