Co-based Amorphous Microwires Research Articles

Superlattice-shelled nanocrystalline core structural design for highly sensitive GMI sensors

Giant magnetic impendence (GMI) sensors are known as a significant pillar of modern technology, and has many applications in smart society such as automation, navigation and bioengineering. Developing high-sensitive GMI sensors operating under extremely weak magnetic fields is highly desirable yet challenging. Here, we proposed superlattice-shelled nanocrystalline core structural design in the cobalt-based amorphous sensor-core microwires for the highly sensitive GMI sensors. Continuous Co-based amorphous microwires with a length up to thousands of meters and uniform diameter is first fabricated by a scalable modified Taylor–Ulitovsky method and then processed by the picosecond lasering heat treatment. As a result, the microwire exhibits a unique nanostructure consisting of ultrafine nanocrystalline cores surround by the thermal-stable Cr-rich superlattice shell, and hence circumferential magnetic domains of the higher density and intensity. The GMI sensors made from nanostructured Co-based microwires show a sensitivity up to 1743 mV/Oe, the maximum GMI ratio up to 78%, increasing the performance by up to twice and nine times respectively over the GMI sensor made from the amorphous counterpart, and a wide linear range of ± 70000 nT. The correlation between the microstructure, magnetic domain structure and GMI performance is also established and discussed in the nanostructured Co-based microwires. Furthermore, we demonstrate the successful application of the highly sensitive GMI sensor in geomagnetic communication to detect underground energy transmission metal pipes with a detection resolution down to pT-level weak magnetic fields.

Advanced structure research methods of amorphous Co69Fe4Cr4Si12B11 microwires with giant magnetoimpedance effect: Part 2 – Microstructural evolution and electrical resistivity change during DC Joule heating

This Part 2 focuses on the processes of atomic regions ordering of the amorphous material and continues study of the processes occurring in Part 1. The formation of pre-crystals / clusters and nanocrystallization of Co-based amorphous microwires during DC Joule heating were investigated, as well as the influence of these processes on the electrical resistance of the samples and the effect of giant magnetoimpedance (GMI). In this paper we investigated the dependence of the change in the maximum response of the GMI diagonal component at different temperatures and holding times in the DC Joule heating process, and discuss the effect of the ductile-brittle transition on the electrical resistance and the GMI effect. Using high-resolution transmission electron microscopy (HR-TEM) and the inverse Fourier transform technique, it was shown that at long exposure during the DC Joule heating, significantly below the crystallization temperature (~ 400 °C), the formation and growth of pre-crystals / clusters occur in various sizes (1 – 8 nm) and shapes. At the initial stage of crystallization, the microwire has three distinct regions of the clusters and nanocrystals formation (surface, pre-surface, and bulk). Moreover, pre-crystals / clusters growth and further bulk formation of nanocrystals (of 10 nm and more) leads to a sharp drop in the electrical resistance and degradation of the GMI microwire diagonal component response. The approach of controlled long-term holding during the DC Joule heating applied in the study allows for creation of various nanoscale structures in an amorphous matrix and provides an opportunity to grow of a given structure in amorphous alloys.

Direct Current Annealing Modulated Ordered Structure to Optimize Tensile Mechanical Properties of Co-Based Amorphous Metallic Microwires

Herein, the ordered structure of Co-based metallic microwires was modulated by direct current-annealing, thereby improving the tensile mechanical properties. Based on the thermophysical parameters of the metallic microwires, the annealing current intensities of 65 mA, 90 mA and 150 mA were determined by the method of numerical calculation. The experimental results indicated that the ordered structure of the metallic microwires was regulated under the action of Joule heating, and with the rising of the annealing current, the ordered structure increased and the distribution tended to be concentrated. The 90 mA current-annealed metallic microwires have favorable tensile mechanical properties and fracture reliability, with the tensile strength and elongation of 4540.10 MPa and 2.99%, respectively, and the fracture threshold is 1910.90 MPa. Both the as-cast and current-annealed metallic microwires were brittle fractures, and the fractures consisted of shear deformation regions and crack extension regions. The improvement of the mechanical properties of metallic microwires is related to the nano-ordered structure and their distribution. Under the condition of 90 mA current annealing, the uniformly distributed nano-ordered structures were formed in the amorphous matrix of the metallic microwires, which can effectively slow down the expansion of the shear bands and reduce the possibility of crack generation. This study provides process reference and theoretical guidance for the application of Co-based metallic microwires in the field of stress sensors.

Excellent soft magnetic properties in Co-based amorphous alloys after heat treatment at temperatures near the crystallization onset

The effect of heat treatment of Co-based amorphous microwires at temperatures near the crystallization (Tcr) onset on micromagnetic structure, magnetostriction, and magnetoimpedance (MI) was investigated. As the annealing temperature approaches Tcr, the hysteresis loops abruptly change the shape from almost rectangular to inclined with a small coercivity and remanence magnetization. This behavior is quite unique since even partial crystallization deteriorates soft magnetic properties. The Kerr-microscopy confirmed the formation of a fine circular domain structure and an easy anisotropy direction of a helical type. Both current and furnace annealing with appropriate high temperatures produce similar magnetic transformation. The origin of the formed easy anisotropy is different from that induced by current annealing at temperatures below the Curie temperature and is associated with a change in the magnetostriction constant, which drops from positive to negative values. This could be explained by structural relaxation leading to amorphous phase decomposition at the very initial stage of crystallization. The effect is not specific for the Co-rich alloy composition and was observed in microwires of two compositions: Co71Fe5B11Si10Cr3 and Co66.6Fe4.28B11.51Si14.48Ni1.44Mo1.69 which in the as-prepared state have axial and circumferential anisotropies, respectively. The obtained combination of magnetic anisotropy, magnetostriction and temperature stability leads to useful MI properties, in particular, at Gigahertz frequencies.

Boosting the Tunable Microwave Scattering Signature of Sensing Array Platforms Consisting of Amorphous Ferromagnetic Fe2.25Co72.75Si10B15 Microwires and Its Amplification by Intercalating Cu Microwires.

The following work addresses new configurations of sensing array platforms that are composed of Co-based amorphous ferromagnetic microwires (MWs) to obtain an enhanced modulation of the microwave scattering effects through the application of low strength DC or AC magnetic fields. An amorphous MW is an ultrasoft ferromagnetic material (coercivity ~0.2 Oe) with a circumferential magnetic anisotropy that provides a high surface sensitivity when it is subjected to an external magnetic field. Firstly, microwave scattering experiments are performed as a function of the length and number of MWs placed parallel to each other forming an array. Subsequently, three array configurations are designed, achieving high S21 scattering coefficients up to about −50 dB. The influence of DC and AC magnetic fields on S21 has been analyzed in frequency and time domains representation, respectively. In addition, the MWs sensing array has been overlapped by polymeric surfaces and the variations of their micrometric thicknesses also cause strong changes in the S21 amplitude with displacements in the frequency that are associated to the maximum scattering behavior. Finally, a new concept for amplifying microwave scattering is provided by intercalating Cu MWs into the linear Co-based arrays. The designed mixed system that is composed by Co-based and Cu MWs exhibits a higher S21 coefficient when compared to a single Co-based MW system because of higher electrical conductivity of Cu. However, the ability to modulate the resulting electromagnetic scattering is conferred by the giant magneto-impedance (GMI) effects coming from properties of the ultrasoft amorphous MWs. The mixed array platform covers a wide range of sensor applications, demonstrating the feasibility of tuning the S21 amplitude over a wide scattering range by applying AC or DC magnetic fields and tuning the resonant frequency position according to the polymeric slab thickness.

Advanced structure research methods of amorphous Co69Fe4Cr4Si12B11 microwires with giant magnetoimpedance effect: Part 1 – Crystallization kinetics and crystal growth

The initial stage processes of nucleation and growth of crystalline phases in Co-based amorphous microwires in a glass shell were studied by processing kinetic curves using various theoretical models. The results of X-ray diffraction for research of the microstructure evolution at Synchrotron Radiation Source and High Resolution Transmission Electron Microscopy were considered. The study of the crystallization process kinetics was carried out by a non-conventional method, in particular based on data of sample magnetization changes over time and with temperature at a unique in situ vibromagnetometer. The analysis of the kinetic curves for isothermal and non-isothermal crystallization process led to the similar conclusions and made it possible to establish the values of the Avrami exponent for the initial stage of surface and bulk crystallization, to establish the dimensionality of crystal growth, and to conclude that there was no nucleation in the process of crystal growth. Based on the analysis of kinetic curves by three methods, the activation energy of the primary crystallization process was determined, its high value is associated with the presence of chromium in the alloy, which determines its nanocrystallization. The microstructure and shape of crystals were analyzed.

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.

Scattering of Microwaves by a Passive Array Antenna Based on Amorphous Ferromagnetic Microwires for Wireless Sensors with Biomedical Applications.

Co-based amorphous microwires presenting the giant magnetoimpedance effect are proposed as sensing elements for high sensitivity biosensors. In this work we report an experimental method for contactless detection of stress, temperature, and liquid concentration with application in medical sensors using the giant magnetoimpedance effect on microwires in the GHz range. The method is based on the scattering of electromagnetic microwaves by FeCoSiB amorphous metallic microwires. A modulation of the scattering parameter is achieved by applying a magnetic bias field that tunes the magnetic permeability of the ferromagnetic microwires. We demonstrate that the OFF/ON switching of the bias activates or cancels the amorphous ferromagnetic microwires (AFMW) antenna behavior. We show the advantages of measuring the performing time dependent frequency sweeps. In this case, the AC-bias modulation of the scattering coefficient versus frequency may be clearly appreciated. Furthermore, this modulation is enhanced by using arrays of microwires with an increasing number of individual microwires according to the antenna radiation theory. Transmission spectra show significant changes in the range of 3 dB for a relatively weak magnetic field of 15 Oe. A demonstration of the possibilities of the method for biomedical applications is shown by means of wireless temperature detector from 0 to 100 °C.

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.

Temperature Effects on the Magnetization and Magnetoimpedance in Ferromagnetic Glass-Covered microwires

The effect of temperature on static and dynamic magnetization in Co-based amorphous microwires was investigated with the aim of potential applications in miniature temperature sensors. The wires of two compositions with different magnetostriction and Curie temperature in glass-cover and after removing the glass layer demonstrated very different temperature behaviour of the magnetization loops and magnetoimpedance. The mechanisms of the temperature effects are related to the residual stress distribution due to fast solidification, the difference in thermal expansion coefficient of metal and glass and the proximity to the Curie temperature. The interplay of these factors may result in a very strong temperature dependence of magnetoimpedance in a moderate temperature range (room temperature −373K). Such elements may be incorporated in various composite materials for a local temperature monitoring.

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.

Tuning of Magnetic Properties and GMI Effect of Co-Based Amorphous Microwires by Annealing

We studied the effect of annealing on the giant magnetoimpedance (GMI) effect, magnetic domain wall dynamics, and magnetic properties of amorphous iron (Fe) and cobalt (Co)-based microwires prepared by the Taylor–Ulitovsky technique. We observed that the properties can be tailored by controlling the magnetoelastic anisotropy of CoFeBSiC microwires during wire formation and also controlling the magnetic anisotropy by further heat treatment. A high GMI effect has been observed in the as-prepared Co-based microwires. High domain wall velocity and rectangular hysteresis loops have been observed in additionally heat-treated microwires. We observed increasing of the wall velocity under stress in some annealed samples. We demonstrated that, for certain annealing conditions, we can observe coexistence of the GMI effect and magnetic domain wall propagation in the same sample.

Tensile properties and fracture reliability of a glass-coated Co-based amorphous microwire

Co68.15Fe4.35Si12.25B15.25 (at%) amorphous microwires with a smooth surface and a circular cross-section were fabricated by the glass-coated melt spinning method. Their mechanical properties were evaluated through tensile tests of the glass-coated amorphous microwires, and their fracture reliability was estimated using two- and three-parameter Weibull analysis. X-ray diffraction and transmission electron microscopy results showed that these glass-coated Co-based microwires were mostly amorphous. The coated Co-based microwires exhibit a tensile strength of 1145 to 2457 MPa, with a mean value of 1727 MPa and a variance of 445 MPa. Weibull statistical analysis showed that the tensile two-parameter Weibull modulus of the amorphous microwires is 4.16 and the three-parameter Weibull modulus is 1.61 with a threshold value as high as 942 MPa. These results indicate that the fabricated microwires exhibit good tensile properties and fracture reliability, and thus appear to be good candidates for electronics reliability engineering applications.

FMR linewidth and the crystallization processes in Co-based amorphous microwires

The ferromagnetic resonance (FMR) technique has been used for many years as a tool to characterize the dynamic magnetic properties of bulk, thin film magnetic materials and microwires. Also, the FMR linewidth is associated to the dissipative processes involved on the magnetization dynamics. Such dissipative channels may be separated in two groups: intrinsic and extrinsic ones. While the first one is related mainly to the Landau–Lifshitz (LL) and eddy-currents damping mechanisms, the second one take in to account contributions coming from the inhomogeneities in the sample. Each one of these relaxation processes contributes to the FMR linewidth in a different frequency and/or magnetic field range and they can be separated by an appropriate FMR linewidth measurement. This work presents FMR measurements, obtained by the impedance method, ranging from 100KHz to 1.8GHz, in Joule annealed Co-based amorphous microwires. It is shown that the crystallization process can be followed by the evaluation of the extrinsic magnetization damping terms, specifically analyzing contributions from inhomogeneities to the FMR linewidth. From the fitting of models which consider LL damping term, anisotropy dispersion and magnon scattering to the experimental data, three ranges of annealing temperatures can be distinguished in terms of spin dynamics on the studied samples: annealing temperatures lower than the Curie temperature, a temperature range between the Curie and the crystallization temperature and another temperature range above the crystallization temperature.

Relating residual stress and microstructure to mechanical and giant magneto-impedance properties in cold-drawn Co-based amorphous microwires

The effect of cold-drawing on the tensile property and giant magneto-impedance (GMI) effect of melt-extracted Co-based amorphous microwires was evaluated through detailed analyses of the distribution of residual stress and microstructural evolution. The tensile ductility and tensile strength increased gradually with cross-sectional area reduction ratio (R) until 51%, and decreased with further deformation. The microwire with R=51% exhibits the highest tensile ductility of 1.09% and tensile strength of 4320MPa. Structural and thermodynamic analyses reveal that it is the mechanical deformation rather than thermal activation that induces the precipitation of nanocrystals and arrests the quick extension of shear bands leading to the enhanced ductility. Interestingly, the GMI effect also attains the maximum value of 160% at 10MHz when R=51% (30% larger than that of the as-cast wires), before decreasing with further cold-drawing. Such an identical evolution trend of both tensile and GMI properties can be ascribed to two underlying mechanisms: the generation of longitudinal and circumferential residual stresses and the growth of deformation-induced nanocrystals during cold-drawing. The role of residual stress is established herein not only as a trigger to accelerate the amorphous-to-nanocrystalline phase transformation but also as a decisive contributor to the mechanical and GMI performance. The unique simultaneous improvement of both mechanical and GMI properties of cold-drawn Co-based microwires opens up new possibilities for a variety of engineering applications, such as high-performance magnetic, stress and biological sensors.

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.

Nanocrystallization enabled tensile ductility of Co-based amorphous microwires

Melt-extracted amorphous Co68.15Fe4.35Si12.25B15.25 microwires were drawn at ambient temperature into different diameters and the dependence of tensile properties and microstructural evolution on the degree of deformation was investigated. The cold-drawn wire with 51% area cross-section reduction exhibits a tensile ductility of 1.64%, with a tensile strength exceeding 4000 MPa. Deformation-induced Co-rich nanocrystals were observed in the cold-drawn wires and proved to be capable of arresting the quick extension of shear bands and leading to the enhanced global plasticity of the microwires.

Effect of a glass shell on the crystallization of Fe- and Co-based amorphous microwires

Crystallization of amorphous Fe73.9B13.2Si10.9C2 and Co73.6B11.2Si13C2 microwires in a glass isolation (shell) upon heating has been studied by X-ray diffraction. Crystallization of the core of an amorphous microwire starts at temperatures above 400°C. No noticeable effect of the glass shell on the crystallization of the Fe-based microwire was observed. The crystallization of the Co-based microwires with a glass shell begins at a lower temperature than the crystallization of those without a shell. There has been observed a difference in the effect of the glass shell on the crystallization of microwires of different chemical compositions.

Magnetic Dynamic Interaction in Amorphous Microwire Array

The magnetic dynamic interaction in ferromagnetic microwire arrays has been studied based on thin Co-based glass-coated amorphous microwires. The longitudinal hysteresis loops of the microwire arrays consisting of different number of wires have been measured statically with no current flowing through the arrays and dynamically with ac current flowing through the arrays. The results show that with the number of wires increases the transverse anisotropy of the arrays has been enhanced, especially when an ac current was applied to the arrays, an additional circumferential anisotropy was induced. The results are explained by a domain unification effect in the form of domain wall displacement. Moreover, the changes in the magneto-impedance effect with the number of wires in the arrays indicate that the domain unification effect is frequency dependant and related to the dimension of the arrays. The domain wall motion contributes to the magneto-impedance up to a critical frequency beyond which only domain rotation occurs. Below the critical frequency, the initial permeability of the arrays increases with the increase of the number of wires while above the critical frequency, the trend inverses. The magneto-impedance response of the microwire arrays is a consequence of the dynamic interaction of domain unification and eddy current.

Magnetoresistance effect in soft magnetic amorphous microwires

Results on the magnetoresistance (MR) effect in Co-based amorphous glass-coated microwires with nearly zero magnetostriction and no granular structure are reported. MR measurements have been performed in parallel and perpendicular configurations, in both low field and high field regimes. The results show that high field resistance variations are caused by the corresponding magnetization processes in both parallel and perpendicular configurations. In case of low parallel field, no resistance variation has been emphasized. A sharp MR response was observed in case of a perpendicularly applied low field and it was attributed to the interdomain wall between the inner core and the outer shell of such microwires. The sharpness of the low field MR peak can be controlled through the microwire dimensions. The results bring new information about the domain structure of amorphous microwires and open up new application opportunities for these materials.