Magnetic properties at elevated temperatures of Co substituted Finemet alloys

(Fe1−xCox)73.5Si13.5B9Nb3Cu1 (x=0–0.8) nanocrystalline alloys were prepared by annealing melt-spun amorphous precursor. According to X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies, the nanocrystalline phase consist of Fe–Co–Si bcc nanocrystals (10–20 nm). The room temperature properties have been studied in the whole compositional range. The coercive field remains low (<10 A/m) up to x=0.2 and jumps to values higher than 40 A/m for higher Co content. However, the annealed Co-rich alloys exhibit a better high temperature behaviour: the induction and coercivity were found to vary little up to 300 °C and even 500 °C in some cases. This result confirms that these alloys are good candidates for high temperature applications provided the composition and treatment are optimized for this purpose.