Abstract
The Stoner-Wohlfarth coherent rotation model has been extended to particle sizes approaching the superparamagnetic critical size S <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> for the case of barium ferrite. It is shown that at particle volumes up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100 S_{c}</tex> , the coercivity factor will be lower than the Stoner-Wohlfarth factor of 0.48. Experimentally, micro-powders of barium ferrite with variable particle size have been formed using the glass-ceramic process. The particle size was determined with X-ray and electron microscope techniques. Although coercivities in excess of 5000 Oe were measured, the comparison between experiment and theory is not good. This has been attributed to the fact that the particles are poorly crystallized even though formation proceeded by a precipitation and crystal growth process.
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