Glass-covered Amorphous Microwires Research Articles

Determination of Magnetic Structures in Magnetic Microwires with Longitudinally Distributed Magnetic Anisotropy.

We studied the magnetic properties of a glass-covered amorphous microwire that was stress-annealed at temperatures distributed along the microwire length. The Sixtus-Tonks, Kerr effect microscopy and magnetic impedance techniques have been applied. There was a transformation of the magnetic structure across the zones subjected to annealing at different temperatures. The annealing temperature distribution induces the graded magnetic anisotropy in the studied sample. The variety of the surface domain structures depending on the longitudinal location has been discovered. Spiral, circular, curved, elliptic and longitudinal domain structures coexist and replace each other in the process of magnetization reversal. The analysis of the obtained results was carried out based on the calculations of the magnetic structure, assuming the distribution of internal stresses.

Helical magnetic structures in magnetostrictive amorphous microwires

Surface magnetic structures have been studied in amorphous glass-covered microwires using the magneto-optical technique. Different types of helical structure have been recognized depending on the external conditions. The simulation of the magnetic structure in magnetic cylinder demonstrates dependence of the type of the structure on the value of the magnetostriction coefficient. The performed comparative analysis of experiment and results of the simulation permitted to establish the direct relation between the value of the magnetostriction coefficient and the type of the helical structure.

Tuning of Magnetic Properties of Magnetic Microwires

Techniques for fine tuning the magnetic properties of glass-covered amorphous microwires are discussed. They include selection of nominal components and geometric proportions, conditions for thermal treatment, and application of mechanical stress and crossed magnetic fields.

The profile of the domain walls in amorphous glass-covered microwires

We have studied the domain wall dynamics in Joule-annealed amorphous glass-covered microwires with positive magnetostriction in the presence of an electric current, in order to evaluate the profile and shape of the moving domain wall. Such microwires are known to present magnetic bi-stability when axially magnetized. The single domain wall dynamics was evaluated under different conditions, under an axially applied stress and an electric current. We have observed the well known increasing of the domain wall damping with the applied stress due to the increase in the magnetoelastic anisotropy and, when the current is applied, depending on the current intensity and direction, a modification on the axial domain wall damping. When the orthogonal motion of the domain wall is considered, we have observed that the associated velocity present a smaller dependence on the applied current intensity. It was observed a modification on both the domain wall shape and length. In a general way, the domain wall evolves from a bell shape to a parabolic shape as the current intensity is increased. The results were explained in terms of the change in the magnetic energy promoted by the additional Oersted field.

Study of left-handed characteristics of parallel microwires based metastructures

The aim of this work is to present a complex experimental and theoretical study of the ferromagnetic resonance frequencies and plasma behavior for CoFe-based, Co-based, Fe-based and FINEMETTM glass covered amorphous microwires (GCAWs), arranged in parallel configurations as free-standing systems, as well as combinations thereof, in order to obtain metastructures with tailored and enlarged left-handed frequency domain. The negative magnetic properties interval of the metastructures is expanded by alternating different types of GCAWs with close natural ferromagnetic resonance (NFMR) and natural antiferromagnetic resonance (NFMAR) domains, respectively.

Left-handed metastructures with selective frequency transmission window for gigahertz shielding applications

Experimental evidence of left-handed properties of the Fe-, CoFe-, and Co-based glass-covered amorphous microwires in free standing systems is demonstrated. A new broadband frequency shielding metastructure with selective transmission frequency in the microwave range is presented. The X-band microwave-guide and the free-space methods were used as experimental techniques in the frequency range from 1 to 12 GHz. The X-band experimental results show that the mixed metastructure presents left-handed behavior between 8.5 and 10.5 GHz. The mixed metastructure for large areas presents broadband frequency domains, with left-handed properties between 6.5 and 10.5 GHz (the sum of intrinsic ferromagnetic and antiferromagnetic resonance frequencies domains of individual microwires) and a transmission window between 7.6 and 10.5 GHz. Double negative characteristics of the metastructures are in agreement with the computed results calculated based on Landau–Lifshitz–Gilbert equations and Nicolson-Ross-Weir analytical method.

Transformation of magnetic structure in amorphous microwires induced by temperature and high frequency magnetic field

The study of the magnetic properties of glass-covered amorphous microwires is reported. This study is focused on the influence of a high frequency electric current and of temperature upon the surface magnetic domain structure. Investigations are performed using the magneto-optical Kerr effect technique, and it is found that the high frequency current and heating process have a great and essential influence upon the surface magnetization reversal and surface domain structure. The induced formation and transformation of the surface magnetic structure are key processes that determine the stable operation of giant magneto-impedance devices.

Effect of Electric Current on Domain Wall Dynamics

In this paper, we determine the domain wall velocity in the low field regime and study the domain wall dynamics in Joule-annealed amorphous glass-covered microwires with positive magnetostriction. Such microwires are known to present magnetic bi-stability when axially magnetized. In order to measure the domain wall dynamics, an electrical current was applied to the wire simultaneously to the driving magnetic field. When the dc current is applied to the sample, an increase or a decrease is observed on the axial domain wall velocity, depending on the current direction. These results are explained in terms of the change in the magnetic energy promoted by the additional Oersted field which, by its time, modifies the length of the conical domain wall, in such a way that the orthogonal domain wall velocity is not changed by the applied current.

In situ microwave characterization of microwire composites with external magnetic field

Rubber composites containing Co68.15Fe4.35Si12.25B15.25 glass-covered amorphous microwires were fabricated. For samples containing 5 mm long wires and randomly dispersed in the polymer matrix, our results demonstrate that the current induces resonance of circumferential permeability (magnetoimpedance resonance) resulting in a significant change of the effective permittivity of the wire-filled composites with application of a high magnetic field. For samples containing longer wires (70 mm) periodically arranged in the polymer matrix, the permittivity spectrum shows two resonance peaks due to, respectively, dipole and magnetoimpedance resonance. The induced magnetoimpedance resonance has a narrower linewidth than the dipole one as magnetic field is increased.

Study of Stress Annealing on Giant Magneto-Impedance in Glass-Covered Co-Based Amorphous Microwires

The glass-covered Co-rich amorphous wires was annealed by an applied stress during the current annealing process. The influence of stress and dc current annealing on the giant magneto-impedance (GMI) effect were investigated by the magneto-impedance. Our experiments showed that the magentic anisotropy is reduced by the annealing treatments, furthermore, the maximum GMI value is increased to 105 %.The GMI value is enhanced by increasing the outer domain volume and inducing a transverse anisotropy due to the changing of inner stress.

Influence of AC Driving Current on Magneto-Impedance Effect in Glass-Covered Co-Based Microwire

The influence of ac driving current on the frequency dependence in the range from 0.1 to 20 MHz of the giant magnetoimpedance effect (GMI) of glass-covered amorphous microwires were investigated. The dependence of GMI for a range of values of ac current (from 0.5 mA to 20 mA) flowing along the microwire has been measured in as-prepared samples. The double-peak feature is changed to the single-peak festure and the magnetic field at which the value of GMI reaches a maximum should increase with the higher the amplitude of the ac at low frequency. It was found that the change of GMI related to circular coercivity.

Estimation of the internal stresses in amorphous glass-covered microwires

A technique is developed to estimate the internal stresses in amorphous glass-covered Co69Fe4Cr4Si12B11 alloy microwires with micron-scale cross sections that are produced by melt quenching by the Taylor-Ulitovskii method. In the developed model, a microwire is considered as a microcomposite to take into account the internal stresses that are induced by the difference between the thermal expansion coefficients of glass and metal during rapid cooling from the solidification temperature to room temperature. The radial, tangential, and axial stresses are calculated in both a glass shell and a metallic core using experimental data on the elastic moduli and the geometric sizes of these structural components of a microwire. The mechanical properties are determined by selective nanoindentation of transverse polished sections of microwires.

Magnetization reversal in thin glass covered amorphous microwires with helical anisotropy

The surface magnetization reversal has been studied in Co-rich amorphous glass-covered microwires. The studies have been performed by the magneto-optical Kerr effect surface loop tracer for the series of the microwires with diameters of 16.8–5.8 μm. The experiments have been carried out in the crossed axial and circular magnetic fields. The variety of the magnetic behaviour has been found in dependence on the diameter of the microwire, that confirms the existence of the helical domain structure in the Co-rich microwires. The special behaviour has been discovered for the wire with the diameter of 10 μm. From this results we can conclude that the axially magnetized inner core disappears for this value of the microwire diameter and that the domain structure of the microwire consists only of the helical domain structure. The experiments are in good agreement with calculations based on the model taking into account the existence of the helical anisotropy in the surface area of the microwires.

Magnetic Properties and the Giant Magneto-Impedance in Joule-Heated Glass-Covered Co-Based Microwire

The influence of DC current annealing on magnetic properties and the frequency dependence in the range from 0.1 to 20 MHz of the giant magnetoimpedance effect (GMI) of glass-covered amorphous microwires were investigated. Under a certain annealing condition (10 min annealing with applied current Ia=110 mA), the maximum change of impedance was about 200% with a maximum slope sensitivity of 0.26%/Am-1 . Further treatment with increased current resulted in a decrease of the maximum MI ratio. Meanwhile, annealing can produce short-range order relaxation and consequently improves the sample's soft magnetic properties. We also found that the GMI ratio increased due to the enhanced induced anisotropy with increasing current under the same Joule energy.

M - H loop tracer based on digital signal processing for low frequency characterization of extremely thin magnetic wires

A high-sensitivity ac hysteresis loop tracer has been developed to measure the low frequency hysteresis loop of soft magnetic materials. It has been applied successfully to characterize straight pieces of amorphous glass-covered microwires with metallic nucleus down to 1.5 microm thick. Based on the electromagnetic induction law, the proposed design is extremely simple and exploits the capabilities of commercially available data acquisition cards together with digital signal processing in order to achieve high-sensitivity without the need of expensive analog equipment.

Mechanical properties of magnetic Fe‐based and Co‐based amorphous wires and microwires

AbstractIn this paper we present measurements of the mechanical properties of magnetically soft Fe‐based and Co‐based conventional amorphous wires (CAW) and glass‐covered amorphous microwires (GCAW) in the as‐quenched state, before and after cold rolling, and after annealing for internal stresses relief. The tensile strength (σ) varies between 2000 MPa and 3400 MPa, strongly depending on CAW diameter and annealing temperature. The Young modulus (E) is following a descendant trend, decreasing with the diameter decrease. The increase of tensile strength (σ) and elongation (ε) values after cold‐rolling and post‐annealing is caused by the release of the internal stresses induced during the preparation. Tensile strengths up to 3900 MPa are achieved for Fe‐based GCAW, depending very strongly on the microwires diameter, but mostly on the glass cover thickness. σ reaches the maximum value of 3150 MPa for a Co‐based microwire having the glass cover of 9 μm and the total diameter of 43 μm, but increases even more after the glass removal (3350 MPa). This behaviour is attributed to the structural disorder and the specific magnetic domains structure in Fe‐based and Co‐based CAW and GCAW. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin relaxation and domain wall dynamics in glass‐coated microwires

AbstractThe dynamic properties of the magnetic domain walls in amorphous glass‐covered microwires were studied. The samples was Joule annealed with an alternate current at different frequencies and the dynamic behaviour of the samples were studied at different applied stresses. The Landau–Lifshitz–Gilbert damping parameter and the domain wall mobility have shown that the dynamic response can be improved by increasing the annealing current frequency. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonlinear magnetoimpedance and parametric excitation of standing spin waves in a glass-covered microwire

The giant magnetoimpedance of an 8.5 μm glass-covered amorphous microwire was investigated in the frequency range of 10 MHz–3.5 GHz. It was found that when the exciting microwave current exceeds some threshold value, a periodic fine structure appears in the frequency dependence of the complex impedance. The appearance of this nonlinear phenomenon is interpreted to be a consequence of the parametric excitation of standing spin waves.

Annealing effects on the magnetization reversal and domain wall dynamics in bistable amorphous glass-covered microwires

We study the mechanism of domain wall depinning and propagation in Fe77.5Si7.5B15 amorphous glass-covered microwires. These samples are ferromagnetic and spontaneously present the striking property of magnetic bistability, with the magnetization reversal process characterized by a single giant Barkhausen jump. This process allows one to easily measure the velocity of a domain wall propagating along the sample as a function of applied magnetic field and temperature and thus determine the wall mobility and the influence of different damping mechanisms on the domain wall dynamics. A discussion on the effects of thermal treatment on the wall mobility is presented. Domain wall velocities of the order of 850 m s−1 are measured. The temperature dependence of the wall mobilities established at 300 and 77 K, for as-cast and annealed samples, clearly shows a damping mechanism arising from the structural relaxation mechanism and from domain wall/atomic defect interactions, the second one playing a major role in the dissipative dynamics of magnetization reversal through depinning and domain wall propagation in Fe77.5Si7.5B15 microwires.

Comparison between ac and dc current annealing in CoFeSiB glass-covered amorphous microwires

In this work, we present a study on the magnetic properties of Joule-heated CoFeSiB glass-covered amorphous microwires under applied stress. A comparison between the magnetic properties of four samples is made: (i) as cast, Joule-heated under (ii) dc current or ac current of (iii) 100 Hz or (iv) 500 Hz. The stress modifies the anisotropy and the domain structure in such a way that it can be studied through magnetoimpedance measurements and ferromagnetic resonance (FMR) dispersion relations (DR) extracted from them. From the fitting of the FMR DR, the magnitude and the orientation of the transverse anisotropy field, as well as an insight into the micromagnetic structure of the microwires were obtained. It was concluded that longitudinal anisotropy dominates the microwire behaviour under zero stress. By applying an increasing stress to the microwires, the inner core with longitudinal anisotropy is surrounded by two concentric outer shells with almost circumferential anisotropies. These magnetic configurations can be explained in terms of the residual stress arising from the wire's manufacturing procedure and the additional applied stress.