Showing papers by "Marcelo Knobel published in 1997"

Giant magnetoimpedance effect in soft magnetic wires for sensor applications

Abstract: The giant magneto-impedance effect (GMI) consists of the large relative change of the impedance (up to around 300%) observed in magnetically very soft ribbon and wire alloys under the application of dc magnetic fields (units of kA m 1 ). The phenomenology of the GMI effect is firstly described including a discussion about its origin which mainly lies in the classical skin-effect. An alternative approach to GMI phenomena considering equivalent circuits is also introduced. The main requirements to detect GMI is to count on a sample with very large circular susceptibility and reduced resistivity provided the frequency of the ac current flowing along the sample (necessary to evaluate the impedance) is high enough (roughly above 0.1 MHz for most samples here considered). The dependence on dc magnetic field, mechanical stresses and particularly on thermal treatments resulting in the induced magnetic anisotropies or in the devitrification of amorphous samples into a nanocrystalline structure are reviewed. First results on GMI in glass-coated amorphous microwires are also reported. The use of the GMI as a tool for studying the inner circular magnetization process or for evaluating the magnetostriction is introduced. Finally, a description on various aspects regarding the development of magnetic field, current, proximity and stress sensor applications is presented.

Influence of the distribution of magnetic moments on the magnetization and magnetoresistance in granular alloys

Abstract: In granular solids, the magnetoresistance is directly related to the macroscopic magnetization, but this relationship is extremelly complex due to the distribution of grain sizes and the intergranular magnetic interactions The dependence of the magnetoresistance on the magnetization is here investigated by means of a theoretical model that is developed taking explicitly into account the magnetic moment distribution and the spin-dependent electron-impurity scattering within magnetic grains and at the interface between the grains and the metallic matrix Using this model, one can explain large experimental deviations from the parabolic behavior of the magnetoresistance vs magnetization curves that are typically expected for equal noninteracting superparamagnetic grains The expressions for the magnetization and magnetoresistance, obtained for general distribution funtions, are tested considering a log-normal-type distribution function by fitting on data obtained from melt-spun Cu${}_{90}$Co${}_{10}$ ribbons after annealing by dc Joule heating The experimental data are well traced using just three parameters that determine the particle size distribution, the particle density, and the ratio of the scattering cross section at the boundaries of the grains to the scattering cross section within the grains

Correlation between magnetic interactions and giant magnetoresistance in melt-spun Co10Cu90 granular alloys

Abstract: The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spun Co10Cu90 granular alloys were investigated. The interaction effects were studied from both remanent magnetization and magnetotransport data, using two different methods to reach the demagnetized state, ac and dc demagnetization. The analysis of the structural evolution and interaction strength between the magnetic clusters clearly shows the role of some structural parameters (particle size and density, interparticle distance) and the degree of magnetic correlation in the magnetic field response of the resistance in these inhomogeneous systems.

Giant magneto-impedance and its relaxation in Co–Fe–Si–B amorphous ribbons

Abstract: Systematic measurements of giant magneto-impedance (GMI) and its relaxation (magneto-impedance aftereffect) have been carried out in a series of Co-rich amorphous ribbons ranging in magnetostriction values from −3.5 to +3.5 ppm. The value of GMI is a maximum for the alloy with the lowest value of magnetostriction, corresponding to a maximum of the transverse permeability. The impedance relaxation between two fixed times depends on the magnetostriction constant (λs) of the samples, and the observed trend is consistent with the theories which predict a direct relationship of the conventional permeability aftereffect (MAE) with the square of λs. Furthermore, the variation of the impedance aftereffect with the driving current resembles the behavior of MAE, and a connection between both effects can be established.

Comparative study of the giant magneto-impedance effect in Fe-based nanocrystalline ribbons

Abstract: We report the results of our studies on the giant magneto-impedance effect in nanocrystalline Fe73.5Cu1Nb3Si13.5B9. Fe90Hf7B3, and Fe90Zr7B3 ribbons. The results are explained by taking into account the basic magnetic and electrical properties, and also the domain structures from these ribbons. Our analysis is important from the practical point of view due to the immediate application possibilities offered by the giant magneto-impedance effect.

Amorphous alloys as anodic and cathodic materials for alkaline water electrolysis

Abstract: The general tendency of operating modern industrial electrolyzers at high current densities has stimulated research on materials with improved catalytic properties for the hydrogen and oxygen evolution reaction in alkaline water electrolyzers. The performances of electrocatalytic materials have usually been improved by increasing the ratio between the real and the apparent surface area. A less common way of improving the electrocatalytic activity is by modifying the microstructure of the alloys. In this paper we describe the electrocatalytic properties of various amorphous alloys. Among those investigated, amorphous FeNiSiB exhibit oxygen potential vs Hg HgO of around 600 mV at 300 mA/cm2 in 30 wt.% KOH at 60 °C. Polycrystalline nickel was less electroactive, indicating that the amorphous state is a better precursor for the formation of an active surface layer. In the alloys tested, metal-metalloid glasses are chemically unstable for high concentrations of iron and the electrode overvoltage decreases with an increase of iron. Consequently, corrosion resistance and electrode performance were found to have opposite dependence on iron concentration.

Magnetoimpedance aftereffect in a soft magnetic amorphous wire

Abstract: A slow relaxation of the high-frequency impedance is observed in a Co68.25Fe4.5Si12.25B15 amorphous wire after nucleation of a new domain pattern in a previously saturated sample. The observed impedance decay follows quasilogarithmic kinetics, and it is probably associated with the low-field ac magnetic permeability aftereffect of the circular domain walls. The impedance drop achieves relative amplitudes up to 1%, 16 s after the removal of the saturating external field. The effect is studied under typical magnetoimpedance experimental conditions, varying both the ac current amplitude and frequency. Although completely undesirable for many technical applications, the unique kinetic features of the impedance aftereffect may be explored to study circular magnetization processes under extreme domain-wall velocities. @S0163-1829~97!50410-3# Although amorphous magnetic materials have been known for more than thirty years, their basic and applied interest has been constantly renewed, owing to the discovery of new and attractive properties. Recently, the observation of large field-induced changes in the high-frequency impedance of soft magnetic amorphous wires 1‐3 and ribbons 4,5 opened enormous perspectives in the use of amorphous materials as cheap and sensitive magnetic-field sensors. When a highfrequency ( f .100 kHz! and low-intensity ( i,15 mA! electrical current flows through a soft magnetic material, its impedance suffers a strong variation if an external magnetic field is applied to the sample. This effect, known as giant magnetoimpedance ~GMI!, can be qualitatively understood in terms of field-induced changes in the magnetic penetration depth d m : 1,6

Giant magneto-impedance in nanocrystalline Fe73.5Cu1Nb3Si13.5B9 and Fe86Zi7B6Cu1 ribbons

Abstract: The field dependence of the real and imaginary components of the impedance was measured in Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 and Fe 86 Zr 7 B 6 Cu 1 ribbons submitted to different thermal treatments. A clear evolution of the so-called giant magneto-impedance (GMI) effect is observed as a function of the degree of magnetic softness of the samples. Furthermore, the measured GMI ratio is larger in the Fe 86 Zr 7 B 6 Cu 1 ribbons, probably related to the lower resistivity attained by this system in the nanocrystalline state.

Small-Angle X-ray Scattering Study of Nanocrystalline and Amorphous States of the Fe73.5CuNb3Si13.5B9 Alloy

Abstract: The structural evolution of the nanocrystalline material obtained from the amorphous compound Fe73.5CuNb3Si13.5B9 was investigated by small-angle X-ray scattering. Four sets of amorphous metallic ribbons were produced by melt spinning using different quenching rates. Samples of each set were annealed above the crystallization temperature; X-ray diffraction analysis showed the formation of an ordered Fe–Si solid solution with average grain sizes ranging from 9 to 11 nm. Small-angle scattering curves of the samples in the initial amorphous states revealed different intensities for different quenching rates, caused by the presence of heterogeneities with dimensions larger than those of the crystallites formed after thermal treatment. In addition, scattering measurements carried out in situ during isothermal annealing showed an intensity evolution as a function of time, attributed to electron-density contrast variation caused by the atomic diffusion process that occurs during crystallization.

Two Phase Magnetic Wire Obtained By High Frequency Current Annealing

Abstract: Alternating current Joule heating is explored to produce magnetic wires consisting of two different magnetic phases, one soft, with coercivities of the order of few A/m, and other harder, with coercivities around 10 kA/m. The effect of current frequency and intensity in the formation of the two-phase composite material is investigated. Although the two phases are formed due to different loss parameters along the wire length (basically due to thermal contacts), there is also an important influence of the skin effect.

Giant Magnetoresistance and Magnetic Interactions in Granular Co10Cu90

Abstract: The discovery of the giant magnetoresistance (GMR) effect in Fe/Cr multilayers [1] has stimulated a remarkable interest in these and other similar multilayers, due to its potential application in magnetic sensors and recording heads. Recently, it was demonstrated that GMR is also exhibited by nonmultilayer heterogeneous samples containing ferromagnetic granules embedded in a nonmagnetic matrix [2, 3]. Here, no coupling between the magnetic entities (as it is the case of multilayers) is necessary to explain the appearance of the GMR; spin dependent scattering originates from randomly oriented single domain regions.