Effects of Cobalt Addition in Nanocrystalline Soft Magnetic Alloy

Amorphous ribbons (~5 mm in width and ~20 μm in thickness) of Fe_83.3-xCo_xSi₄B₈P₄Cu_0.7 (x = 0-40 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 °C-550 °C). Addition of Co leads to increase in primary crystallization temperature (T_x1), and the difference between the primary and the secondary crystallization temperature (T_x2) 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 Fe_79.3Co₄Si₄B₈P₄Cu_0.7 alloy are similar to Fe rich Fe₈₅Si₂B₈P₄Cu₁ nanocrystalline alloy.