Amorphous ultrafine powders in (Fe, Co, Ni)-B binary systems were prepared in different reduction conditions of metal ions in an aqueous solution by use of KBH4, with the aim of clarifying the effect of reaction conditions on the composition, thermal stability, and magnetic properties of the resultant amorphous powders. As the mol ratio of KBH4 to metal ions decreases, the structure of the ultrafine powders changes from amorphous to crystalline phase. The morphology of these powders is in a nearly spherical shape with a particle size of about 20 nm for the amorphous phase and changes to the chain-like or net-like shape for the crystalline phase. The B content in the Fe-B amorphous powder decreases with a decrease of the ratio of KBH4 to metal ions, and the powder size decreases with an increase of the reduction temperature.
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