Synthesis of iron-nanocrystalline clusters from micron-sized iron oxides, hydroxides, and organic iron salts by reduction of gas-solid reaction

Abstract Micron-sized clusters composed of nanocrystallites are proposed as a promising structure for achieving air-stable soft magnetic powders with high saturation magnetization and low eddy current losses, and their synthesis is demonstrated. FeC 2 O 4 ·2H 2 O, α-FeOOH, α-Fe 2 O 3 , and Fe 3 O 4 are employed as starting materials. The starting material and reduction temperature affect the resulting microstructure, ranging from nanocrystalline clusters to sintered structures, while a saturation magnetization exceeding 200 emu/g is achieved. Analysis of the rotational magnetization processes reveals that the effective anisotropy field in the high-field region larger than 16 kOe is nearly constant at approximately 24 kOe and is not correlated with coercivity. This large field, compared with the magnetocrystalline anisotropy field, is attributed to exchange interactions between directly contacting crystallites. In contrast, the effective anisotropy field in the low-field region near 0 Oe ranges from 1.74 to 3.50 kOe and strongly correlates with coercivity, suggesting that it originates from flux-closure configurations induced by dipolar interactions among nanocrystallites. Since suppressing flux-closure configurations is therefore crucial for achieving soft magnetic properties, slightly sintered nanocrystalline clusters are suggested as a promising structure.