Maghemite spindle-type

The coercive force of a powder is an extrinsic property, which is influenced by many factors such as size and shape and packing density. Maghemite particles prepared from directly precipitated spindle-type hematite particles showed a tendency of increase in coercive force with decrease in particle size, and increase in their aspect ratios (31). However, a quantitative relationship between the coercive force and size and/or aspect ratio is still not available. 

Monodispersed spindle-type maghemite can be produced by first transforming spindle-type hematite (see chap. 10) into magnetite (by heating the hematite in a H2 gas stream at 340-400 °C for 1-3 h) and then oxidizing the magnetite to maghemite with air at 240 °C for 1-2 h (Ozaki and Matijevic, 1985). 

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