Behavior of glassy Fe77B23 upon anneal in the absence of externally applied fields

Glassy Fe77B23 when annealed at 623 K, in vacuum and in the absence of external magnetic fields, shows a deterioration of its soft magnetic properties. The coercive field Hc is found to increase sharply with time of anneal. (Hc = 220 A/m after 60 minutes of anneal, compared to the ’as cast’ value of 9 A/m). X-ray diffraction (Mo-Kα) is unable to detect crystallinity in samples annealed for up to 60 min. However, TEM studies show the emergence of small crystallites occupying less than 1% of the total volume. The size and number of these crystallites increase with time till 90 min. X-rays then show the presence only of Fe3B. The TEM results indicate that the increase in Hc is the result of eutectic crystalliztion. At this same temperature, Fe76B24 also shows a similar effect with anneal time, but to a lesser extent.