Amorphous ribbons (∼5 mm in width and ∼20 μm in thickness) of Fe83.3-xCoxSi4B8P4Cu0.7 (x = 0C40 at%) alloys were made by single roller melt-spinning technique in air. Crystallization of ribbons takes place in two steps, i.e., primary and secondary. Nanocrystalline bcc Fe-Co grains (size ∼15-30 nm) were precipitated from the amorphous matrix during primary crystallization (temperature range ∼350 °550 °C). Addition of Co leads to increase in primary crystallization temperature (Tx1), and the difference between the primary and the secondary crystallization temperature (Tx2) tends to broaden with the increase in Co content. Lowest coercivity (∼5 A/m), highest saturation magnetization (∼195 emu/g) and a very low core loss of ∼0.32 W/kg (at 50 Hz and 1.5 T) were obtained for the ribbons with Co content ∼4 at%. Although bcc Fe has high solubility of Co, the soft magnetic properties were found to degrade for the Co contents >10 at%. Soft magnetic properties of Fe79.3Co4Si4B8P4Cu0.7 alloy are similar to Fe rich Fe85Si2B8P4Cu1 nanocrystalline alloy.
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