Metallic Nucleus Research Articles

Magnetization reversal process in bistable microwires and its temperature dependence

The temperature dependence of the magnetic behaviour of Fe-based glass-coated amorphous microwires with positive magnetostriction has been studied in the temperature range from 5 to 300 K. For long enough microwires, hysteresis loops are squared shaped in the whole temperature range denoting the existence of a large domain with axial easy axis. For short microwires, magnetic bistability is partially lost near room temperature but may be recovered at low temperature. This is interpreted considering the influence of the shape anisotropy: in short wires the demagnetizing field may become large enough to destroy the single domain structure. The influence of coating is also considered by studying microwires with different coating thickness. Transverse magnetoelastic anisotropy up to around 400 MPa can be induced at low temperature as a consequence of the difference in thermal expansion coefficients between metallic nucleus and insulating coat. Additional changes in the low-temperature loops have been identified to be a consequence of the way how the microwires are fixed to the sample's holder. Under particular circumstances transverse anisotropy induced at low temperature may destroy the bistable behavior.

Multilayer Microwires: Tailoring Magnetic Behavior by Sputtering and Electroplating

AbstractA novel technique for preparing multilayer microwires with controlled magnetic behavior has been developed. This technique involves combining sputtering and electroplating procedures to deposit (magnetic or non‐magnetic) metallic nano‐ and microlayers onto glass‐coated amorphous magnetic microwires. A suitable choice of magnetostrictive amorphous metallic nucleus, together with the specific stresses induced by the deposited layers, allows the tailoring of specific magnetic behavior. In this way, the preparation of multilayer microwires characterized either by square‐shaped hysteretic loops (typical of magnetically bistable microwires with longitudinal easy axes), or by nearly non‐hysteretic loops (for those microwires with a circumferential magnetization easy axes), can be achieved.

Magnetostriction in glass-coated magnetic microwires

The hysteretic magnetic properties of glass coated magnetic microwires depend on the magnetostriction constant: Co-rich microwires with negative magnetostriction constant present an almost non-hysteretic loop with relatively high magnetic anisotropy field up to around 8 kA/m. In contrast, Fe-rich microwires with positive magnetostriction show rectangular hysteresis loops with switching field depending on diameter of the metallic nucleus and the thickness of the glass coating. The softest magnetic properties, such as large magnetic permeability, are observed in nearly zero magnetostrictive alloys. It is then obvious that the experimental determination of the saturation magnetostriction λ s of glass-coated microwires is very important to predict their magnetic behaviour. Different methods for the determination of the saturation magnetostriction λ s of tiny glass coated microwires have been reviewed and compared in this manuscript. Small angle magnetization rotation (SAMR) method and change of the giant magneto-impedance spectrum under applied stress have been employed in nearly zero magnetostrictive in as-prepared and current annealed glass-covered microwires. The conditions of applicability of these methods to the microwires have been analysed, taking into account the domain structure expected for vanishing magnetostriction constant of the metallic nucleus. These different techniques give similar saturation magnetostriction constant values. Heat treatment results in a significant change of λ s.

Sensitive magnetoelastic properties of glass-coated CoMnSiB amorphous microwires for magnetoelastic sensors

Magnetoelastic properties of glass-coated CoMnSiB amorphous microwires with nearly zero magnetostriction constant have been studied. Effects of external tensile stress on magnetic bistability and on GMI effect exhibited by microwires with adecuated CoMnSiB compositions of metallic nucleus have also been investigated. These dependencies, used in the design of new magnetoelastic sensors, are related with the magnetoelastic anisotropy induced by the external stress via magnetoelastic coupling.

Correlation of magnetic and structural properties of glass-coated Cu-based microwires

The microstructural characteristics of Cu-based glass coated microwires (Cu 70Fe 30 and Cu 80Co 20) are studied considering their composition and geometry determined by their fabrication. The parameter describing the geometry is the ratio ρ between the diameter of the metallic nucleus and the external one of the layer of the Pyrex glass. αFe and Co phases (nanocrystals of some 40 nm) coexist with a Cu phase in each system. The relative content of each phase shows a non-linear ρ-dependence with a critical maximum at around ρ=0.2 for the αFe and Co phases. Hysteresis measurements show that the ratio ρ is also critical in the magnetic characterization of the microwires.

Preparation and properties of glass-coated microwires

Technological aspects of the Taylor–Ulitovsky method for fabricating glass-coated microwires with different structures of metallic nucleus are discussed. The main parameters of the process can be distinguished as follows: a casting rate of microwire and its limits, cooling rate of the metal core, geometrical characteristics and alloy composition. The casting rate is important in determining the upper and lower limits of the diameter of microwire. Depending on the critical quenching rate (10 4–10 7 K/s), metastable, amorphous or supersaturated state phases are obtained. Mixed structures consisting of micro- and nanocrystallites embedded into the amorphous matrix can also be formed. During the fabrication process, strong internal stresses are generated, which determine some of the most interesting properties of these materials. In particular, magnetic properties depend mostly on the magnetoelastic anisotropy arising from the coupling of the internal stresses with magnetostriction constant.

Switching field fluctuations in a glass-coated Fe-rich amorphous microwire

Spontaneous experimental spread, Δ H s, of the switching field, H s, (switching field fluctuations) during the re-entrant magnetization process of Fe-rich (Fe 65Si 15B 15C 5) microwire (metallic nucleus diameter of around 10 μm) exhibiting rectangular hysteresis loop has been observed at room temperature. Shape of the switching field distribution has been measured and analyzed in terms of a thermoactivational mechanism. The model predicts the lineal dependence of the logarithm of Barkhausen jump probability, d w, on reduced switching field fluctuation, Δ h=Δ H s/ H sm. Observed d w(Δ h) dependence fits satisfactorily to the predicted semi-logarithmic dependence.

Structural study of glass coated Cu-based microwires

The microstructural characteristics of Cu based glass coated microwires are studied considering the different factors determined by their fabrication (composition and geometry). Two different sets of binary systems (Cu x Fe 100− x and Cu x Co 100− x ) and Cu 63Fe 36P and Cu 80Co 19Ni samples have been studied. The parameter that describes the geometry is the ratio ρ between the diameter of the metallic nucleus and the external one corresponding to the layer of the Pyrex glass. It straightforwardly relates to the fabrication process of the samples owing to the strong internal stresses in the samples because of the different thermal expansion coefficients of the glass coating and the metallic nucleus. The structural study, carried out by means of wide angle X-ray scattering (WAXS) and a subsequent identification of the crystalline peaks, reveals that Feα and Co phases coexist with a Cu phase in each system. In the case of the samples containing P or Ni atoms, a third phase, Cu 3P or Cu 3.8Ni, respectively, appears. An evaluation of the relative content of each phase shows in each system a sort of a master curve with a non-linear ρ-dependence with a critical maximum at around ρ=0.25 for the Feα and Co phases in the two systems. The calculation of the grain size shows that nanocrystals around 40 nm in size are formed in all the cases by Feα, Co and Cu phases. Magnetic measurements are now in progress in order to complement these structural results.

Evaluation of the saturation magnetostriction in nearly zero magnetostrictive glass-coated amorphous microwires

Measurements of the saturation magnetostriction λs of glass-coated covered (Co1−xMnx)75Si10B15 (x=0.09, 0.10 and 0.105) and Co56.5Fe6.5Ni10B16Si11 amorphous microwires carried out by means of the stress dependence of initial magnetic susceptibility method are reported. The conditions of applicability of this method to the microwires have been analyzed, taking into account the domain structure expected for very low negative magnetostriction of the metallic nucleus. The experimental data of λs obtained in those amorphous microwires are in agreement with those reported in amorphous ribbons with similar compositions.

Magnetoelastic sensor of liquid level based on magnetoelastic properties of Co-rich microwires

The effect of external tensile stress on the hysteresis loops of nearly-zero magnetostrictive glass-coated amorphous microwires of (Co 100− x Mn x ) 0.75Si 10B 15 (10< x<11) and Co 56.5Fe 6.5Ni 10B l6Si 11 compositions of metallic nucleus has been studied. All hysteresis loops have a rectangular shape without applied stress. Significant changes in the shape of the hysteresis loop are produced by the application of tensile stress, particularly the hysteresis loop of the sample (Co 92Mn 8) 75Si 10B 15 under a loading of 10 g, becoming flat with reduced values of both remanence and coercivity. These changes of the hysteresis loop with the external strain have been used to design a magnetoelastic sensor of liquid level.

Coercivity of glass-coated Fe 73.4-xCu 1Nb 3.1Si 13.4+xB 9.1 (0≤x≤1.6) microwires

The effects of the geometry, thermal treatment and the amplitude of applied magnetic field on the coercivity behaviour of glass covered Fe 73.4-xCu 1Nb 3.1Si 13.4+xB 9.1 (x = 0; 1.1 and 1.6) have been investigated. Variations of the coercivity, H c, with the geometry (d/D ratio, d –diameter of the metallic nucleus, D-total diameter, d/D ranging from 0.1 to 0.9) are attributed mainly to the internal stress acting on the metallic nucleus owing to the glass coating. Annealing temperature (300–700 °C) dependencies of H c exhibit some peculiar differences with respect to that of the classical FINEMET ribbon, especially in the temperature range of 400–500 °C (very beginning of the nucleation of the nanocrystallization process) and at 650–700 °C (magnetic hardening owing to the iron borides segregation) depending in both cases on ratio d/D. With decreasing of d/D ratio the transition soft-hard magnetic character takes place in a narrower annealing temperature range with a rectangular hysteresis loop of H c≈2400 A/m. The evolution of H c with the applied magnetic field amplitude, H o (<2500 A/m) for the as-prepared and treated at low temperature samples results to be quite linear. This linear behaviour of H c (H o) allows to assign a nucleation of domain walls mechanism mainly for the magnetization process connecting with a magnetic bistable behaviour, while the non-linear H c (H o) curves of samples with x = 1.6 treated above 650 °C and 700 °C suggest that different mechanism takes place.

Glass coated microwires with enhanced coercivity

Glass coated Cu–Ni–Co microwires with a metallic nucleus 3 μm in diameter were prepared by the Taylor–Ulitovsky technique. The effects of heat treatments on the magnetic and structural properties were studied. At room temperature, the coercivity increases from 65 Oe in the as-prepared samples to 800 Oe after annealing the material at 973 K during 1 h.

Ferromagnetic resonance, magnetic behaviour and structure of Fe-based glass-coated microwires

Natural ferromagnetic resonance (NFMR), hysteresis loops and X-ray diffraction have been studied in the Fe-rich microwires with a ratio ρ of the metallic nucleus diameter to the total diameter of the microwires ranging between 0.1 and 0.4. Both coercivity, H c, and frequency of the NFMR, ω res, increase as ρ decreases. The splitting of ω res was explained from the coexistence of crystalline and amorphous phases.

Stress dependence of the switching field in Co-rich amorphous microwires

Rectangular hysteresis loops have been observed in Co 68Mn 7Si 10B 15 amorphous glass-coated microwires of different geometry with ratio r of metallic nucleus radius to total radius of the microwires ranging between 0.62 and 0.17. Switching field H* decreases from 200 to 40 A/m when r increases from 0.17 to 0.62. Dependence of H* on external tensile stresses, σ a, with different r was studied in as-prepared state and after glass-coating removal by chemical etching in HF. H* increases with σ for each sample after certain roughly stress-independent region. Chemical etching generally results in a decrease of H*. Dependence of magnetostriction constant on the strength of the total external and internal stresses was taken into account to explain the experimental results.

The stress dependence of the switching field in glass-coated amorphous microwires

Glass-coated amorphous microwires of two different compositions, and , with square hysteresis loops have been prepared and studied. The effect of applied tensile stress on their magnetic properties is reported for both kinds of microwires with radii of the metallic nucleus in the range from 1.9-m and thicknesses of the coating of 3.8-m. The switching field of the studied microwires depends on the sample's geometry (the radius of the metallic nucleus and the thickness of the glass coating), exhibiting a decrease with decreasing thickness of the coating owing to the induced internal stresses. For Fe-rich compositions an increase of the switching field with the applied tensile stress in the range of 0-750 MPa was observed, such an increase being more significant for a thin coating. The classical square root dependence of the switching field on the applied stress is found for Fe-rich compositions for the range of thicker metallic nuclei. Co-rich microwires exhibit quite a different stress dependence of the switching field, exhibiting first roughly stress-independent behaviour and then an increase with the stress. Removal of the glass coating results in a drastic change of the switching fields for both compositions, indicating the existence of strong internal frozen-in stresses. The observed experimental dependences were explained in terms of a dependence of the nucleation field both on external and on internal stresses and by taking into account the dependence of the magnetostriction constant on the strength of the total stresses.

Magnetic properties of Fe-based glass-coated microwires

Axial hysteresis loops of glass-coated amorphous Fe 70B 15Si 10C 5 microwire have been measured as a function of both the diameter of metallic nucleus (from 3.7–14.9 μm) and the thickness of the coating (4.0–9.6 μm). They exhibit low-field rectangular hysteresis loops with a single and large Barkhausen jump even for samples as short as 5 mm long. Coercivity remarkably increases (roughly from 1 to 10 Oe) and remanence decreases (from 1 to 0.45 T), respectively, as the ratio of metallic nucleus radius to the total radius of the wire decreases from 0.63 to 0.16. The strength of the internal stresses induced during the fabrication depends on this ratio, and the easy axes of the corresponding magnetoelastic anisotropies determine the actual value of coercivity and remanence for these microwires.

Nature and origin of chemical zoning in the metal nucleus and oxide cortex of cosmic spherules from the Tuamotu Archipelago, French Polynesia

Chemical analyses of two cosmic spherules with a FeNi metal nucleus and an off-centre wüstite and magnetite cortex revealed Ni, Co, and Cr gradients that delineate a marked chemical zoning. In the cortex, the Ni and Co concentrations increase from the rim to the centre, with a maximum at the contact with the nucleus. The zoning within the nucleus is vesicular, with an increase in concentration towards the front part of the spherule. Contrary to Ni and Co, Cr is more abundant in the oxide cortex than in the metallic nucleus. We assume that the chemical zoning has been present prior to solidification and indicates high-temperature NiFe alloy oxidation during the traverse of the particle through the Earth's atmosphere. According to this hypothesis, the concentration levels in the cortex reflect successive stages of the fractionation of metals between alloy and oxide according to their respective oxidation rates. Calculated chemical gradients agree with the measured gradients.

Magnetic properties of amorphous and devitrified FeSiBCuNb glass-coated microwires

Abstract The evolution of magnetic properties and structure with annealing temperature of glass-coated microwires with composition Fe73.5B22.5 − xSixCu1Nb3 (x = 11.5, 13.5 and 16.5) has been studied for samples with diameter of metallic nucleus ranging from 4 to 22 μm and fixed total diameter of around 26 μm. Magnetic bistability characterized by a single and large Barkhausen jump is observed in all the cases in the as-obtained state as well as after devitrification into nanocrystallized state (annealing at around 550 °C), while this bistable character is finally lost after annealing at around 750 °C when the final second crystallization stage proceeds. It is noticeable that squared loops can be observed in the range of switching field values between 1 and 100 Oe, depending on the structure of the material with the consequent possibility of tailoring the magnetic bistability.

Magnetoelastic anisotropy distribution in glass-coated microwires

Amorphous microwires, obtained by the glass-coated melt-spinning method having diameters in the range of micrometers, can exhibit perfectly square (single and large Barkhausen jump) or quasi-anhysteretic hysteresis loops, depending on the easy magnetization direction determined by the intrinsic magnetoelastic anisotropy. The thermoelastic internal stressed frozen-in during the fabrication that model the domain structure are here calculated by considering the classical theory of elasticity. A complex stress distribution is obtained having magnitude of 103 MPa. Circular stresses turn out to be predominant, which arises from the composite nature of the microwire (metallic nucleus and insulating glass coating having different mechanical and thermal properties).

Magnetic properties of glass-coated amorphous and nanocrystalline microwires

The magnetic properties of Fe- and Co-based amorphous and nanocrystalline glass-coated microwires are reviewed. Such wires are composed of a metallic amorphous nucleus (3 to 20 μm diameter) and an insulating glass cover (2 to 12 μm thick). Interest is mainly focused on samples exhibiting squared hysteresis loops (magnetically bistable) which exhibit positive magnetostriction (bistability can be observed for 2 mm long samples). The internal stresses are much stronger than for in-rotating-water quenched amorphous wires as a consequence of the presence of the glass coating and, coupled with magnetostriction, give rise to strong magnetoelastic anisotropies determining the magnetization process. Upon heating, the structure evolves to nanocrystalline and microcrystalline and although the magnetic behavior remains bistable a large influence on the switching field is observed (variations of switching field between 0.3 and 100 Oe). These magnetic properties make such microwires very promising for technological applications.