The GMI effect was investigated in microwires based on a Fe70Si18B9Cu1Nb2 alloy (rho = d/D = 0.5, d = 15 mu m) after heat treatment at 550 degrees C resulting in the formation of a non-uniform nanocrystalline structure. It was shown experimentally that short heat treatments (<= 2 min 40 s) led to the emergence of alpha-Fe(Si) nanocrystals near the surface of a microwire while the central part of the microwire remained amorphous. Simultaneous processes of stress relaxation and formation of the nanocrystalline structure induced an enhancement of the GMI effect. A maximum value of Delta Z/Z = 150 % was observed at a frequency of 200 MHz. With an increase in the heat treatment time, the soft magnetic properties of the surface layer degenerated, resulting in the degradation of the observed GMI effect.

The magnetic domain structure of the surface of microwires with composition Fe73.9B13.2Si10.9C2 was studied by magnetic force microscopy. It has been found that the removal of glass shell by chipping leads to distortion of the original magnetic domain structure. Chemical etching of the glass shell makes it possible to observe the magnetic domain structure due to the stresses that have arisen due to the microwire production. In the absence of an applied magnetic field, a magnetic domain structure of the surface layer is observed, consisting of domain layers inclined to the microwire axis by 45 or 135 degrees. This structure has a shape close to a zigzag. The thickness of the domain layers is not constant and varies from 3 to 5 μm. It has been found that the application of a constant magnetic field along the microwire axis causes the formation of ring domain layers of various thicknesses (from 1 to 5 μm) with different orientations of the magnetic moment relative to the microwire surface. In a field of 60 oersteds along the axis of the microwire, the domain magnetic structure consists of only ring layers of domains. Magnetic field inversion leads to almost complete inversion of the observed domain structure. In this case, the complete removal of the magnetic field leads to the formation of a new domain structure of the surface layer. Such a structure is close in shape and position of the domains to the original one, but does not repeat it.

The magnetic domain structure of the surface layer of Fe-based amorphous microwires with a diameter of 20 mu m (after glass removal) was investigated. An approach has been developed to study the domain structure of the curved surface of a microwire by magnetic force microscopy. It was found that in low magnetic fields the domain structure of the surface layer consisted of zigzag-shaped domains elongated along the microwire axis. These domains are inclined to the microwire axis at an angle of 45 degrees. An increase in the magnetic field along the microwire axis resulted in the transformation of the magnetic structure to the structure consisting of ring domains with an opposite orientation of the magnetic moment. The geometric parameters of both types of the domain structure were determined. The width of zigzag-shaped domains was 2 mu m. The width of ring domain was 2.5-5 mu m.

A change in the structure of an amorphous Zr55Cu30Al10Ni5 alloy under deformation by high-pressure torsion (HPT) was studied by X-ray diffraction, high-resolution electron microscopy, scanning electron microscopy, and atomic force microscopy. It was found that the uneven distribution of deformation along the radius of the sample, characteristic of deformation by high-pressure torsion, led to the formation of an inhomogeneous structure. The formation of nanocrystals begins at the periphery of the sample. The threshold value of deformation required for crystallization onset was established; the formation of nanocrystals begins in areas with true deformation e = 4.83 or more. An increase in the deformation degree led to an increase in the height of steps on the deformed sample surface and an increase in the roughness of the surface. The thickness of an elementary step that was formed when one shear band came out to the surface was 10 nm, and its height was about 1 nm. It was found that large steps on the deformed surface of the sample had a complex structure and consisted of a large number of elementary steps. The results obtained are important for analyzing the stress distribution and the concentration of free volume in a deformed material, which affect the parameters of the amorphous-nanocrystalline structure formed.

The shear bands formed under rolling in an amorphous alloy and their relation with the steps on the surface have been investigated. The studies were carried out by scanning, transmission electron, and atomic force microscopy. It was shown that a shear band began on a surface step. The thickness of the shear band was about 10 nm and varied from place to place. Secondary shear bands were found, which were located at an angle of 70 degrees to the shear band come out to the surface. The regions of the shear band had light and dark contrast, which indicated a change in the density of an amorphous phase along the shear band. The steps on the surface had a complex shape and consisted of elementary steps. The thickness of the elementary steps corresponded to that of the shear band.

Amorphous microwires of Fe 73.8 Si 13 B 9.1 Cu 1 Nb 3.1 andFe 77.5 Si 13.5 B 9 composition fabricated by the Ulitovsky--Taylor method were studied. The samples with the glass shell removed were heated at temperatures of 753 K and 703 K for 20 min, afterwards, their structure was examined using X-ray diffraction. Subsequently, the thermally treated samples were chemically etched and X-ray diffraction study of the structure was again carried out. Experimental results on the predominant crystallization of near-surface regions were discussed assuming that mechanical stresses affect the nucleation and growth of nanocrystals. Keywords: amorphous materials, stress distribution, nanocrystallization, X-ray diffraction.

Amorphous microwires of Fe73.8Si13B9.1Cu1Nb3.1 and Fe77.5Si13.5B9 composition fabricated by the Ulitovsky–Taylor method were studied. The samples with the glass shell removed were heated at temperatures of 753 K and 703 K for 20 minutes, afterwards, their structure was examined using X-ray diffraction. Subsequently, the thermally treated samples were chemically etched and X-ray diffraction study of the structure was again carried out. Experimental results on the predominant crystallization of near-surface regions were discussed assuming that mechanical stresses affect the nucleation and growth of nanocrystals.

Within the framework of this work, a new method for tensile fatigue testing of thin microwires and wires has been developed in accordance with GOST 25.502-79. The developed method has been tested on amorphous microwires of composition Fe 77.5 Si 7.5 B 15 in glass insulation. It has been established that the deformation mode in the stress range from 0 to 700 MPa corresponds to the long-term use of the studied microwires (millions of cycles). In addition, the studied microwires are capable of withstanding hundreds of thousands of tensile cycles at stresses from 700 to 1100 MPa.

The bulk-inhomogeneous crystallization of amorphous microwires of the composition Fe73.8Cu1Nb3.1B9.1Si13 is studied. An assumption is put forward concerning the influence of the inhomogeneous distribution of tensile and compressive stresses in the bulk of microwires on their crystallization. It is established that, at the initial stages of crystallization, crystallization occurs in the surface region of the microwire with a thickness of about 2.5 μm. It is established that the sizes of the nanocrystals in the surface region of the microwire are about 10 nm. It is found that the formation of an amorphous nanocrystalline layer on the microwire surface leads to an increase in the Mr/Ms ratio (ratio of remanent magnetization to saturation magnetization), which is associated with a decrease in the magnetic anisotropy due to a decrease in the stress level during heat treatment and nanocrystallization. The chemical etching of annealed microwires leads to a significant increase in the Mr/Ms ratio, which is due to an increase in the relative volume of the central domain layer. The results obtained indicate the potential for creating composite amorphous-nanocrystalline structures based on microwires. In the case of microwires of Fe73.8Cu1Nb3.1B9.1Si13 composition, the predominant crystallization of the surface layer can increase the effect of the giant magnetic impedance. Such objects may have potential applications in sensorics, in particular, in magnetic field and strain sensors.

The early stages of nanocrystallization in amorphous Fe73.8Si13B9.1Cu1Nb3.1 ribbons and microwires were compared in terms of their internal stress effects. The microstructure was investigated by the X-ray diffraction method. Classical expressions of crystal nucleation and growth were modified for microwires while accounting for the internal stress distribution, in order to justify the XRD data. It was assumed that, due to the strong compressive stresses on the surface part and tensile stresses on the central part, crystallization on the surface part of the microwire proceeded faster than in the central part. The results revealed more rapid nanocrystallization in microwires compared to that in ribbons. During the initial period of annealing, the compressive surface stress of a microwire caused the formation of a predominantly crystallized surface layer. The results obtained open up new possibilities for varying the high-frequency properties of microwires and their application in modern sensorics.

The investigation of domain structure of Co-based microwires with negative magnetostriction was provided by magnetooptical method of indicator films. The thickness of the domain layer and the width of the surface domains were determined. It has been suggested that in thin microwires (with the diameter of about several tens of microns) with negative magnetostriction, the Neel-type of domain walls between the domains of the surface layer are observed. A number of assumptions about the regularities of the surface domain structure formation of microwires with negative magnetostriction have been put forward.

A method for measuring adhesion between the glass shell and the metallic nucleus of an amorphous microwire was developed. It was revealed that the stress of separation of the glass shell from the metallic nucleus (pull-off stress) can vary from hundreds of MPa to units of GPa. It was found that the adhesion value between the glass shell and the metallic nucleus is 270 MPa. It was found that the annealing of microwires below crystallization temperature can lead to a more than 2-fold increase in the adhesion. The effect of adhesion on the stress state and coercivity of microwires under tension was analyzed.

The effect of tensile stresses on the magnetic properties of amorphous microwires with positive magnetostriction has been studied. It has been determined that the experimental dependence of coercivity on the tensile stresses is reversible and consists of linear and square root parts. The evolution of an average stress level in the "core" and surface domain layer has been theoretically estimated depending on the external tensile stress. The value of internal stress level may vary from 150 to 600 MPa in "core" for microwires with different ratio of amorphous metallic nucleus diameter to thickness of glass shell. Crucial differences of tension of glass-coated and uncoated microwires have been investigated. The analysis of these differences with relation to effect under tension has been performed. (C) 2020 Elsevier B.V. All rights reserved.

The dependence of the magnetic structure and the hysteresis properties of the amorphous microwires on the applied load is investigated by the magnetic-optical method of the indicator films and the inductive pickup coils. We examine the amorphous microwires of Fe73.5Cu1Nb3Si13.5B9 and Fe77.5Si7.5B15, with a positive magnetostriction and different thickness of the glass coating and the diameter of the metal core. An increase in the stress level can be achieved by stretching along a microwire axis. The hysteresis properties and magnetic domain structure are examined during in situ stretching. Coercivity versus tension curves have been plotted for the examined microwires. It is shown that an increase in the stress leads to an increase in the coercivity.

Magnetic microwires are unique materials with a wide range of magnetic and high-frequency properties that are of considerable interest from the point of view of the application for magnetic sensors based on microwires. The magnetic properties of microwires directly depend on the level and distribution of stresses. Therefore, the study of the magnetic properties of microwires, depending on their stressed state, is of considerable interest. This paper describes a method for measuring the hysteresis properties of amorphous microwires under stretching in situ. An experimental facility has been developed. It allows measuring the dependences of the coercivity of microwires under stretching in situ with an error of no more than 8 A/m with an external magnetizing field of 2000 A/m and a frequency of 2.5 kHz.

The current research describes a non-contact method for stress monitoring based on stress dependence of magnetic properties of ferromagnetic microwires as well as composites with embedded microwires. We investigated the change of various magnetic properties of Fe-based amorphous microwires under tensile stress. The amplitude of the electromotive force (EMF) signal due to the Barkhausen jumps, the saturation magnetostriction and the coercivity were studied. It was found that the dependence of the EMF amplitude on the axial tensile stress of microwire exhibits a maximum. The value of internal tensile stresses that arise during the preparation of glass-coated microwires was estimated (about 200 MPa). It was shown that a wider range of measuring stresses under the proposed method yields the use of Finemet type microwires in the amorphous-crystalline state as well as amorphous microwires without glass coating. (C) 2018 Elsevier B.V. All rights reserved.