Co-based Amorphous Wires Research Articles

Off-Diagonal Impedance in Amorphous Wires and Its Application to Linear Magnetic Sensors

We investigated the magnetic-field behavior of the off-diagonal impedance in Co-based amorphous wires under sinusoidal (50 MHz) and pulsed (5 ns rise time) current excitations. For comparison, we measured the field characteristics of the diagonal impedance as well. In general, when an alternating current is applied to a magnetic wire, the voltage signal is generated not only across the wire but also in a pickup coil wound on it. These voltages are related to the diagonal and off-diagonal impedances, respectively. We demonstrate that these impedances have a different behavior as functions of axial magnetic field: the diagonal impedance is symmetrical, whereas the off-diagonal one is antisymmetrical with a near-linear portion within a certain field interval. For the off-diagonal response, the dc bias current is necessary to eliminate circular domains. In the case of the sinusoidal excitation without a dc bias current, the off-diagonal response is very small and irregular. In contrast, the pulsed excitation, combining both high- and low-frequency harmonics, produces the off-diagonal voltage response without additional biasing. This behavior is ideal for a practical sensor circuit design. We discuss the principles of operation of a linear magnetic sensor based on a complementary metal-oxide-semiconductor transistor circuit.

Magnetization reversal of Co-based amorphous wires induced by longitudinal AC magnetic field

The remagnetization process in CoFeSiB amorphous wires under influence of a high-amplitude AC longitudinal magnetic field is studied. The frequency spectra of the voltage at the wire ends are measured as a function of a longitudinal DC magnetic field and the AC field amplitude. A high sensitivity of the voltage harmonics to the DC magnetic field is demonstrated. The experimental results are interpreted within a simple rotational model.

Origin of asymmetrical magnetoimpedance in a Co-based amorphous microwire due to dc bias current

Permeability spectra and asymmetrical giant magnetoimpedance (AGMI) have been studied in a Co-based amorphous microwire upon the application of a dc biasing current together with a shift of axial hysteresis loops. The permeability from wall motion of core domains is nearly constant, but the permeability from rotational magnetization of shell domains increases with the bias current. AGMI and its field sensitivity are realized for the bias current due to the influence of a unidirectional bias field on wall motion in the circumferential direction. In addition, helical internal anisotropy in this microwire also plays a significant role in responding differently to the axial magnetization process and thus to AGMI. The helical residual stress is estimated to be smaller in Co-based amorphous wire than that in Fe-based amorphous wire.

Effect of helical-induced anisotropy on the magnetoinductance response of Co-based amorphous wires

In this letter, the effect of helical anisotropy, induced by torsion strain, on the circular permeability μφ, of a vanishing negative magnetostrictive CoFeSiB wire is presented. As a function of a dc applied magnetic field, Hdc, μφ shows an asymmetric behavior dependent on the direction of the applied torsion strain: for counterclockwise torsion angles (−), a monotonous decrease in the μφ (Hdc) double peak magnetoimpedance response is observed together with a linear increase in the wire’s circular anisotropy field Hk. In contrast, for clockwise torsion angles (+), a gradual suppression of the μφ (Hdc) maxima was evident, for which a magnetic softening of the wire is assumed. Results are interpreted in terms of a counterbalance/enhancement effect of the wire’s circular anisotropy for + and − torsion angles, respectively.

Internal magnetoimpedance of amorphous wires

Analysis of the usual experimental method to quantify the giant magnetoimpedance (GMI) effect indicates that at high enough working frequencies the values of the impedance can be largely influenced by parasitic impedances of the circuit that affect significantly the analysis of the data. A discussion is introduced on the intrinsic impedance for a Co-based amorphous wire and the actual intrinsic value when parasitic impedances are cancelled.

Nonlinear magnetoimpedance effect in soft magnetic amorphous wires extracted from melt

The nonlinear mode of the magnetoimpedance in Co-based amorphous wires extracted from the melt is studied. The frequency spectrum of a voltage induced in a pick-up coil wound around the wire is analyzed as a function of an ac current amplitude and a longitudinal dc magnetic field. It is observed that at sufficiently high ac current amplitudes even harmonics dominate in the pick-up voltage spectrum. A high sensitivity of even harmonics to the dc magnetic field is demonstrated. The studied nonlinear magnetoimpedance is promising for the design of weak magnetic field sensors.

The temperature dependence of the magneto-impedance effect in the Co-based amorphous wires

Abstract We used CoFeSiB amorphous wires to investigate the influence of heating and tensile stress on the magneto-impedance value. Measurements were performed with as-cast wires without cover of 100 μm diameter and 7 mm length. Experiments were carried out at current frequencies 10, 30 and 50 MHz, in temperature range from 23 to 180 °C.

비정질 와이어를 센서헤드로 이용한 금속의 미세결함 검출

미소결함을 가진 금속체에 교류자기장을 인가하면 결함부근에는 와전류의 분포가 달라져 이에 의해 금속체를 관통하는 교류자기장은 결함부근에서 차이를 가진다. 이 교류자기장을 비정질 와이어로 된 센서헤드로 측정하여 센서헤드의 유기전압 크기로부터 결함 유무를 검출하는 방법에 대해 검토하였다. 비정질 와이어는 Co-based재료로 자왜가 거의 0이며 고투자율 자성체이고 비정질 와이어는 길이가 15mm, 직경이 100<TEX>$\mu\textrm{m}$</TEX>인 원주형 자성체이다. 실험대상 금속체로 0.5mm의 단일 직선 갭을 가진 두께 1mm의 동판과 0.1mm의 갭이 규칙적으로 배열된 두께 25<TEX>$\mu\textrm{m}$</TEX>의 Al 판를 이용하였다. 스파이럴 코일에 인가하는 교류자기장의 주파수는 100KHz~600KHz였다. 본 실험의 결과에서 동판에서는 유기전압의 최대치와 최소치의 차가 약 2.5㎷ 얻어졌고, Al판에서는 500KHz에서 0.4㎷가 얻어져 직선 갭의 유무를 유기전압의 크기만으로 확인할 수 있었다. Ac magnetic field was changed in the vicinity of a flaw because of the distribution of eddy current within a conductor, when the magnetic field was applied to a conductor having a flaw. The flaw detection was performed by using Co-based amorphous wire sensor head. The wire has almost 0 magneto-striction and high permeability. An comparative uniform magnetic field was applied to a 1㎜ thick copper plate and a 25㎛ thick aluminum sheet conductor using spiral typed coil. The size of the coil has 40㎜<TEX>$\times$</TEX>40㎜ outer width and 8㎜<TEX>$\times$</TEX>8㎜ inner width. The copper plate and the aluminum sheet has 0.5㎜ and 0.1㎜ wide gap, respectively. The frequency range of applied field was 100㎑∼600㎑. The induced voltage difference of 2.5㎷ was obtained in the maximum voltage and minimum one measured across the gap of the 1mm thick conductor. In the case of aluminum sheet, 0.4㎷ was obtained. From this results, the effectiveness of Co-based amorphous wire was confirmed in the ECT technique.

Magnetoimpedance effect in Co-rich metallic glasses

Some aspects of the magnetoimpedance (MI) effect in amorphous ribbons and wires are analysed. In concrete, the explanation of the phenomenon based on the frequency and the driving current amplitude dependencies is given. Also, the different MI responses depending on the external parameters that play an importat role in this effect, such as applied magnetic field and tensile or torsional stresses are discussed. Results obtained in different samples are classified in two sections: Drive current dependence of MI and Torsion impedance in Co-based amorphous wires. The results give evidence of the great interest displayed by these soft ferromagnetic materials for technological applications using the MI effect under different external conditions. The importance of the adequate thermal treatments in order to configure the magnetic anisotropy of the materials is also underlined.

High-frequency magnetic aftereffect in Co-based amorphous wires and ribbons

In this work, the magnetoimpedance relaxation in Co 68.25Fe 4.5Si 12.25B 15 systems has been measured in the frequency interval 500 kHz–3 GHz. A comparison between ribbons, amorphous and glass-covered wires of nominal composition Co 68.25Fe 4.5Si 12.25B 15 was made. The time variation of the calculated characteristic impedance after field removal displays a similar behavior to the classic magnetic permeability aftereffect measured at lower frequencies by means of conventional fluxmetric techniques. Thermal treatments are shown to affect the impedance relaxations in melt-spun alloys. Conversely, no substantial effect has been observed in glass-covered wires.

Magnetoelastic behavior in Co-based glass-covered amorphous wires

The effects of Fe content as well as the dc or ac stress current annealing conditions (current density, annealing time, and tensile stress) on the magnetoelastic behavior of amorphous Co/sub 72.5-x/Fe/sub x/Si/sub 12.5/B/sub 15/ (x=0, 3, 4.35, 5, and 7. at.%) glass-covered wires have been investigated. The saturation magnetostriction at zero applied stress /spl lambda//sub s/ (0) in the as-cast state of the samples is about -2.8/spl times/10/sup -6/; -0.6/spl times/10/sup -6/; -0.11/spl times/10/sup -6/; 0.36/spl times/10/sup -6/, and 0.9/spl times/10/sup -6/ for x=0, 3, 4.35, 5, and 7 at.% Fe, respectively. For nearly zero magnetostrictive amorphous Co/sub 68.15/Fe/sub 4.35/Si/sub 12.5/B/sub 15/ glass-covered wire, after stress current annealing /spl lambda//sub s/ (0) changes from small negative to small positive values. The positive /spl lambda//sub s/ (0) values are obtained after about 3-8 min annealing time for current densities higher than 130/spl times/10/sup 6/ A/m/sup 2/. The ac stress current annealing leads to an increase with maximum 60% of the maximum saturation magnetostriction value /spl lambda//sub s/ (0)/sub max/ (depending on the frequency value) in comparison with dc annealing for the same current density, treatment time, and tensile stress. For alloys with x=0, 3, 5, and 7 at.% Fe, after dc or ac stress current annealing with (60-180)/spl times/10/sup 6/ A/m/sup 2/, under 300-800 MPa, a modification up to 10% in /spl lambda//sub s/ (0) values was observed (the absolute value for negative magnetostrictive samples decreases while for positive magnetostrictive samples increases).

Giant magnetoimpedance of Fe- and Co-based amorphous wires up to 2 GHz

In this work, results on the magnetoimpedance (MI) of magnetic amorphous wires with nominal compositions Fe 77.5Si 7.5B 15, Co 68.15Fe 4.35Si 12.5B 15 and Co 72.5Si 12.5B 15 are presented. The impedance Z= R+ iX has been investigated as function of frequency (from 100 kHz up to 1.8 GHz) and magnetic field, using a HP4396B impedance analyzer and an appropriate coaxial microwave cavity. The effects of the length of the samples (from 7 to 80 mm) and of thermal treatments for anisotropy induction (Joule heating, 600 mA for 30 min) have been investigated.

Asymmetrical giant magneto-impedance (AGMI) in amorphous wires

The discovery of giant magneto-impedance (GMI) in soft magnetic amorphous wires and films has opened new horizons in the development of micromagnetic sensors operating at nano-Tesla range. Asymmetrical GMI (AGMI) is very important to further improve sensor performance in terms of linearity and sensitivity. This paper presents a review on AGMI in Co-based amorphous wires with two types of anisotropy: circumferential and helical. Physical concepts of AGMI supported by theoretical and experimental results and problems of significant practical importance are discussed.

Magneto-impedance measurements in amorphous Co-based magnetic wires at high frequency

An alternative method to evaluate the magnetic response of soft magnetic systems in the high-frequency region (1 MHz–6 GHz) is presented here. The proposed experimental set-up is based on the measurement of the four scattering parameters S ij of a coaxial line by means of a vectorial network analyzer. The magnetic sample, either in wire or ribbon form, is used as the inner conductor of a coaxial line. The characteristic impedance of the line section is given by an analytical formula taking into account the geometrical structure of the device and the sample physical properties. Transmission line theory has been successfully employed to extract from the raw experimental data the information concerning the magnetic sample, discarding the contribution of the terminations. Magneto-impedance is obtained by measuring the characteristic impedance under different applied magnetic fields. The proposed method provides a natural extension of the usual characterisation techniques from lower frequencies up to 6 GHz. As an example, the magneto-impedance response of Co-based amorphous and glass-covered wires in the high-frequency region will be discussed.

Influence of stress on Matteucci and search coil voltages in amorphous wires

The influence of tensile, compressive and torsional stresses on the magnetic properties of as-cast and treated high positive Fe-based and nearly zero Co-based magnetostrictive amorphous wires has been studied. The application of tensile and compressive stresses determine the decrease of the Matteucci voltage, V m, as a result of cancelling of helical stresses, the effect being more important in the Fe-based wires. However, it increases with torsional stresses applied on preloaded samples, reaching a maxima at a certain number of turns that corresponds to the maximum alignment of vector components. V m was found to be more sensitive and linear to the applied torsion than the induction voltage detected from the pick-up coil, V s. This investigation also highlighted the occurrence and the stress dependence behaviour of the second harmonic component of the induced EMF voltages in amorphous wires. This extensive study was undertaken so that industrially viable sensors could be designed based on these stress-dependence effects.

Magnetic domain imaging using orthogonal fluxgate probes

This article presents an application of a miniature orthogonal fluxgate magnetometer to magnetic domain imaging of coated grain-oriented silicon steels. The method is based on a common technique which maps stray magnetic fields emerging from the surface of the silicon steel. An orthogonal fluxgate made of a thin amorphous wire core is bent in a J-shape so that it may work as a gradiometer, in which the longer leg of the J-shaped wire core is made sensitive to stray magnetic fields whereas the shorter one is effectively located at a distance from them. Experiments were carried out with a thin Co-based amorphous wire used as a wire core and a highly grain oriented silicon steel as a sample on which the domain refinement by laser irradiation is made. Well-defined stripe domains with some spike domains, and grain boundaries were clearly imaged with this method without any assisting magnetic field to enhance stray magnetic fields.

Orthogonal fluxgate mechanism operated with dc biased excitation

A mode of operation is presented for an orthogonal fluxgate built with a thin magnetic wire. By adding a proper dc bias to the wire excitation, the new mode is easily established. In this case, the fundamental component of the induced voltage at the sensing coil (secondary voltage) is made sensitive to the axial magnetic field, compared to the second harmonic in a conventional orthogonal fluxgate. The operating principle is explained using a magnetization rotation model. A method is proposed to cancel the offset that is inevitable when the magnetic anisotropy is present in a magnetic wire at an angle to its circumference. Experimental results are shown for a sensor head consisting of a 2-cm-long Co-based amorphous wire 120 μm in diameter with a 220-turn sensing coil. The sensitivity obtained is higher than that obtained using a conventional type of the orthogonal fluxgate built with the same sensor head. It is also demonstrated that the proposed method for canceling the offset works well.

Magneto-impedance in Co-based amorphous wire and ribbon

Magneto-impedance of Co-based amorphous alloys with low magnetostriction ( λ) and in the form of wire and ribbon is measured as a function of longitudinal d.c. magnetic field ( H) for frequencies up to 100 KHz. Nearly 100% drop in reactance is found in wire and ribbon with small and negative λ for H≈30 Oe. The field sensitivity of impedance goes down for alloys with higher values of λ. The longitudinal external stress reduces the reactance.

High-frequency giant magnetoimpedance in joule-heated Co-based amorphous ribbons and wires

The giant magnetoimpedance (GMI) effect in Co 68.25Fe 4.5Si 12.25B 15 melt-spun amorphous wires and ribbons was studied in the microwave region ( 45 MHz≤f≤6 GHz ). Selected samples were submitted to dc joule-heating in vacuum, exploiting the heat released to the sample by a dc electrical current flow, to induce circular and transverse magnetic anisotropy. The high-frequency impedance was measured using a technique based on the S-parameter measurements of a specific transmission line by means of an automatic vectorial network analyser. Annealed materials, either in wire or ribbon form, display a linear increase of Δ Z/ Z ratio with the applied magnetic field up to reaching a maximum (i.e. Δ Z/ Z≈60% at H≈400 A/m) at low field values.

Asymmetrical magnetoimpedance in as-cast CoFeSiB amorphous wires due to ac bias

Asymmetrical giant magnetoimpedance (AGMI), which utilizes a high frequency bias field hb, is realized in a Co-based amorphous wire having a circumferential anisotropy in the outer region. No asymmetry in the dc magnetic configuration is needed in this case. AGMI is discussed in terms of the surface impedance tensor, demonstrating that the effect of hb is related to the role of the off-diagonal component of the impedance in the voltage response measured across the wire. This effect is important for developing autobiased linear magnetic sensors. Using two oppositely biased wires, a near-linear voltage output (±4 mV) is obtained in the range of ±5 Oe for the sensed dc field at a frequency of 8 MHz.