We studied the magnetic properties of Co73Fe4Si12B11 ferromagnetic microwires in initial amorphous and crystalline state achieved by thermal treatment with direct current. A microwire in amorphous state is found to be characterized with quasilinear hysteresis loop with small coercive force less than 0.05 kA/m. Once crystallized, the microwire loses its soft magnetic properties, its hysteresis loop radically expands, and the coercive force becomes equal to 15 kA/m.

A method for the determination of the thermal conductivity of polymer composite materials by using Co-based ferromagnetic microwires is proposed. Microwire segments were integrated into the samples of studied materials during their manufacture and used as current microheaters and resistance thermometers. As a representative material, we used a material based on nitrile butadiene rubber filled with hexagonal boron nitride after its low-temperature carbonization and a significant increase in thermal conductivity. The thermal conductivity values of composite samples determined during experiments varied from 1.0 W/(m·K) to 1.8 W/(m·K) depending on the percentage of boron nitride. The thermal conductivity values obtained are in good agreement with the estimates obtained by the standard laser flash method.

A scanning magnetometer based on a magnetoimpedance sensor with a 1 mm spatial resolution and 10 nT sensitivity was used to study stray magnetic fields of Fe74B13Si11C2 amorphous ferromagnetic microwires. Spatial magnetic images and vertical component profiles of stray magnetic fields of the studied microwires were obtained in a longitudinal homogeneous magnetic field of Helmholtz coils with a strength in the range of ±600 A/m. A magnetic calculation method is suggested that allows for using the measured magnetic fields to determine the magnitude and pattern of magnetization for the microwire. Characteristic values of the microwires’ average magnetization and width of closure domains for various values of bias fields were found.

In this work, a scanning magnetometer based on a giant magnetoimpedance sensor was used to study the toner material of a black-and-white laser printer that contains iron oxide nanoparticles. We measured the local perpendicular magnetic field component of various sizes black toner strips, magnetized in a perpendicular magnetic field. Our calibration samples were two-wire current lines, which are the current analogue of magnetized toner strips. Magnetic images of the samples were obtained. It was found that investigated toner samples have an average concentration of magnetic nanoparticles per unit area of order of 6 mu g/mm(2). At this concentration of nanoparticles, the remnant magnetic moment of the sample 1.6 x 5 mm(2) in size was about of 0.046 mu A.m(2) and could be reliably registered by the scanning magnetometer. The magnetic image of the toner sample was compared with a hysteresis loop obtained using a vibrating sample magnetometer.

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 degrees C. During heating by a power up to 2.5 W (-400 degrees 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 T-M similar to 163 degrees C. This minimum shifted to a temperature value of -260 degrees C after annealing. The research shows that the temperature dependence of resistivity in the temperature range below T-M obeys the f(T) = -ln(T) law.

A low-frequency model of the magnetization reversal of a microwire is developed for those cases when the microwire magnetization response can go beyond the linear approximation. The analysis of the influence of external magnetic fields on the process of magnetization reversal of the microwire, including the hysteresis mode, was performed. The characteristic dependences of the amplitude of the electromotive force, U2f, arising in the pick-up coil wound around the microwire are obtained. It was established that, in the region of relatively small-acting circular and longitudinal magnetic fields, the U2f signal could have a region with the opposite sign. An extended small-angle magnetization rotation method was used to verify the proposed model and test glass-coated, amorphous, Co-rich microwires. During the experiments, the amplitude of the second harmonic, U2f, arising in the pick-up coil when an alternating electric current with the frequency f flows through the microwire, was measured as a function of the applied longitudinal magnetic field at various mechanical tensile stresses. The effective anisotropy field, the magnetostriction constant, and the residual quenching stress of the investigated microwires were determined by comparing the theoretical and experimental data.

The results of comparative studies of the electrically conductive properties of Co69Fe4Cr4Si12B11 glass coated amorphous microwires obtained during their heat treatment in a conventional furnace and by the Joule heating method are presented. The fully crystallized microwire dramatically changes its electrically conductive properties. We found that the crystallized microwire has a temperature coefficient of resistance, alpha = 315*10(-6) 1/degrees C. The crystallized microwire was used as a reference resistance thermometer under Joule heating for determining the temperature of the microwire as a function of the applied thermal power T(P). This dependence obtained was used to determine the temperature dependence of the resistance R-M(T) of other microwire samples in an amorphous or partially crystallized state of the same series. The proposed method allows to select thermal modes during Joule annealing of microwires and to compare the resistive, magnetic and structural-phase properties of microwires after thermal effects.

A scanning magnetic microscope based on an off-diagonal magnetoimpedance sensor is presented. As a sensor, we use a 4 mm segment of glass covered microwire, having a metallic core diameter of 13.5 mu m and CoFeCrSiB composition and a pick-up coil wound around the microwire containing 70 turns. The magneto-impedance sensor is fixed perpendicular to the surface of the sample, in such a way that the distance between the tip of the microwire and the sample is about 200 mu m. The relative movement of the sample and the GMI sensor is carried out using a non-magnetic X-Y positioner. Measurements are taken inside the magnetic screen. For test measurements, samples in the form of the letters IWMW - of the conference abbreviation are used. The samples were made based on thin copper wire, Fe-rich amorphous ferromagnetic microwires and printed by laser-jet printer. Clear magnetic images were obtained on all samples, which demonstrates the high practical potential of the proposed method.

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 mm 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. (C) 2020 Elsevier B.V. All rights reserved.

Amorphous Co69Fe4Cr4Si12B11 glass-coated microwires after heat treatment in the temperature range of 250-600 degrees 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 degrees C. At 450 degrees 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 tau-phase with a Me23B6 type structure. Thermal stability of the tau-phase was analyzed.

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.

The results of qualitative and quantitative changes in phase composition as well as microstructure changes of amorphous Co69Fe4Cr4Si12B11 microwires at three stage crystallization process of are presented. After first and second stages the amount of crystalline Co-phase was 36 wt%. To study magnetization of the alloy during heating, exposure and cooling a unique vibration magnetometer with a chemical microreactor was used. The unique vibration magnetometer was used for low inertia real-time measurements under controlled atmosphere. The Curie temperatures of phases formed in the crystallization process were determined. Based on the analysis of magnetization kinetic curves, nucleation and growth mechanisms were established on the stage of primary crystallization. It was found that a one-dimensional mechanism is responsible for the growth of Co-based crystals and leads to the formation of a specific needle-shaped structure. At the second crystallization stage of a metastable phase with a Co23B6 structure type was formed. The Curie temperature of this phase was 165 degrees C. (C) 2018 Elsevier B.V. All rights reserved.

In this paper, we present the resistance measurements of the amorphous ferromagnetic Co-rich microwires (metallic nucleus diameters 15-30 mu m) 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 A. V. Popova1, V. I. Odintsov1, I. V. Kozlov2, G. N. Elmanov2, E. V. Kostitsyna3, E. S. Gorelikov3, and S. A. Gudoshnikov1,3 1Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, (IZMIRAN), Troitsk, Moscow, Russia 2National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Kashirskoe shosse 31, Moscow, 115409, Russia 3National University of Science and Technology “MISiS”, Moscow, Russia

The spatial period of magnetization perturbations that occur near the surface of magnetic nanotube or nanowire under the influence of surface magnetic anisotropy is determined by means of numerical simulation as a function of the sample geometry and material parameters. The surface magnetization distribution obtained is then used to estimate the period of the surface magnetic texture in amorphous microwire of several micrometers in diameter by means of appropriate variational procedure. The period of the surface magnetic texture in amorphous microwire is found to be significantly smaller than the wire diameter.

The magnetostatic and electrodynamics properties of amorphous Co-rich microwire with a weak helical anisotropy has been studied taking into account the distribution of the residual quenching stresses over the microwire cross section. The components of the Giant Magneto- Impedance tensor have been obtained taking into account the necessary electromagnetic boundary conditions at the wire surface. A weak helical anisotropy of the microwire has been introduced through a small off-diagonal correction to the residual stress tensor. In this way an adequate description of the results of experimental measurements of the diagonal and off-diagonal components of the wire Giant Magneto- Impedance tensor at moderate frequencies has been obtained.