•Amorphous powder of Fe-Si-B-Nb-Cu alloy was produced by helium gas atomisation.•Proper annealing resulted in structural relaxation and nanocrystallisation.•Powder cores were manufactured using the ...as-atomised and annealed powders.•Cores exhibit excellent low coercivity and low core losses.•The small particle size and good insulation can explain the low core losses.
The present work demonstrates the high-frequency core loss performance of Fe-based amorphous and nanocrystalline powder cores, initially produced by gas atomised powder, consolidated using sieved particles ≤20 µm, and isolated by a precise insulating layer of polymer to limit the inter- and intra-particle eddy currents to attain enhanced performance. The large glass forming ability (GFA) of the gas atomised powder, reflected by different glass forming instruments, such as the supercooled region (ΔTX = 54 °C) and the reduced glass transition temperature (Trg = 0.56), is consistent with the substantial amorphisation capability of the alloy. To the best of our knowledge, this is the first-ever report to reveal a large ΔTX in the Finemet-type alloy powders, an essential parameter to gas-atomise the amorphous powders with significantly lower cooling rates compared to the melt-spun ribbons. Further, subsequent annealing of the amorphous powders, between the exothermic events guided by differential scanning calorimetry (DSC), lead to the growth of a fine nanocrystalline structure of grains ≤15 nm, thanks to the positive enthalpy of mixing of Cu with the constituents to act as a nucleation agent, to retain the excellent soft magnetic properties. The DC soft magnetic properties of the powders were significantly improved on thermal annealing, confirmed by hysteretic loops, quantified by reduced coercivity HC <1 Oe of annealed powders at <575 °C, and attributed to the reduced magnetoelastic contribution due to zero/near-zero magnetostriction anisotropy, attained due to the homogenous nanocrystalline structure. The amorphous and nanocrystalline powder cores, consolidated by compression moulding, show ultra-high loss performance, due to the ultra-low coercivity attained on nanocrystallisation, and negligible eddy currents loss, owning to efficient insulation of small particles, for high-frequency power conversion applications, such as voltage regulator (VR), and resonant converters, in automotive industry and data storage centres.
There is a pressing need for improving of the high-frequency magneto-impedance effect of cost-effective soft magnetic materials for use in high-performance sensing devices. The impact of the ...stress-annealing on magnetic properties and high frequency impedance of Fe-rich glass-coated microwires was studied. Hysteresis loops of Fe-rich microwires have been considerably affected by stress- annealing. In stress-annealed Fe- rich microwire we obtained drastic decreasing of coercivity and change of character of hysteresis loop from rectangular to linear. By controlling stress-annealing conditions (temperature and time) we achieved drastic increasing (by order of magnitude) of giant magnetoimpedance ratio. Coercivity, remanent magnetization, diagonal and of-diagonal magnetoimpedance effect of Fe-rich microwires can be tuned by stress-annealing conditions: annealing temperature and time. Observed experimental results are discussed considering relaxation of internal stresses, compressive "back-stresses" arising after stress annealing and topological short range ordering.
Processing by Joule-heating the Co67Fe3.9Ni1.5B11.5Si14.5Mo1.6 glass-coated microwire allows considerable improvement of magnetic softness and giant magnetoimpedance (GMI) effect. At optimal joule ...heating conditions we observed GMI ratio up to 650%. Observed magnetic softening and GMI ratio improvement have been discussed considering magnetic anisotropy induced by Oersted magnetic field during current annealing, internal stresses relaxation and radial distribution of magnetic anisotropy. The analysis of the GMI ratio as a function of the frequency (10–1000 MHz) opens new lights to understand the distribution of the magnetic anisotropy inside the glass-coated microwire.
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•Remarkable mprovement of GMI effect in Co -rich microwires current-annealing.•Considerable enhancement of magnetic softness in current-annealed Co-rich microwires.•Interpretation of current annealing effect considering internal stresses relaxation and induced anisotropy.•Evaluation of magnetic field dependence of skin depth in as-prepared and current-annealed microwires.
We present an overview of the factors affecting soft magnetic properties and giant magnetoimpedance (GMI) effect as well as of post processing tools allowing optimization of magnetic softness and GMI ...effect of thin amorphous wires. Generally low coercivity and high GMI effect have been observed in as-prepared Co-rich microwires. The magnetoelastic anisotropy is one of the most important parameters that determine the magnetic softness and GMI effect of glass-coated microwires. Annealing at adequate conditions can be very effective for manipulation the magnetic properties of amorphous ferromagnetic glass-coated microwires. After annealing of Co-rich we can observe transformation of inclined hysteresis loops to rectangular and coexistence of fast magnetization switching and GMI effect in the same sample. Using stress-annealing, GMI effect of Co-rich microwires can be increased. We demonstrated that the switching field value of microwires can be tailored by annealing in the range from 4 to 200 A/m. Similarly stress-annealing allows induction of transverse magnetic anisotropy and improvement of magnetic softness and GMI effect in amorphous Fe-rich microwires. On the other hand in Fe-rich FeCuNbSiB microwires after appropriate annealing we observed considerable magnetic softening and GMI effect enhancement.
•Trends in optimization of GMI effect in amorphous and nanocrystalline microwires.•Engineering of magnetic properties and GMI effect in as-prepared amorphous microwires.•Magnetic softening of Fe-rich microwires after stress-annealing.•Improvement of GMI effect after stress-annealing of Fe- and Co-rich microwires.•Improvement of GMI effect after nanocrystallization of Fe- -rich microwires.
Fe–Si–B–Nb–Cu alloy powders, with and without P additions, were produced by gas atomization. The particles smaller than 20 μm are fully amorphous, exhibiting good soft magnetic properties. The ...crystallization process was studied by differential scanning calorimetry, demonstrating that its kinetics changes dramatically with small variations in the composition. The (Fe0.76Si0.09B0.10P0.05)97.5Nb2.0Cu0.5 (at. %) alloy was annealed in the supercooled liquid region (480 °C) and at the first crystallization peak (530 °C). The structural characterization by means of differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy provided information that explained the excellent soft magnetic properties. Annealing at 480 °C produced an amorphous relaxed structure with improved soft magnetic properties. At 530 °C, a two-phase material formed by nanocrystals with an average grain size of 16–17 nm embedded in an amorphous matrix was developed. Partial nanocrystallization increased the saturation magnetization from 139 to 144 emu/g and reduced the coercivity from 2.24 to 0.69 Oe. These results can be understood in terms of the algebraic contribution of both phases to the magnetization and the application of the random anisotropy model to nanocrystalline soft magnetic materials.
•Amorphous powder of Fe–Si–B–P–Nb–Cu alloys was produced by He gas atomization.•Proper annealing resulted in structural relaxation and nanocrystallization.•Crystallization kinetics was different depending on composition.•The annealed powders exhibit excellent soft magnetic properties.•Random anisotropy model can explain the very low coercivity.
► Systematic studies of the optimization of magnetic properties and GMI effect of microwires. ► Systematic study of the effect of magnetoelastic anisotropy on magnetic properties of microwires. ► ...Contextual discussions of the impact of magnetoelastic anisotropy on GMI effect of microwires. ► Systematic study of effect of magnetostatic interactions on magnetic properties of microwires. ►Contextual discussions of magnetic microwires applications for magnetic microsensors.
We present the results on tailoring of soft magnetic properties and GMI effect in thin microwires paying special attention on achievement of low hysteretic high GMI effect in the extended frequency range (up to 4GHz). We observed considerable dependence of the GMI ratio and magnetic anisotropy field, Hk, of Co-rich amorphous microwires with vanishing magnetostriction constant on the internal and applied stresses. For low magnetostrictive Co-rich composition we obtained microwires low coercivity values (generally below 10A/m). Field dependence of the off-diagonal voltage response measured in pulsed regime (pulsed GMI) exhibits anti-symmetrical shape.
The magnetic anisotropy of Co and Fe-rich microwires can be tailored by stress or magnetic field annealing. Particularly stress annealed Fe-rich microwires exhibit stress-sensitive GMI effect and hysteretic properties. Varying the time and the temperature of such stress annealing we are able to tailor both magnetic properties and GMI. Additionally, magnetic response of linear microwires arrays and GMI effect of the system containing few microwires can be tailored through the magnetostatic interaction between the microwires.
We found, that if the surface anisotropy is not circumferential, then the MI curve Z(H) presents hysteresis. This hysteresis can be suppressed by application of sufficiently high DC bias current IB that creates a circumferential bias field HB.
We present an overview of the factors as well as post processing tools allowing optimization of magnetic softness and GMI effect of rapidly quenched materials: microwires and ribbons. Generally, low ...coercivity and high GMI effect have been observed in as-prepared Co-rich compositions. Annealing at adequate conditions can be very effective for manipulation of the magnetic properties and GMI effect of amorphous and nanocrystalline rapidly quenched materials. After annealing of Co-rich compositions, we can observe transformation of inclined hysteresis loops to rectangular. However, at certain annealing conditions GMI effect can be improved. Using stress-annealing, GMI effect of both Fe-rich and Co-rich microwires as well as of amorphous ribbons can be improved. On the other hand, in Fe-rich FeCuNbSiB microwires after appropriate annealing we observed considerable magnetic softening and GMI effect enhancement. The other promising post-processing allowing GMI effect optimization is Joule heating.
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•Trends in optimization of GMI effect in amorphous and nanocrystalline materials.•Magnetic softening of Fe-rich microwires after stress-annealing or nanocrystallization.•Improvement of GMI effect after Joule heating of Co-rich microwires.•Optimization of GMI effect after post-processing of Co-rich amorphous ribbons.•Improvement of GMI effect after annealing and stress-annealing of Co-rich microwires.
In the current study, we have succeeded to fabricate Co2FeSi Heusler alloy glass-covered microwires with magnetic core nucleus diameter d = 4.36 µm and total diameter D = 17.55 µm, with high Curie ...temperature (Tc>1100 K) and well-defined magnetic anisotropy for high temperature spintronic devices application. The magnetic properties of as-prepared and annealed at different temperature (873 K, 973 K and 1073 K for 1 h) of Co2FeSi Heusler alloy glass-covered microwires have been investigated. Strong dependence of the magnetic properties on the annealing conditions has been indicated. Anomalous magnetic behavior for annealed samples at 873 K and 973 K has been found and structural properties of such samples have been analyzed. Critical temperatures 155 K and 250 K have been detected for annealed samples at 873 K and 973 K, respectively, where the behavior of M-H loops and coercivity changed. Below the critical point the M-H curve shows “kink or wasp-waisted” magnetic behavior and complex magnetic reversal mechanism is supposed. The anomalous magnetic behavior is due to the martensitic phases induced by annealing conditions below the room temperature. This unusual magnetization behavior provides opportunities to understand the phenomena of different types of magnetic domain structures in the preferred crystallographically oriented Co2FeSi Heusler alloy glass-coated micro-wires, essentially helpful for designing the devices based on magnetization switching.
•Production of Co2FeSi Heusler alloy glass-coated microwires with high Curie temperature.•Strong dependence of the magnetic properties Co2FeSi Heusler alloy glass-coated microwires on the annealing conditions.•Anomalous magnetic behavior below the room temperature for Co2FeSi glass-coated microwires annealed at 873 K and 973 K.•Critical temperatures where changes in the magnetic and structure phases appear were detected for annealed samples.•Suitability of Co2FeSi microwires for the design of devices based on magnetization switching.
We studied the correlation between magnetic softness and magnetostriction coefficient for as-prepared and annealed Co–Fe-rich microwires. We found that the hysteresis loops and magnetostriction ...coefficients of Co and Fe-rich microwires depend not only on the chemical composition of the metal but also on internal stress. Consequently, both hysteresis loop and magnetostriction coefficient can be adjusted by annealing. We varied the time and temperature of annealing and observed changes of the character of the hysteresis loops. These changes correlated with evolution of the magnetostriction coefficient. Drastic changes of the hysteresis loop for Co-rich microwires were attributed to changes of the sign and value of the magnetostriction coefficient.
As-prepared and annealed Heusler-type NiMnGa glass-covered microwires present considerable dependence of magnetization on magnetic field values attributed to the magnetic and atomic disorder. We ...observed that annealing conditions strongly affect structure, temperature dependence of magnetization and Curie temperature of microwires. After annealing a magnetic phase transition is observed at about 270–300 K which is beneficial for magnetic solid state refrigeration. Exchange bias effect, enhanced coercivity values at low temperature and features that can be attributed to coexistence of two magnetic phases are observed after long annealing of NiMnGa glass-covered microwires.
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•Tunability of magnetic properties of Heusler-type microwires by annealing.•Magnetic phase transition near room temperature beneficial for magnetic solid state refrigeration.•An exchange bias of about 20 Oe at 5 K in annealed NiMnGa microwires.•An enhanced coercivity at low temperatures in annealed NiMnGa microwires.
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