Coercivity of Fe–Mn powders prepared through hydrogen reduction of wet-process-synthesized oxide powders

Abstract

In this study, metallic Fe–Mn soft magnetic powders were produced via hydrogen reduction of the oxide powders consisting of nanoferrites produced by a wet process. The effects of solution concentration and pH value during the wet process on the magnetization and coercivity after the hydrogen reduction were investigated. The experimental results indicated that the solution concentration exhibited no effect on the coercivity, whereas the pH produced a large effect on the coercivity. In the wet process at pH greater than 12, irregular-shaped oxide powders were precipitated. The reduced irregular-shaped metallic Fe–Mn powders showed a small coercivity of approximately 90 A/m. In contrast, at pH = 7–8, both irregular- and plate-shaped oxide powders were precipitated, and the reduced Fe–Mn powders showed a large coercivity of ∼ 250 A/m. The reduced plate-shaped Fe–Mn powders exhibited smaller grain sizes (diameter: 15.9 µm) than those of the irregular-shaped Fe–Mn powders (diameter: ∼120–140 µm), indicating that the plate-shaped Fe–Mn powders exhibited increased coercivities. The plate-shaped metallic powders were produced by the reduction of the plate-shaped oxide powders, which contained ferrihydrite and green rust. These results reveal that the pH value of the wet process changes the oxide shape, which in turn affects the shape and grain size of the metallic Fe–Mn powders. By optimizing the wet process conditions, the coercivity of the Fe–Mn powder was improved to 80.9 A/m, although this value is lower than that of the atomized pure Fe powders.