Structural and magnetic characterization of FeNbBCu alloys as a function of Nb content

In this work we describe the changes in the crystallization behaviour and the magnetic properties with variation of the Nb content in Fe 79-xNb5+xB15Cu1 (x = 0,2,4) alloys. The microstructure of the samples, as-quenched and after several heat treatments, is analysed by transmission Mössbauer spectroscopy (TMS), x-ray diffraction, differential scanning calorimetry and vibrating sample magnetometry. The saturation polarization, Curie temperature, magnetic entropy change and the maximum value of the hyperfine field distribution of the amorphous phase are composition dependent and are enhanced with a reduction in Nb content. Devitrification is produced by the nanocrystallization of bcc-Fe followed by the precipitation of iron borides. The calorimetric analysis indicates that Nb stabilizes the alloy against nanocrystallization. Both the amount of bcc-Fe precipitates and their mean grain size decrease significantly with increasing Nb content, suggesting a higher level of disorder at the interface between the amorphous matrix and the nanocrystal that reduces grain growth. The variation in the Fe environments after crystallization events deduced from TMS measurements are analysed in terms of the amount of Nb in the alloy. Both amorphous and nanocrystallized alloys show a soft magnetic behaviour with coercivity values in the range 4-10 A m-1. © 2009 IOP Publishing Ltd.