A method for the preparation of amorphous Fe 3 O 4 powder with a particle size of ca. 25 nm is reported. Amorphous Fe 3 O 4 powder can be simply synthesized by slowly exposing amorphous Fe powder to the air. The amorphous properties of Fe 3 O 4 nanoparticles were characterized by X-ray diffraction, Mössbauer spectroscopy, transmission electron micrography, differential scanning calorimetry and quantum design SQUID magnetization measurements. This amorphous Fe 3 O 4 powder is superparamagnetic and its magnetization at room temperature is very low (<1.5 emu g -1 ); it crystallizes at 285±15 °C.
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Cao et al. (1997) studied this question.