Co-rich Amorphous Microwires Research Articles

Bending annealing induced transformation of magnetic structure in Co-rich amorphous microwires

For the first time, the magnetic structure of a glass-coated microwire, previously subjected to bending annealing was investigated. Magnetic and magneto-optical analyses were performed after the sample was annealed and returned to a straightened state. Changes in the magnetic structure, induced by the bending annealing, were observed both within the microwire and on its surface. The spatial distribution of the magnetic structure was primarily attributed to the spatial distribution of the stress gradient resulting from bending stress. Special emphasis was placed on the transformation of surface domain structures, with the diversity of these structures indicating a significant influence of bending-annealing-unbending procedure on their formation process.

Influence of Joule heating on electrical resistivity in Co-rich amorphous microwires

The paper investigates the electrical resistivity of two types of the glass-coated Co-rich amorphous ferromagnetic microwires during the Joule heating by direct current. We measured the relationships between the microwire’s resistivity and the power, which applied to the samples in as-prepared and crystallized conditions. Both types of microwires were demonstrated to completely crystallize on exposure to a temperature of more than 600 °C. During heating by a power up to 2.5 W (~400 °C), the microwires with Mo-Ni content were characterized by a monotonic resistivity increase with the temperature. But the microwires with Cr content had a well-marked resistivity minimum near the temperature Tм ~ 163 °C. This minimum shifted to a temperature value of ~260 °C after annealing. The research shows that the temperature dependence of resistivity in the temperature range below Tм obeys the f(T) = -ln(T) law.

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The paper investigates the electrical resistivity of two types of the glass-coated Co-rich amorphous ferromagnetic microwires during the Joule heating by direct current. We measured the relationships between the microwire’s resistivity and the power, which applied to the samples in as-prepared and crystallized conditions. Both types of microwires were demonstrated to completely crystallize on exposure to a temperature of more than 600 °C. During heating by a power up to 2.5 W (~400 °C), the microwires with Mo-Ni content were characterized by a monotonic resistivity increase with the temperature. But the microwires with Cr content had a well-marked resistivity minimum near the temperature Tм ~ 163 °C. This minimum shifted to a temperature value of ~260 °C after annealing. The research shows that the temperature dependence of resistivity in the temperature range below Tм obeys the f(T) = -ln(T) law.

Correlation of electrical and magnetic properties of Co-rich amorphous ferromagnetic microwires after DC Joule heating treatment

The present work describes the investigation of electrical and magnetic properties of the glass-coated Co-rich amorphous ferromagnetic microwires, having typical metallic core diameters d = 13–20 μm and tailoring by the direct current Joule heating. During this treatment, a continuous monitoring of the microwire condition was carried out, measuring its resistance by means of a DC bridge circuit. The heating of the microwire sample was provided by a direct current in the range from 20 mA to 63 mA. In this heating currents range, the microwires show a slight change in their resistivity. After annealing the resistivity of microwires may increase of about 1% with respect to as-prepared one. Depends on the heating current, the hysteresis loops of annealed microwires demonstrated the transition from a quasi linear type to a bistable one and vice versa. The maximum of the giant magnetic impedance ratio was observed in such microwires, which had a hysteresis loops describable by a near-zero anisotropy field.

Evolution of the magnetic anisotropy and magnetostriction in Co-based amorphous alloys microwires due to current annealing and stress-sensory applications

Abstract In this paper, the transformation in easy-axis anisotropy and magnetostriction in Co-rich amorphous microwires by dc current annealing has been studied with the aim to realize stress-sensitive magnetization process and magnetoimpedance (MI). With increasing the annealing current intensity I a n , the anisotropy and magnetostriction undergo non-monotonic changes owing to different processes occurring during annealing: structural relaxation, atomic pair ordering, atomic rearrangements, internal stress relief and phase transformation at the crystallization onset. At moderate values of I a n the induced anisotropy sets a near-circumferential easy direction whilst the magnetostriction parameter increases from a very low negative value in the as-cast state to positive values. At optimal combination of the induced anisotropy and positive magnetostriction a highly stress-sensitive MI behavior is observed which is important for stress-sensory applications. At these conditions, the impedance vs. magnetic field plots may change shape in the presence of tensile stress which at zero field is accompanied by a huge impedance change in the range of 70–162% per 100 MPa.

Dependence of the noise of an orthogonal fluxgate on the composition of its amorphous wire-core

In this paper we study the effect of magnetostriction of Co-rich amorphous microwire to the noise of the orthogonal fluxgates based on such wires. The magnetostriction was modified by changing the relative amount of iron x with the respect of the total amount of cobalt and iron in the alloy. Specifically we changed x in the composition (Co1−xFex)75Si15B10 casting wires with the following values of x: 0.05, 0.055, 0.06, 0.062, 0.065, 0.07 and 0.08. We found out that the noise indeed depends on the composition of the wire: while it is minimum (2.5 pT/Hz) for x between 0.06 and 0.062 where the magnetostriction is vanishing (λs ≈ 10−7) it significantly increases to tens of pT/Hz for both positive and negative magnetostriction when λs becomes one order of magnitude bigger. We verified that once the composition returns a magnetostriction low enough (around x = 0.06) then the noise does not depend on mechanical stress anymore. In fact the noise of a sensor with x = 0.06 is the identical in its natural curved state and when bended straight; for vanishing magnetostriction the bending does not affect at all the noise, as on the contrary it happens with larger magnetostriction. This suggests that once the wire has composition with x = 0.06 the remaining noise is not caused by mechanical stress. Finally we show how to overcome the problem of offset arising after annealing if continuous annealing current is used. We explain how this could be due by the fact that magnetostriction changes with temperature and even a wire with vanishing magnetostriction at room temperature can became significantly magnetostrictive at annealing temperature. For this reason we propose a method of annealing consisting rising the temperature of the wire while the wire is kept in its natural curved state. In this way no mechanical stress is applied to the wire during the annealing process, when the temperature is high enough to make it magnetostrictive even if it is non-magnetostrictive at room temperature. We show how this method suppresses the offset and significantly reduces the noise.

Co-rich Amorphous Microwires with Improved Giant Magnetoimpedance Characteristics Due to Glass Coating Etching

Glass-coated Co-rich amorphous microwires are very promising for the development of tiny magnetic sensors that can be used in portable electronic devises. In this study, a substantial decrease in the residual quenching stress in Co-rich microwires is achieved by reducing the thickness of the glass coating by means of precise etching of the wire in a specially designed gel. This effect is confirmed experimentally by means of the small-angle magnetization rotation method as well as by direct measurement of the off-diagonal component of the giant magnetoimpedance (GMI) tensor of wires with different thicknesses of glass coating as a function of the applied magnetic field. A reduction in the thickness of the glass coating to the range of 0.5–2.0 µm resulted in a nearly twofold increase in the steepness of the off-diagonal GMI component of the studied Co-rich microwires. Therefore, this method can be used to improve the sensitivity of miniature magnetic sensors to weak external magnetic fields.

Stress dependence of the magnetic properties of glass-coated amorphous microwires

We studied the effect of applied tensile stresses on magnetic properties of Fe- and Co-rich amorphous glass-coated microwires. Rising of magnetic anisotropy field upon tensile stress in Co-rich glass-coated microwires is described by a linear dependence. Furthermore, decrease of magnetic susceptibility upon tensile stress is observed in Co-rich microwires. A monotonous increase in the coercivity and the switching field upon tensile applied stress is observed for as-prepared Fe-rich microwire. Stress induced magnetic bistability consisting of transformation of hysteresis and onset of rectangular hysteresis loop upon tensile stress, σ, of about 22.5 MPa is observed in stress-annealed Fe-rich microwires. Such stress-induced magnetic bistability results in considerable growing of coercivity and remanent magnetization in stress-annealed Fe-rich microwires. The rising in the coercivity and the switching field upon applied tensile stresses in stress-annealed Fe-rich microwires with stress induced magnetic bistability (at σ > 22.5 MPa) is roughly described by the σ½ dependence, as previously reported for the wires with spontaneous magnetic bistability. Consequently, stress-annealing of Fe-rich microwire allows beneficial enhancement of the stress dependence of coercivity. Observed dependencies are discussed considering stress dependence of the magnetostriction coefficient in Co-rich glass-coated microwires, internal stresses redistribution and domain structure rearrangement in stress-annealed Fe-rich microwires upon applied tensile stresses.

Continuous control of a resistance in Co-rich amorphous ferromagnetic microwires during DC Joule heating

In this paper, we present the resistance measurements of the amorphous ferromagnetic Co-rich microwires (metallic nucleus diameters 15–30 μm) during Joule heating by direct current. The heating currents were provided by a programming power supply, which allowed setting up a different heating and cooling regimes. The Wheatstone bridge circuit was used for continuous resistance monitoring during the heating process. We observed the dependences of the microwire resistance versus the time and the applied power of heating for the samples having common composition Co69Fe4Cr4Si12B11 and different diameters. It was shown that the irreversible changes of the microwire resistance were occurred during heat treatment. The microwires have gone through a crystallization phase under linear varying power in the range between 0 and 4.5 W and their relative resistance reduced by up to 10% relative to the initial values. The one-time annealing run increases the relative resistance of the microwires samples from 0.1% to 1% at the various values of maximum power. The repeated heating processes result to the reversibility of the resistance values; in that case each value of maximum applied power provides finite rise of the microwire resistance value.

Influence of Technological Parameters on Magnetic Properties of Co-Rich Amorphous Ferromagnetic Microwires

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Glass shell etching to control residual quenching stress in Co-rich amorphous ferromagnetic microwires

The magnetic properties of amorphous glass coated microwires are determined mainly by the distribution of residual quenching stresses over the microwire cross section. In this paper a control of the residual quenching stresses in amorphous ferromagnetic microwires is achieved by changing the microwire glass shell thickness by etching. To this end, a special gel is synthesized for precision glass shell etching of series of Co-rich microwires. The use of the gel provides a possibility of uniform etching of the microwire glass coating at a rate of 0.2–3.2 μm/h depending on the amount of glass already removed, a surface roughness being about 40–100 nm. It is also possible to completely remove the glass shell of the microwire without damage its metallic nucleus. The small angle magnetization rotation method has been used to determine the change in the amplitude of the residual quenching stress with a gradual decrease of the glass shell thickness of microwires of composition Co67Fe4Ni2Mo2B11Si14 and Co71Fe4Si10B15. It is found that the amplitude of the residual quenching stress deceases by 40–50% in average after removing of the layer of 1 μm thickness from the glass coating.

Temperature Dependence of Giant Magnetoimpedance in Amorphous Microwires for Sensor Application

Temperature dependence of giant magnetoimpedance (GMI) amorphous wires has been investigated in a quasi-industrial temperature range of [−20 °C; 120 °C]. Co-rich amorphous microwires, from Unitika Ltd, of $100~\mu $ m diameter and about 50 mm length were placed inside a regulated temperature chamber. The impedance modulus, as a function of the magnetic field, was measured using an impedance analyzer. The excitation current of the GMI wire was about 3 mA of amplitude and 1 MHz of frequency. In addition to the GMI ratio, particular attention was paid to the behavior of the intrinsic sensitivity and the offset, which are the most relevant quantities in a real GMI sensor implementation. The experimental results have showed that the impedance curve was changed with the temperature. The maximum sensitivity and the GMI ratio increased with temperatures ranging from −20 to 80 °C. Then, beyond 80 °C, the trend seemed to be reversed. Over the considered temperature range, and at typical bias field of H = 35 A/m, the intrinsic sensitivity varied with a temperature coefficient of 0.4%/°C, while the temperature coefficient of the offset was about 200 nT/K.

Current induced domain wall propagation in Co-rich amorphous microwires

We studied the influence of the AC current flowing through the magnetically bistable Co-rich microwire on the magnetization dynamics. We used previously developed Sixtus-Tonks modified set-up for evaluation of the domain wall (DW) velocity within the microwire, but instead of the magnetizing solenoid we used the current flowing through the microwire. We observed that the AC current flowing though the sample produces the remagnetization of the sample by fast domain wall propagation along the microwire. Estimated DW velocity is about 4.5 km/s, being similar and even higher to that reported for the magnetic field driven domain wall propagation in Fe and Co-rich microwires. Observed current induced DW propagation through the microwire we explained considering the magnetostatic interaction between the outer circumferentially magnetized shell and the inner axially magnetized core.

Mechanical properties and internal quenching stresses in Co-rich amorphous ferromagnetic microwires

Abstract The whole set of geometrical, mechanical and magnetic parameters of Co-rich amorphous ferromagnetic microwires very promising for the development of sensitive stress and Giant Magneto-Impedance sensors have been determined by means of scanning electron microscopy, selective nanoindentation and small angle magnetization rotation method. The magnetostriction constants obtained are sensitive to the true microwire compositions. The experimentally estimated amplitudes of the residual quenching stress are mainly determined by the ratio of the inner and outer wire diameters. They are found to be in good correspondence with the theoretical values calculated based on the theory of viscoelasticity.

Features of Amorphous Microwires With Spontaneous and Induced Magnetic Bistability

We demonstrate that in Co-rich amorphous microwires with vanishing magnetostriction coefficient, a conventional annealing is quite an efficient method for tailoring of magnetic properties. We observed induced magnetic bistability allowing the observation of fast domain-wall (DW) propagation and giant magnetoimpedance effect simultaneously in the same sample. Moreover, DW dynamics in microwires with induced magnetic bistability presents unusual dependence on applied stress, exhibiting a subsequent growth of DW velocity. For interpretation of these observed changes of hysteresis loops by annealing, we consider stress dependence of the magnetostriction.

Enhanced high-frequency magneto-impedance response of melt-extracted Co69.25Fe4.25Si13B13.5 microwires subject to Joule annealing

We report an enhancement of low-field, high-frequency magneto-impedance (MI) response and circumferential domain structures of melt-extracted amorphous Co69.25Fe4.25Si13B13.5 microwires by dc Joule annealing. The current amplitudes of 5 mA, 10 mA, and 20 mA were applied to amorphous microwires for different time treatments. The magnetic field dependence of impedance and circumferential domain structure of the microwires were explored using an impedance analyzer over a high frequency range (20 MHz–1000 MHz) and magnetic force microscopy (MFM), respectively. Consequently, the optimum value of 5 mA current for 20 min markedly improved the MI ratio up to 610% (1.7 times of 380% from the as-cast state) at a working frequency of 20 MHz. Moreover, the magnetic field sensitivity has been achieved up to 500%/Oe (more than two times of 225%/Oe from the as-cast one) at a frequency of 40 MHz. MFM images clearly show the development of a well-defined circumferential domain structure of the annealed microwire comparing with the as-cast one. The improvement of the high-frequency MI response in Co-rich amorphous microwires is promising for sensitive magnetic microsensors for low field detection.

Engineering of magnetic properties and GMI effect in Co-rich amorphous microwires

We studied the effect of annealing conditions on magnetic properties of amorphous CoFeNi-based glass-coated microwires. We showed that annealing can be very effective for manipulation of the magnetic properties of amorphous ferromagnetic glass-coated microwires. Low coercivity and high Gaint magnetoimpedance (GMI) effect have been observed in as-prepared Co-rich microwires. After annealing of Co-rich we can observe transformation of linear hysteresis loops to rectangular and coexistence of fast magnetization switching and GMI effect in the same sample. We demonstrated that the coercivity of microwires can be tailored by annealing in the range from 4 to 200 A/m. We found that the annealing considerably affects the magnetostriction coefficient and observed correlation between the magnetic hardening of studied microwires and the magnetostriction sign change.

Studies of High-Frequency Giant Magnetoimpedance Effect in Co-Rich Amorphous Microwires

We studied giant magnetoimpedance (GMI) effect and magnetic properties of various Co-rich amorphous microwires. We measured the values of magnetic field dependence of the GMI ratio up to gigahertz frequency range, and observed quite high GMI ratio in as-prepared Co-rich microwires even at gigahertz frequencies. We observed the different values of the magnetic anisotropy field obtained from the hysteresis loops and the impedance curves. Observed dependence can be explained considering different magnetic anisotropy in the surface layers and in the inner part of metallic nucleus and skin depth effect. Features of high-frequency GMI effect have been analyzed using ferromagnetic resonance-like approximation.

Investigation of the properties of Co-rich amorphous ferromagnetic microwires by means of small angle magnetization rotation method

The amplitude of the second harmonic of the electro-motive force occurring in the receiving pick-up coil when alternating electrical current is flowing through the microwire is measured as a function of applied external magnetic field, at different mechanical tensile stresses. In addition, an analytical expression for the amplitude of the second harmonic of the electro-motive force is derived. Comparing the experimental and theoretical data the saturation magnetization, the magnetostriction constant and the amplitude of the residual quenching stress have been determined for a family of Co-rich glass-coated microwires.

Asymmetric giant magnetoimpedance of Co-rich melt-extraction microwires

The giant magnetoimpedance(GMI) effect of Co-rich microwires makes an opportunity to design sensitive GMI weak magnetic meter sensor. Optimization of magnetic meters needs to improve the GMI response, especially the field sensitivity of microwires. In this study, Co-rich amorphous microwires each with an average diameter of 32 μm are prepared by melt-extracted technique and their GMI characteristics are investigated at frequencies ranging from 0.1 to 10 MHz with and without bias direct voltage applied. Experimental results indicate that the GMI effect of these wires has asymmetric features with the increases of frequency and driving current. It is found that the intrinsic asymmetric GMI (AGMI) response results from the helical anisotropy and magnetization hysteresis of the Co-rich microwires. Furthermore, it is found that there is a pronounced improvement in AGMI response when a bias voltage is applied. In theory, the factor which induces an increase in circular magnetic field causes successive changes in magnetization reversal of the quickly quenched Co-rich microwires with multiple domains and helical anisotropy. As a consequence, the circular magnetization process is enhanced, leading to higher circular permeability and stronger GMI response. Meanwhile, a bias voltage inducing the given circular magnetic field reinforces the magnetization process in a certain direction, which intensifies the asymmetric characteristic of GMI response. For example, the asymmetric ratio between two impedance peaks rises from 1.46% to 12.06% at 1MHz and 3 mA after applying a 1 V bias voltage. Simultaneously, the circular field inclines the magnetization off the axial direction which makes the axially induced magnetization reversal more difficult and occur at a higher switching field. This effect broadens the linear impedance zone; however, it reduces the slope of the impedance with the external field and the field sensitivity increasing to some extent. The balance between these two sides proves that AGMI response is related to the magnetization reversal process which is sensitive to the circular magnetic field. Experimental results indicate that the field sensitivity rises from 616 to 5687 V/T with the impedance linear zone broadening from 0.65 to 1.16 when a 1 V bias voltage is applied, while it decreases to 4525 V/T when the bias voltage futher increases to 2 V at 10 MHz and 5 mA. This reveals that the GMI effect of these amorphous Co-rich microwires with high field sensitivity can be optimized by applying proper bias voltage.