Giant Magnetoimpedance Profile Research Articles

Impact of uniaxial stress on soft-magnetic and magneto-impedance properties of vitrified magnetostrictive microwires

Abstract The investigation addresses the impact of stress on soft magnetic and giant magneto-impedance (GMI) of rapidly quenched Fe77.5Si7.5B15, (Co94Fe6)72.5Si12.5B15 and (Co94Fe6)72.5Si12.5B12.5Nb0.5Cr2 microwires. Differential scanning calorimetry revealed interesting phase stability in the later alloys. The vitrified microwires contained dominant Co and a lean Fe content showed much superior soft magnetic properties and high magnetoimpedance. The alloy microwires incorporated with Nb and Cr manifested lowest coercivity value of 0.034 Oe with a high GMImax value of 425% in the as-quenched state. The uniaxial stress applied on the microwires modified the shape of the hysteresis loops and GMI plots. The hysteresis loops showed stress dependence of coercivity based on alloy chemistry, phase stability and consequent magnetostriction. The GMImax revealed sensitive change with respect to the applied stress in all the alloy microwires. In addition to GMImax interesting features were observed in the GMI profile pertaining influence of stress on the anisotropy field. The anisotropy field shifted systematically in one of the alloy microwires which displayed symmetric dual GMI peaks. The stress was also found to modify the asymmetric characteristics in a microwire.

Influence of Aspect Ratio on Giant Magnetoimpedance Effect for Fe67Co18Si11B4 Amorphous Ribbons

The effect of sample geometry aspect ratio (l/w) on the giant magnetoimpedance (GMI) in Fe67Co18Si11B4 amorphous ribbons was investigated systematically. The GMI profiles were measured as a function of the external magnetic field at different frequencies up to 110 MHz. The results show that there exists a critical aspect ratio ((l/w)0 = 5.4) below which the maximum GMI effect and sensitivity η decrease with decreasing l/w and above which the maximum GMI effect keeps almost constant and η decreases with increasing l/w. The observed dependence on aspect ratio as (l/w) < (l/w)0 is correlated with the magnetization process: Complex domain structures emerged near the ribbon ends to decrease the magnetostatic energy, modify the transverse permeability and consequently GMI response. Contributions from transverse permeability and resistance may dominantly determine the change of GMI effect as (l/w) > (l/w)0.

Cryogenic Joule annealing induced large magnetic field response of Co-based microwires for giant magneto-impedance sensor applications

We have presented herein the results of microstructure, surface magnetic domains (SMDs), and giant magneto-impedance (GMI) effect of melt-extracted Co68.15Fe4.35Si12.25B11.25Nb2Cu2 amorphous wires for the first time employed by using a cryogenic Joule annealing (CJA) technique with large DC current amplitude. Compared with the conventional JA method, experimental results indicate that the maximum GMI ratio [ΔZ/Z0]max achieves up to 425% at 8.1 MHz with monotonic increase of the axial magnetic field Hex up to 6.5 Oe for 300 mA (equal to around 1.06 × 106 A/dm−2) CJA-ed wire, which is about 75% larger than the [ΔZ/Z0]max for the 100 mA (nearly 3.53 × 105 A/dm−2) JA-ed microwires. The remarkable features of large and linearly sensitive response field (2.5 ∼ 6.5 Oe) and the sensitivity of 99.4%/Oe with higher GMI ratio simultaneously make the CJA tailored melt-extracted microwires promising candidate materials for miniaturized GMI sensors. Another interesting result of GMI profiles of 200 mA (appropriately equal to 7.07 × 105 A/dm−2) CJA-ed wire show a linear response to Hex (ranging from 10 to 80 Oe or more), this behavior of GMI curves can be explored to fabricate bi-sensor. Large response field proves to originate from the intensive coupling between the radial stress field and the circumferential magnetic field during CJA process. The effect of outer-shell microstructure and complex SMD for 300 mA CJA-ed microwire is attributed to the fact that liquid nitrogen hinders the evolution of circumferential domain structure to some extent and protects the amorphous structure in the shell region.

Investigation on the relation between the length of magnetic core and the giant magnetoimpedance effect of solenoid with FeCo-based magnetic core

The relation between the length of the magnetic core (l) and the giant magnetoimpedance(GMI) effect of solenoid with magnetic core of Fe36 Co36 Nb4Si4.8 B19.2 ribbon which annealed with current 32 A/mm2 for 10 min, has been investigated. The result shows that there is a sensitive relation between l and GMI. There is a linear relation between the l and the maximum of the GMI (Z/Z)max when l is smaller than the length of the solenoid l0. The linear correlation coefficient can be deduced from the electromagnetism theory. It should deviate from the linear relation when l is greater than l0. There is a maximum value of the (Z/Z)max when the l reaches the best value. There is a similar relation between the height of the tip like part of the GMI profile (Z/Z)T and l. There is a maximum value of the (Z/Z)T when the l is the best value. The tip like part of the GMI profile, which responds to the weak magnetic field sensitively, is influenced by the demagnetizing field. There is a negative exponential relation between the demagnetizing field and the length of the magnetic core l.

Giant magneto-impedance and stress-impedance effects of microwire composites for sensing applications

Composites consisting of glass-coated amorphous microwire Co 68.59Fe 4.84Si 12.41B 14.16 and 913 E-glass prepregs were designed and fabricated. The influences of tensile stress, annealing and number of composite layers on the giant magneto-impedance (GMI) and giant stress-impedance (GSI) effects in these composites were investigated systematically. It was found that the application of tensile stress along the microwire axis or an increase in the number of composite layers reduced the GMI effect and increased the circular anisotropy field, while the annealing treatment had a reverse effect. The value of matrix–wire interfacial stress calculated via the GMI profiles coincided with the value of the applied effective tensile stress to yield similar GMI profiles. Enhancement of the GSI effect was achieved in the composites relative to their single microwire inclusion. These findings are important for the development of functional microwire-based composites for magnetic- and stress-sensing applications. They also open up a new route for probing the interfacial stress in fibre-reinforced polymer (FRP) composites.

Wire-length effect on GMI in Co 70.3Fe 3.7B 10Si 13Cr 3 amorphous glass-coated microwires

The sample length dependence of giant magnetoimpedance (GMI) in Co 70.3Fe 3.7B 10Si 13Cr 3 amorphous glass-coated microwires has been studied experimentally and theoretically in the frequency range of 1–10 MHz. It has been shown that there exists a critical microwire length, below which the GMI effect is favored in shorter microwires and above which longer microwires possess a more sensitive dependence of the real component of the impedance to the frequency variation. It is demonstrated that once the resistance of the microwires of varied length is determined, it is possible to evaluate the change in GMI profiles with respect to the change of wire length. The simplified skin-effect model has been found to well explain the observed behaviors.

Large Magnetoimpedance Effects in Nanostructured Fe-Rich Composite Materials for GMI Sensor Applications

In this letter, a systematic study of the giant magnetoimpedance (GMI) effect in nanostructured Ferich composite materials reported. The nanostructure of Fe-rich composite materials consisting of ultra-fine nanoscale Fe(Si) grains embedded in a residual amorphous matrix was attained by annealing their amorphous precursors at a proper temperature (550 °C). The impedance measurements were conducted in the frequency range of 1-10 MHz for a dc magnetic field varying within ±300 Oe. We demonstrated that the nanocrystallization of the alloys led to an improved magnetic softness of the material and, hence, to the GMI effect. It was obvious that, at low frequencies (f ≤ 4 MHz), the maximum value of GMI was observed at near zero field (H dc 0) and the GMI profiles exhibited a typical single-peak feature. At high frequencies (f ≥ 5 MHz), however, a two-peak feature was observed. More interestingly, the GMI ratio and its field sensitivity reached the highest values of 170% and 64%/Oe at a frequency of 2 MHz, respectively. This result is very beneficial for developing highly sensitive GMI sensor applications. The observed GMI features in nanostructured Fe-rich composite materials can be interpreted by adapting the skin-effect model in conjunction with the magnetic-field dependence of the longitudinal permeability. Author

Intermittent deposition and interface formation on the microstructure and magnetic properties of NiFe/Cu composite wires

In this work, magnetron sputtering in the form of continuous deposition or intermittent deposition modes was used to obtain NiFe/Cu composite wires. Based on the results, intermittent deposition mode led to the formation of an interface between deposited layers. A better crystallite and a little grain growth were found for the intermittently deposited wire. Good soft magnetic properties and large giant magneto-impedance (GMI) effect were exhibited by both composite wires. The GMI profile for the intermittently deposited composite wire was characterized by two peaks, which could be attributed to the difference in the magnetic properties of the inner and the outer magnetic layers. Annealing was introduced to the intermittently deposited wire so that the inherent stresses were partially relaxed. As a result, its GMI effect was enhanced and the magnetic properties of the two magnetic layers became similar. Hysteresis loops of the composite wires displayed different magnetic behaviors and hence further affirmed the GMI results.

Anomalous magnetoimpedance in Co–Fe–Al–O thin films

AbstractThe high‐frequency giant magnetoimpedance (GMI) effect in a Co47.5Fe21.9Al17.2O15.2 thin film was systematically investigated. In the frequency range of 100–500 MHz, GMI profiles showed a double‐peak characteristic, reflecting an evident existence of the transverse magnetic anisotropy in the film, which coincided with the magnetization data. Interestingly, there was an anomalous transformation in the GMI profile from “positive” to “negative” and this was associated with the complex phase transformation of permeability. The GMI ratio decreased with decreasing film‐thickness, due to an increase of the electrical resistivity and a decrease of the magnetic softness. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of Annealing Temperature on the Permeability and Magneto-Impedance Behaviors of Fe68.5Mn5Si13.5B9Nb3Cu1Amorphous Alloy

The effect of annealing temperature on the permeability and giant magneto-impedance (GMI) behaviors of <TEX>$Fe_{68.5}Mn_{5}Si_{13.5}B_9Nb_3Cu_1$</TEX> amorphous alloy has been systematically investigated. The nanocrystalline <TEX>$Fe_{68.5}Mn_{5}Si_{13.5}B_9Nb_3Cu_1$</TEX> alloys consisting of ultra-fine <TEX>$(Fe,Mn)_3Si$</TEX> grains embedded in an amorphous matrix were obtained by annealing their precursor alloy at the temperature range from <TEX>$500^{\circ}C\;to\;600^{\circ}C$</TEX> for 1 hour in vacuum. The permeability and GMI profiles were measured as a function of external magnetic field. It was found that the increase of both the permeability and the GMI effect with increasing annealing temperature up to <TEX>$535^{\circ}C$</TEX> was observed and ascribed to the ultrasoft magnetic properties in the sample, whereas an opposite tendency was found when annealed at <TEX>$600^{\circ}C$</TEX> which is due to the microstructural changes caused by high-temperature annealing. The study of temperature dependence on the permeability and GMI effect showed some insights into the nature of the magnetic exchange coupling between nanocrystallized grains through the amorphous boundaries in nanocrystalline magnetic materials.

Effect of annealing on the microstructure and magnetic properties of Fe-based nanocomposite materials

The influence of annealing on microstructure, magnetic properties including the giant magnetoimpedance (GMI) effect of a Fe-based nanocomposite has been investigated. The nanocomposite structure composed of ultra-fine Fe(Si) grains embedded in an amorphous matrix was attained by annealing the Fe-based amorphous alloy prepared by rapid quenching method. The GMI profiles were measured for samples annealed at different temperatures ranging from 350 to 650 °C in vacuum and for 30 min. It is found that the mean grain size of the α-Fe(Si) crystallites in the order of 12 nm remains almost unchanged until the annealing temperature reached 540 °C. A decrease of anisotropy field and an increase of GMI with increasing annealing temperature up to 540 °C were observed and ascribed to the increase of the magnetic permeability and the decrease of the coercivity, whereas the opposite tendency was found for the sample annealed above 600 °C which is likely due to the microstructural change caused by high-temperature annealing. This indicates that variation in the magnetic characteristic of the amorphous phase upon annealing changed the intergrain exchange coupling. This altered both the magnetic softness and the effective anisotropy and consequently modified the GMI features. The study of the temperature dependence of the GMI effect provides further understanding of the magnetic exchange between these crystallized grains through the amorphous boundaries in Fe-based nanocrystalline materials.

Impedance property of thin film GMI sensor with controlled inclined angle of stripe magnetic domain

The giant magnetoimpedance (GMI) effect of controlled-inclination magnetic stripe domain for a thin-film GMI sensor is investigated. The inclined stripe domain structure has an inclined angle of domain walls against the transverse direction of the rectangular-strip element. It is obtained that GMI profile changes from a profile with a dip point at the zero magnetic field to a profile with one maximum impedance only at the zero field at an angle of 70°. In the case of GMI profile with a dip point, increasing the inclination angle decreases the bias field. This reduction of the bias field more than triples the sensor gain. An excellent linear property extending for wide range is obtained. A steplike impedance change property is also realized by this method.

A model for asymmetric giant magnetoimpedance in field-annealed amorphous ribbons

A phenomenological model for the asymmetric giant magnetoimpedance (GMI) in field-annealed amorphous ribbons is developed. The effect of a surface crystalline layer on the GMI response is described in terms of an effective bias field appearing due to a coupling between the crystalline layer and amorphous phase. It is shown that the presence of the bias field changes drastically the GMI profile. At low frequencies, the domain-walls motion leads to a steplike change in the GMI response. At high frequencies, the domain-walls motion is damped, and the GMI profile exhibits asymmetric two-peak behavior. The calculated dependences are shown to be in a qualitative agreement with results of experimental studies of the asymmetric GMI in field-annealed Co-based amorphous ribbons.

The GMI profiles of surface-removed amorphous ribbon

Results are reported on the giant magnetoimpedance (GMI) in soft-magnetic Co-based amorphous ribbons in view of their high potential for magnetic sensor applications. The thickness dependence of the GMI profile has been examined in surface-removed amorphous ribbons. The GMI valve characteristic decreases with increasing frequency, and asymmetric GMI profiles in the as-received state change into symmetrical ones with increasing etching time. The maximum of the GMI ratio, Δ Z/ Z, appears at a frequency of 1 MHz and decreases with increasing frequency. The frequency shift of Δ Z/ Z reveals the dominant process controlling MI effect in the amorphous region.

Helical shell-domain structure in laser-annealed Co-based amorphous microwires analyzed by giant magnetoimpedance

A commercial glass-covered, Co-based amorphous microwire (Co 67Fe 3.8Ni 1.4B 11.5Si 14.6Mo 1.7) is etched in order to remove its glass cover in a 60.51% hydrofluoric acid solution, and annealed in air by illuminating with a pulsed Nd:YAG laser beams with an energy E of 48 mJ/pulse. Giant magnetoimpedance (GMI) profiles at a frequency f are measured as function of the angle θ of the external field, H, with respect to the wire axis. The sign of the H values at the peak of the GMI profiles for f=100 kHz and at the dip of the GMI profiles for f=10 MHz, H p and H d, respectively, change at θ=85–90°, reflecting that the tilt angle of the helical domains is between 0° and 5° from the circular direction for the as-etched samples. The variations of H p and H d with θ for the sample with E=48 mJ/pulse and H a=20 Oe show a change in sign of H p and H d at θ=90–100°, reflecting that the tilt angle of the helical domains is decreased by about −10° compared to that of the as-etched sample.

The role of exchange coupling on the giant magnetoimpedance of annealed amorphous materials

The characteristics of giant magnetoimpedance (GMI) profiles were measured in annealed amorphous Co 66Fe 4B 15Si 15 ribbons in the open air as functions of annealing time, temperature and field. The GMI profile measured at 0.1 MHz exhibits a drastic step-like change, the so-called “GMI-valve” in samples produced with the optimum annealing parameters: time t a⩾8 h, temperature T a≈380°C and in the weak-field range, 0.5 Oe⩽H a ⩽3 Oe . The GMI-valve is related to an exchange coupling of a bias-field with the magnetization of the soft amorphous phase, where the bias-field is caused by HCP-Co, FCC-Co and/or Co 2Si crystalline phases in a B and Si depleted layer.

Hysteretic characteristics of giant magnetoimpedance due to the exchange coupling in annealed amorphous materials

Experiments of the giant magnetoimpedance (GMI) profile have been performed in annealed amorphous Co66Fe4B15Si15 ribbons in open air to characterize the role of the bias field on the GMI. The GMI ratio profile measured at 0.1 MHz exhibits a drastic step-like change, the so-called GMI valve, in an 8 h annealed sample at 380 °C. The GMI valve is related to exchange coupling of the bias field with magnetization of the soft amorphous phase, where the bias field is caused by hcp-Co and fcc-Co crystalline phases on the surface B and Si depleted layer. The bias field is stable for an external field less than 100 Oe, but its direction is entirely changed according to the external field over 400 Oe. The GMI profile begins to reveal hysteresis for increasing and decreasing cyclic fields when the maximum field strength exceeds 200 Oe. The peak for the increasing field is positioned in the negative field region, but in the positive field region for decreasing field. This behavior is opposite to general magnetic hysteretic characteristics, suggesting that there is an antiferromagnetic coupling of the bias field with magnetization of the inside amorphous phase.

Role of surface crystalline phases in magnetoimpedance in Co-based amorphous ribbons

The giant magnetoimpedance (GMI) profile at 0.1 MHz shows one-peak behavior in vacuum-annealed samples at T=380°C, irrespective of annealing time t a from 20 to 480 min. For the samples annealed in open air, the GMI profile shows two peaks for t a=20 min annealed sample. However, one of the peaks disappears and the asymmetric GMI profile exhibits a drastic step-like change near zero field for t a⩾300 min. Such an asymmetric GMI characteristic is probably due to the magnetic coupling of amorphous phase with a relatively hard magnetic phase on surface microstructures of FCC-Co, HCP-Co and Co 2Si crystalline phases.

Bias-current effect on giant magnetoimpedance in Co-based amorphous microwire

The giant magnetoimpedance (GMI) effect has been measured in Co 83.2B 3.3Si 5.9Mn 7.6 amorphous microwire as a function of direct current (dc)-biased current and measuring frequency, f. The GMI profile has a single peak for f⩽5 MHz, and two peaks at higher frequency. The maximum GMI ratio increases continuously with frequency up to 45% at 10 MHz. The influence of superimposed dc biasing is shown to result in asymmetrical GMI as well as in a shift of the axial hysteresis loop. Both effects are interpreted in terms of the relative contribution of domain wall and rotational processes to the transverse susceptibility.

Validity of the Stoner–Wohlfarth model in hysteretic giant magnetoimpedance of annealed amorphous materials

The hysteretic characteristics of giant magnetoimpedance (GMI) profiles have been measured in Co-based amorphous ribbon with various anisotropy angles θk, and they have been analyzed by using the Stoner–Wohlfarth model. A two-peak behavior with a dip near zero field is revealed in the measured GMI profile at 10 MHz, irrespective of θk. The negligible hysteresis of the field for the dip is in close agreement with that calculated assuming a magnetization jump from a metastable to a stable state. However, the hysteretic asymmetries for the increasing and decreasing fields in the samples with the angle range of 20°⩽θk&amp;lt;60° are well described by a divergence in the calculation without a magnetization jump. The asymmetry for the sample with θk⩾60° may also be due to the divergence. But the two peak of the measured profiles, which are due to the anisotropy distribution in the actual materials, are different from the calculated profiles, which have a single peak near zero field. This indicates that the Stoner–Wohlfarth model adequately describes a hysteretic asymmetry of GMI profiles for the anisotropy angle in the range of 20°⩽θk&amp;lt;60° at a frequency of 10 MHz in amorphous ribbons.