Soft magnetic behaviour of nanocrystalline Fe-based glass-coated microwires

Microstructural and magnetic behaviours of two thin Fe-rich glass-coated microwires (Fe73.5CU1Nb3Si13.5B9 and Fe79Hf7B12Si2) with a nanocrystalline structure of such materials are presented. The nanocrystalline state is obtained by heating (300 - 700 degrees C, 1 hour) the precursor amorphous microwires. Structural characteristics of the as-cast and annealed samples were determined, at room temperature, by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Such techniques allow to obtain the evolution of the grain size and relative fraction volume (5 - 65%) of the nanograins. Thermal dependencies of H, are qualitatively similar in both compositions and results to be also qualitatively similar to that reported for the classical Finemet ribbon. In fact, H, decreases from the as-cast (relaxation process) showing a maximum around 450 degrees C (pre-nucleation of nanograins) decreasing significantly between 500-600 degrees C (exchange coupling of the nanograins). It is remarkable to note that the H-c values are larger in these glass-coated microwires as comparing with the nanocrystalline Finemet ribbon of similar composition. Such effect should be ascribed to the presence of the glass insulating sheath inducing strong and complex internal stresses on the metallic nucleus which affects dramatically the soft magnetic character.