Soft nanocrystalline Fe-based alloys are excellent soft magnetic nanocomposite materials with the grain size of their primary crystallization products much smaller than their magnetic exchange length. Precursor materials are produced usually by rapid quenching with ribbon-shape with amorphous character. Recently, the Taylor-Ulitovsky technique has resulted very useful to produce glass-coating microwires with the metallic nucleus composition similar to that of the amorphous ribbons. In fact, in ref. [1] we reported on the coercivity behaviour of Finemet-type glass-coated microwires.

In the present work we deal with the fabrication of thin of Fe₇₉Hf₇B₁₂Si₂ (low Si content) glass-coated microwire with a nanocrystalline structure and structural and coercivity characterization of such samples which can be considered as a new family of these nanocrystalline materials. Pieces of 10 cm of this microwire were annealed (300-600^ºC during 1 hour). The structural characteristics of the as-cast and annealed samples were determined, at room temperature, by X-ray Diffraction (XRD) technique. XRD measurements allow to obtain the evolution of the grain size (15-35 nm) and relative volume fraction (5-60%) of the nanograins as a function of the annealing temperature in the annealed samples. Coercive field (Hc) of the as-cast and annealed samples has been evaluated from the hysteresis loop of the samples obtained by a conventional induction method at 100 Hz. Thermal dependence Hc is quite similar to that reported in other nanocrystalline Fe-based alloys. It decreases from the as-cast state (relaxation process) showing a maximum at around 450^ºC (pre-nucleation of nanograins) decreasing significantly between 500-600^ºC (exchange coupling of the nanograins).

[1] V. Zhukova, A.F. Cobeño, A. Zhukov, J.M. Blanco, V. Larin and J. Gonzalez., Nanostructured Materials, 11 (1999)1319

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1. MAGNETOIMPEDANCE EFFECT AT HIGH FREQUENCY IN GLASS-COATED AMORPHOUS WIRES