The control of nano-grain size and magnetic properties of Fe73Si16B7Nb3Cu1 soft magnetic powder cores

Melt-spun amorphous Fe73Si16B7Nb3Cu1 (numbers indicate at.%) alloy strip was pulverized to make flake-shaped fine powders. The powders were ball-milled to control the powder shape and to mix with polyimide-based binder of 0.5–3 wt.%, followed by cold compaction to form toroid-shaped bonded powder-metallurgical magnets. The powder cores were heat-treated to crystallize the amorphous structure and to control the nano-grain structure. Well-coated polymer binder on the powder surface decreased the coercivity and the core loss with the insulation of each powder. The analyses by differential scanning calorimetry, magnetization cores and transmission electron microscopy revealed that the best magnetic properties of powder cores was obtained after annealing for 1 h at 550–600 °C. The annealing condition to obtain the aimed grain size of 10–20 nm should be optimized considering the stored energy generated during the powder-forming process.