Magnetic Properties and Microstructures of Newly Developed Iron-Based Soft Magnetic Powders

We report the synthesis and characterization of a high-magnetization Fe-Mn powder that is easily solidified using conventional powder-metallurgy processes rather than the conventional method of using rolled electrical steel sheets. Fe-Mn powders doped with 0.1 and 33 at% manganese, referred to as “Mn0.1” and “Mn33”, respectively, were fabricated by the reduction of Mn-doped-ferrite (Fe 1−x Mn x ) 3 O 4 nanopowders with hydrogen gas at 900–1100 °C. The starting manganese-doped-ferrite nanopowder, with particles in the 5–50 nm size range, were prepared using an aqueous process. The Mn0.1 sample exhibited a saturation magnetization of ~219 emu/g, which is comparable to that of pure iron powders. The Mn0.1 and Mn33 powders, featuring crystal sizes of 0.1–10 μm, exhibited coercivities of 0.1–1 Oe; these values are much lower than that of similarly sized iron powders. To study the fine microstructures of these powders, transmission electron microscopy augmented with energy-dispersive X-ray spectroscopy was used, which revealed grains 20–100 nm in size, despite the Mn0.1 and Mn33 specimens having different manganese contents. As these sizes are rather large for coercivity to be controlled by random anisotropy, we propose that a novel magnetic-reversal mechanism operates in these Fe-Mn powders.