Amorphous Glass-coated Microwires Research Articles

Modeling the Giant Magnetoimpedance Effect in Amorphous Microwires with Induced Magnetic Anisotropy

The giant magnetoimpedance effect in a glass-coated Fe-based amorphous microwire annealed under stresses is theoretically studied. The magnetization distribution in a microwire is described within a model that takes into account the presence in the sample of two regions with different types of magnetic anisotropy. It is assumed that, in the central part of the microwire, the anisotropy is longitudinal and, in the surface region, an induced helicoidal anisotropy arises as a result of annealing. An expression for the impedance of the microwire with allowance for different permeabilities of the two regions is found. The obtained theoretical dependences of the impedance on an external field and frequency make it possible to qualitatively describe the results of experimental studies of the magnetoimpedance in glass-coated Fe-based amorphous microwires.

Highly oriented ferromagnetic polymers based on Co- and Fe-rich amorphous microwires

In this paper highly oriented magneto-polymer composites have been obtained by means of Co and Fe-rich glass-coated amorphous microwires with various metal nucleus and outer diameters. The microwire segments with length L ~ 0.5–0.6 cm were oriented in the magnesium alloy matrix and were poured with a liquid polymer whose composition was a mixture of methyl methacrylate methacrylic acid and methacrylate copolymers in a ratio of 37/63% by mass. The magneto-polymer composites obtained possess high electric resistance, chemical stability and mechanical strength. The high average saturation magnetizations, <Ms> = 20–30 emu/cm3, have been obtained for Co and Fe based composites with the volume fraction of the magnetic material in the polymer matrix within the range 0.02–0.05. Both Co and Fe based composites show highly anisotropic magnetic properties. The magneto-polymer samples can be cut into pieces of arbitrary size and shape. This facilitates their use in various technical applications.

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.

The role of uniaxial magnetic anisotropy distribution on domain wall tilting in amorphous glass-coated microwires

Magnetic properties of highly magnetostrictive amorphous glass-coated microwires are strongly correlated to the presence of a glass coating that introduces a spatially inhomogeneous stress field distribution. We investigate the influence of mechanical stresses on the inclination of magnetic domain walls in magnetic microwires. Magneto-optical Kerr effect imaging is used to compare the tilted orientation of the domain wall shape in as-cast and annealed microwires. Angular dependencies of magnetization loops measured by alternating gradient field magnetometry reveal that the change of domain wall tilting with annealing is related to the decrease of magnetic anisotropy with axial orientation. Finally, micromagnetic simulations are used to show that sufficiently high uniaxial magnetic anisotropy gives rise to the presence of observed charged domain walls with tilted orientation.

Glass-coated ferromagnetic microwire-induced magnetic hyperthermia for in vitro cancer cell treatment.

Limitations in effectiveness and the invasive nature of current cancer treatment options emphasize the need for further clinical advancements. Among other approaches, targeted hyperthermia is as a new strategy aimed at targeting cancerous cells to improve the efficacy of radiotherapy or cytotoxic drugs. However, the testing of magnetic vehicles has mainly focused on the use of nanoparticles. In this work, Fe77B10Si10C3 glass-coated amorphous magnetic microwires were assessed for the first time as magnetic vehicles with high potential for the localized heating of osteosarcoma cells by means of an AC magnetic field. The results from the in vitro assays performed inside a microfluidic device demonstrated the ability of these magnetic microwires to induce malignant cell death. Exposing the system to different magnetic fields for less than 1 h provoked a reduction up to 89% of the osteosarcoma cell population, whereas healthy myoblastoma cells remained nearly unaffected. The proposed technology demonstrates in vitro the effectiveness of these microwires as vehicles for targeted magnetic hyperthermia.

Impact of Stress Annealing on the Magnetization Process of Amorphous and Nanocrystalline Co-Based Microwires.

The domain wall (DW) dynamics of amorphous and nanocrystalline Co-based glass-coated microwires are explored under the influence of stress annealing. Different annealing profiles have enabled remarkable changes in coercivity and magnetostriction values of Co-based amorphous microwires with initially negative magnitude, allowing induced magnetic bistability in stress-annealed samples and, consequently, high DW velocity has been observed. Similarly, Co-based nanocrystalline microwires with positive magnetostriction and spontaneous bistability have featured high DW velocity. Different values of tensile stresses applied during annealing have resulted in a redistribution of magnetoelastic anisotropy showing a decreasing trend in both DW velocities and coercivity of nanocrystalline samples. Observed results are discussed in terms of the stress dependence on magnetostriction and microstructural relaxation.

The evolution of structure and magnetoimpedance characteristics of amorphous Co69Fe4Cr4Si12B11 microwires under heat treatment

Abstract Amorphous Co69Fe4Cr4Si12B11 glass-coated microwires after heat treatment in the temperature range of 250–600 °C during 30 min were investigated. Changes of microstructure, phase composition, fracture morphology, and giant magnetoimpedance (GMI) properties were shown. It is confirmed that a significant increase in the GMI effect was possible not only due to the structural relaxation, but also as a result of phase transformations at the temperature close to the onset of crystallization. It is shown that at the very initial stage of the nucleation of Co nanocrystals, a sharp increase in circumferential diagonal GMI component was observed, a further increase in the amount of the crystalline Co phase was accompanied by degradation of this effect. The most significant GMI ratio was obtained for microwires annealed at 430 °C. At 450 °C and above, an irreversible decrease of the GMI ratio took place. It was caused by the formation of Co crystals, decrease in amount and composition change of the amorphous phase during primary and secondary crystallization. At the final stage of the Co crystals segregation the secondary crystallization of the residual amorphous phase occurred with formation of a metastable τ-phase with a Me23B6 type structure. Thermal stability of the τ-phase was analyzed.

Magnetic Anisotropy and Super-Sensitive Stress-Magnetoimpedance in Microwires with Positive Magnetostriction

In glass-coated Co71Fe5B11Si10Cr3 amorphous ferromagnetic microwires subjected to current annealing, a record sensitivity of the magnetoimpedance (MI) to mechanical tensile stresses (stress MI) up to 100% at 100 MPa in the absence of additional magnetic bias fields is achieved. The current annealing, combining the effect of Joule heating and a circular magnetic field, induces a specific helicoidal/circular-type magnetic anisotropy and, thus, allows one to control the behavior of the MI and stress MI, making the wires more suitable for use in sensor devices. As a result of changing the direction of the easy anisotropy axis, external mechanical stresses lead to a change in the direction of the static magnetization, which causes an increase in the sensitivity of stress MI.

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.

Field and Current Controlled Domain Wall Propagation in Twisted Glass-Coated Magnetic Microwires

The torsion effect on the field and current driven magnetization reversal and the associated domain wall velocity in cylindrical amorphous and nanocrystalline glass-coated microwires is reported. Samples from three representative compositions have been investigated: (1) amorphous Fe77.5Si7.5B15 with positive magnetostriction, λ ≅ 25 × 10−6, (2) amorphous Co68.18Fe4.32Si12.5B15 with nearly zero negative magnetostriction, λ ≅ −1 × 10−7, and (3) nanocrystalline Fe73.5Si13.5B9Cu1Nb3 (FINEMET) with small positive magnetostriction, λ ≅ 2.1 × 10−6, all having the diameter of the metallic nucleus, d, of 20 µm and the glass coating thickness, tg, of 11 µm. The results are explained through a phenomenological interpretation of the effects of applied torque on the anisotropy axes within the microwires with different characteristics. Among all the complex mechanical deformations caused by the application of torque on magnetic microwire samples, the most important are the axial compression – for axial field-driven domain wall motion, and the circumferential tension – for electrical current/circumferential field-driven domain wall motion. The Co68.18Fe4.32Si12.5B15 microwire, annealed at 300 °C for 1 hour and twisted at 168 Rad/m exhibits the optimum characteristics, e.g. the lowest switching current (down to 9 mA~2.9 × 10−3 A/cm2) and the largest domain wall velocity (up to 2300 m/s).

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.

Characterizing and Testing a Fabric Containing Amorphous Glass-Coated Ferromagnetic Microwires for its Magnetic and Electric Shielding Properties in the Frequency Range (1-3) Ghz

Abstract By applying a non-standardized methodology and by using electric- and magnetic-field probes of small dimensions (&lt; 1cm), we experimentally characterized the electromagnetic shielding properties of a fabric containing ferromagnetic microwires weaved on a single direction. Electronic microscopy and X-ray spectroscopy revealed the structure, dimensions and chemical elements content of the amorphous magnetic material. Electric shielding factor proved to be very low in the investigated frequency range, but magnetic shielding factor was high, especially when the weaving direction of the magnetic wires corresponded to the polarization direction of the emitting antenna, and showed some resonances. The magnetic shielding efficiency, if checked against an increasing incident magnetic flux density, proved not to change up to 200 nT. The investigated fabric have been previously proved to be very efficient in shielding the field emitted by a mobile phone in its near field, but present results show that near and far field shielding properties are different.

Magnetic anisotropy and stress-magnetoimpedance (S-MI) in current-annealed Co-rich glass-coated microwires with positive magnetostriction

Abstract Effects of a dc current annealing, in terms of magnitude and inflow time, on magnetic hysteresis properties, magnetoimpedance (MI) and stress-magnetoimpedance (S-MI) in glass-coated amorphous Co71Fe5B11Si10Cr3 microwires with a small positive magnetostriction were investigated in this work. The annealing process combining Joule-heating and circular magnetic field leads to the creation of a specific magnetic anisotropy of a helical or circular type, and consequently, is helpful to control the MI and S-MI behaviours making them more suitable for particular sensing applications. After annealing, the magnetoimpedance plots transformed from a bell-shaped typical of an axial easy anisotropy to the shape with two symmetrical peaks, the magnitude and position of which are controlled with the annealing current parameters. Applying a tensile stress on the annealed wires changes the easy anisotropy direction which leads to the potential for highly sensitive S-MI without use of dc bias fields. A stress sensitivity of more than 260% at σ ex = 250 MPa at zero magnetic field is achieved.

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

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Control of reversible magnetization switching by pulsed circular magnetic field in glass-coated amorphous microwires

Magnetization reversal in magnetic microwires was studied in the presence of external mechanical stress and helical magnetic fields using the magneto-optical Kerr effect. It was found that a combination of tuned magnetic anisotropy and a direct current or pulsed circular magnetic field activated different types of magnetization reversal scenarios. The application of the pulsed magnetic field of 10 ns time duration induced a transient controlling action to switch the magnetic states without activating a domain wall motion. This created a promising method for tuning the giant magneto-impedance effect.

Measurement of magnetic noise in magnetoimpedance sensing element

A method for measurement of magnetic noise in magnetoimpedance sensing element is proposed. Glass-coated Co-based amorphous microwires with a slightly negative magnetostriction were used as the sensing element. The operation principle of the sensing element was based on the nonlinear off-diagonal magnetoimpedance, when field dependent higher harmonic components appeared in voltage in the pick-up coil wound around the microwire. The magnetic noise for the second harmonic in the pick-up coil voltage was studied. The dependences of the magnetic noise and the signal-to-noise ratio in the sensitive element on the current amplitude were analyzed. The noise performance of the second harmonic in the pick-up coil voltage was compared to that for a flux-gate sensing element.

Stress Effects on Magnetic Properties of Amorphous Microwires Subjected to Current Annealing

Effects of current annealing on magnetic hysteresis properties and magnetoimpedance (MI) of glass-coated amorphous microwires with nominal composition of Co71Fe5B11Si10Cr3 and small positive magnetostriction were investigated with the purpose to control the magnetic anisotropy. We have demonstrated that current annealing can produce a controllable change in the easy anisotropy direction from almost axial to circular, depending on the annealing time. The induced magnetization configuration is very sensitive to the applied tensile stress. The combination of positive magnetostriction and helical anisotropy makes it possible to realise large stress-magnetoimpedance (S-MI) effect without use of any dc bias fields owing to the directional change in magnetization. This is especially important for microwave frequency S-MI effect and can be very interesting for developing stress-sensors operating at the high frequency region.

The change of domain structure of the amorphous microwire of Fe73.5Cu1Nb3Si13.5B9 composition under thermal treatment

The investigation on domain structure evolution under variation of magnetostriction, phase composition and internal stresses was provided. Glass-coated amorphous microwires of Fe73.5Cu1Nb3Si13.5B9 composition with positive magnetostriction were studied. The decrease of internal stresses and controlled crystallization leading to changes in the phase composition and reduction of magnetostriction were observed under different thermal treatment. The domain structure of the as-prepared and annealed microwires was studied by the method of magneto-optical indicator films. The estimation of the domain size was realized by Fourier transform from the domain contrast of magneto-optical images. The regularities of the influence of thermal treatment on the magnetic domain structure of microwires have been estimated.

Temperature Effects on the Magnetoimpedance in Glass-Coated Amorphous Wires

The temperature dependence of magnetoimpedance (MI) in glass-coated amorphous microwires of two compositions Co66.94Fe3.83Ni1.44B11.51Si14.59Mo1.69 and Co23.67Fe7.14Ni43.08B13.85Si12.26 having high (613 K) and low (335 K) Curie temperatures, respectively, was investigated. The wires of the first type have a small negative magnetostriction and are widely used as MI sensing elements. For sensor operation, the temperature stability is of primary importance, however, it was found that the MI change at moderate temperatures of 20 °C–90 °C and low magnetic fields can be nearly 200%. This is explained by high temperature sensitivity of the residual stress and stress-dependent magnetostriction, which resulted in abrupt change in the easy anisotropy direction. The wires of the second type with low Curie temperature ${T}_{c}$ and positive magnetostriction demonstrate large and sensitive change in impedance only in the vicinity of ${T}_{c}$ due to a corresponding drop in the magnetization saturation and magnetostriction, whilst the bistable type of the magnetization reversal remains. The frequency dispersion of the dynamical permeability shifts toward lower frequencies when approaching ${T}_{c}$ suppressing the high-frequency properties and MI. Such temperature dependence of MI is well controllable and is proposed for applications in temperature sensors.

Structure of domain wall in cylindrical amorphous microwire

The structure of the domain wall (DW) in the inner core of thin circular cross-section amorphous glass-coated bistable ferromagnetic microwires is studied. The influence of the outer shell is accounted either phenomenologically via Dzyaloshinskii-Morija-like interaction or/and by letting the relevant anisotropy constant to be dependent on the distance from the axis of the wire. All previously analytically found families of the DWs for the considered model are revisited, enabling the explanation of their more complex structure observed in experiments comparing to the known analytical results. Based on the found DW textures, an explicit value of the domain wall propagation velocity under the action of a time-dependent longitudinal applied magnetic field is found. Within the considered model the DW retains its shape while propagating, hence the introduced solutions determine the dynamics of the system.