P. Rudkowski

The fabrication of fine metallic fibers by continuous melt-extraction and their magnetic and mechanical properties

Abstract The method of melt-extraction has been developed to continuously manufacture metallic fibers down to 3 μm diameter. Fibers of ferrous-based alloys, both amorphous and crystalline, show outstanding soft magnetic properties in their as-cast form. Furthermore, amorphous alloys show extraordinary yield strengths which increase with decreasing diameter, reaching values of up to 1000 kg mm −2 .

HIGH FREQUENCY BEHAVIOR OF SOFT MAGNETIC WIRES USING THE GIANT MAGNETOIMPEDANCE EFFECT

We have investigated the high frequency properties of several amorphous and polycrystalline wires mounted as inner conductors in coaxial lines. A static magnetic field was applied along the wire axis. The impedance spectra of the wires, measured using a network analyzer, show peaks in the real part of the impedance, which shift to higher frequency with the strength of the static field, a behavior typical of ferromagnetic resonance. The theoretical resonance condition predicts a straight line on an f02−H0 plot, where f0 is the resonance frequency and H0 is the resonant field, whose slope depends only on the saturation magnetization, Ms, of the material. All our wires obey this relation, and the values of Ms calculated from the slopes are in good agreement with those measured directly using a vibrating sample magnetometer.

The magnetic properties of sub‐20‐μm metallic fibers formed by continuous melt extraction

Melt‐extraction technology has been developed which allows the continuous casting of fine metallic fibers down to less than 5 μm in diameter. Outstanding soft magnetic properties are found both for amorphous and crystalline alloys manufactured by this method. The technique will be discussed, as will the magnetic properties, both in ac and dc driving fields.

Fine metallic and ceramic fibers by melt extraction

Abstract Techniques are described for the fabrication of metal and ceramic fibers of diameter below 20 μm by a method of melt extraction. The process is containerless and has been successfully applied to reactive metals and high temperature ceramics. The properties of the fibers, where exceptional, are reviewed.

The properties of sub-20-micron Permalloy fiber formed by melt extraction

Fibers of Permalloy of different compositions have been manufactured in diameters from 5-20 mu using a technique of melt-extraction. The molten material is raised on to the edge of a sharpened rotating molybdenum wheel whose tangential velocity is typically about 60 ms/sup -1/. The surface of fibers is free formed and is almost without defects. The radial cooling and high quenching rates result in a unique crystalline and magnetic microstructure. The fibers show outstanding soft magnetic properties and require no annealing to optimize performance. They also exhibit strong resistance to degradation under mechanical stress and are especially attractive for sensor applications. >

Rf and uhf giant magnetoimpedance effects in soft magnetic fibers and wires

We have investigated the giant magnetoimpedance (GMI) effect in fibers of different soft magnetic materials and in Permalloy wires. The fibers have been tested in the radio frequency (rf) range, by measuring the voltage drops in different parts of the circuit. The wires have been tested in the ultra high frequency (uhf) range, by measuring the transmission coefficient s/sub 21/ of coaxial lines having as central conductor a HyMu80 wire. The two methods provided quite similar results showing a strong GMI effect for very different magnetic samples.

Magnetic and magnetoelastic properties of Fe-Si-B metallic fibers

Amorphous metallic Fe/sub 78/Si/sub 9/B/sub 13/ fibers, 50 /spl mu/m in diameter, produced by the melt extraction method, have been studied in the as-quenched state and after furnace annealing at 455/spl deg/C and 480/spl deg/C for times between 0.5 min and 50 min. Measurements of the M-H loops and field dependence of Youngs modulus have shown that they behave in a way very like that of amorphous wires. The minimum coercivity of 3 A/m is greater than that found in wires, and the minimum Youngs modulus (/spl sim/0.45 times the saturation value) is greater. It is suggested that the core-shell domain model applies in the as-quenched state and that stress relief followed by surface crystallization dominate the anisotropy.

Study of the complex permeability of amorphous wires using microwave impedance spectroscopy

We have measured (10 MHz-6 GHz) the magnetoimpedance of ferromagnetic amorphous wires placed as internal conductors in shorted coaxial lines. Three types of materials have been investigated: CoFeSiB wires from Unitika (Japan), cast by rapid quenching in water, and NiCoFeSiBMn and CoFeSiNbB wires from MXT (Canada), cast by melt extraction. In order to investigate magnetic properties, we used a complex permeability formalism directly obtained from impedance by means of a transformation which leads to an exchange in components from real to imaginary and vice-versa. From the analysis of Cole-Cole plots of the relative permeability, ferromagnetic resonance was identified as the magnetization process occurring when an axial dc magnetic field of several hundred oersteds was applied to the line. An equivalent circuit of the magnetic wire was used to demonstrate that the field variation of the inverse of the equivalent resistance was quite similar to those of the line width vs resonance frequency ratio and of the maximum of the imaginary part of the relative permeability.

The magnetoimpedance effect in rapidly solidified soft magnetic fibers

Abstract Soft magnetic fibers, 20 to 50 μm in diameter, have been cast by melt extraction. A giant magnetoimpedance effect (GMI) of about 60% was observed in a (NiCo) 70 FeSiBMn fiber driven by a rf current. The magnetic field and frequency responses of fibers depend on the density and the frequency of the drive current. The GMI effect at saturation decreases from 70% at 3 μ A/ μ m 2 to 55% at 30 μ A/ μ m 2 . The saturation field increases with an increase in frequency, and is larger for lower current densities. These fibers can be used as sensing elements for a new generation of magnetic field sensors.

Microstructure and magnetic properties of semi-hard Fe-Co-B-Cu alloys formed by rapid solidification

We present the magnetic and microstructural properties of a semi-hard magnetic material Fe-Co-B-Cu produced by two rapid solidification processes: melt-extraction and melt-spinning. The as-quenched material (in the ribbon or fiber form) has coercive field H/sub c/ in the range of 8-12 kA/m and remanence B/sub r/ of 0.7 T. The semi-hard magnetic properties were obtained as a result of the combined effects of composition and special microstructures produced in the rapid solidification process. >