Nanocrystalline Glass-coated Microwires Research Articles

Field and Current Controlled Domain Wall Propagation in Twisted Glass-Coated Magnetic Microwires

The torsion effect on the field and current driven magnetization reversal and the associated domain wall velocity in cylindrical amorphous and nanocrystalline glass-coated microwires is reported. Samples from three representative compositions have been investigated: (1) amorphous Fe77.5Si7.5B15 with positive magnetostriction, λ ≅ 25 × 10−6, (2) amorphous Co68.18Fe4.32Si12.5B15 with nearly zero negative magnetostriction, λ ≅ −1 × 10−7, and (3) nanocrystalline Fe73.5Si13.5B9Cu1Nb3 (FINEMET) with small positive magnetostriction, λ ≅ 2.1 × 10−6, all having the diameter of the metallic nucleus, d, of 20 µm and the glass coating thickness, tg, of 11 µm. The results are explained through a phenomenological interpretation of the effects of applied torque on the anisotropy axes within the microwires with different characteristics. Among all the complex mechanical deformations caused by the application of torque on magnetic microwire samples, the most important are the axial compression – for axial field-driven domain wall motion, and the circumferential tension – for electrical current/circumferential field-driven domain wall motion. The Co68.18Fe4.32Si12.5B15 microwire, annealed at 300 °C for 1 hour and twisted at 168 Rad/m exhibits the optimum characteristics, e.g. the lowest switching current (down to 9 mA~2.9 × 10−3 A/cm2) and the largest domain wall velocity (up to 2300 m/s).

As-cast nanocrystalline glass-coated microwires

As-cast nanocrystalline Fe74.5−xSi9B13.5CuxNb3 (x=1 and 2.5at.%) glass-coated microwires have been prepared by drawing from the melt in a single step process. The cooling conditions and Cu content were modified in order to facilitate the growth of α-(Fe,Si) nanograins directly from the wire formation process. By increasing the cooling distance, the microstructure evolves from a fully amorphous one at low cooling distances to a nanocrystalline central area and an amorphous shell towards the surface at higher cooling distances. Changes of the composition and cooling conditions allow an extensive tailoring of the magnetic properties and structure of the as-cast glass-coated microwires.

Magnetization Process and GMI Effect in As-Cast Nanocrystalline Microwires

Results on the axial and circumferential magnetization processes in as-cast nanocrystalline glass-coated microwires are reported for the first time. Glass removal induces a peculiar change in the shape of the axial hysteresis loop. The initially rectangular axial hysteresis loop becomes almost anhysteretic after glass removal as a consequence of a change in the mechanism of axial magnetization, which has been explained by taking into account the changes induced by stress relief in the exchange length. Therefore, the nanosized crystalline grains can be coupled or uncoupled, depending on the presence of the glass coating. The change in the shape of the hysteresis loop is also observed on the circumferential direction. Results of giant magneto-impedance measurements support the explanation given, as the magneto-impedance response at 50 MHz increases about 9 times after glass removal due to both stress relief and to the phase consisting of coupled crystalline grains, which becomes the phase that preponderantly determines the magnetic behavior of the microwire after glass removal.

Domain Wall Propagation in Nanocrystalline Glass-Coated Microwires

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The characteristics of domain wall propagation in Fe<formula formulatype="inline"> <tex Notation="TeX">$_{73.5}$</tex></formula>Cu<formula formulatype="inline"> <tex Notation="TeX">$_{1}$</tex></formula>Nb<formula formulatype="inline"> <tex Notation="TeX">$_{3}$</tex></formula>Si<formula formulatype="inline"> <tex Notation="TeX">$_{13.5}$</tex></formula>B<formula formulatype="inline"> <tex Notation="TeX">$_{9}$</tex></formula> amorphous and nanocrystalline glass-coated microwires are investigated in order to determine the changes induced by structural transformation in wall velocity and mobility. An improved method for sensing the wall presence and direction, and for measuring its velocity, is also presented. Amorphous samples are bistable, and their wall velocity displays typical values for microwires with positive magnetostriction. Nanocrystallized samples are not bistable in general, but they become bistable either if the glass coating thickness is large enough to induce a strong axial anisotropy, or if their metallic nucleus diameter is large enough to increase the contribution of the magnetostatic term. Bistable nanocrystalline microwires display smaller switching field values, together with larger wall velocity and mobility values as compared to bistable amorphous samples with the same composition. The results have been explained considering the changes induced in magnetostriction by the nanocrystalline phase formation and the magnetization reversal mechanism by means of wall propagation. </para>

Engineering of the magnetic properties of Finemet based nanocrystalline glass-coated microwires

The magnetic properties of Finemet nanocrystallized ribbons have been extensively studied and well understood. However, results obtained on microwires of such compositions are ambiguous considering the form of the hysteresis loop after the crystallization thermal treatment. In this paper, we show results on the static characterization of annealed Finemet based microwires (Fe73.2Si15.8B6.9Cu1.2Nb2.9) of different geometries and illustrate how the magnetic anisotropy of Finemet based nanocrystalline microwires depends on the annealing temperature and the geometric characteristics of the microwires.

Annealing induced evolution of structure and high frequency magnetic properties in nanocrystalline glass-coated microwires

The annealing induced evolution of structure and magnetic properties of glass-coated microwires (GMW) made of FeNbCuBSi alloy were studied. The microwires were produced by a modified Taylor method. The annealing temperatures were varied from 800 to 890 K. In their as-cast state, all microwires featured perfect square hysteresis loops. After annealing, the GMW hysteresis loop width decreased thus indicating the alloy devitrification and the nanocrystalline phase growth. The grain size measured in TEM studies was 10–15 nm. The hysteresis loops of GMW with 13% Si content remained square-shaped in the whole range of annealing temperatures, while in the case of GMW with 16% Si content, increasing T a caused the loop transformation to a flat, anhysteretic one. After the hysteresis loop underwent the square-to-flat transformation, an enhancement in GMW high frequency properties had been observed.

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.