Theoretical and experimental aspects of coercivity versus particle size for barium ferrite

The Stoner-Wohlfarth coherent rotation model has been extended to particle sizes approaching the superparamagnetic critical size Scfor the case of barium ferrite. It is shown that at particle volumes up to , 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.