Stress dependence of coercivity in nanocrystalline Fe79Hf7B12Si2 glass-coated microwires

The stress dependence of the coercivity of thin Fe79Hf7B12Si2 glass-coated microwires exhibiting a nanocrystalline microstructure is presented. Such nanocrystalline microstructure was developed by means of 1 h annealing at temperatures between 450 and 600 degrees C. The evolution of the grain size (15-35 nm) with the annealing temperature was obtained from x-ray diffraction measurements. The applied tensile stress dependence of the coercive field of both as-cast and annealed samples has been measured. The coercivity monotonically decreases with the applied tensile stress for the as-cast and the low annealing temperature (T-ann < 500 degrees C) samples. This fact should be ascribed to the positive magnetostrictive character of these samples. Nevertheless, the samples annealed above 500 degrees C behave in the opposite sense, the coercivity increases with the applied tensile stress, as a consequence of the negative effective magnetostriction of the annealed samples with a massive presence of alpha-Fe(Si) and alpha-Fe nanograins. (C) 2006 American Institute of Physics.