Annealing effects on magnetic properties of acicular hematite nanoparticles

The effect of transformation on the structure and magnetic properties of -Fe₂O₃ acicular nanoparticles (major axis, 330 nm; minor axis, 70 nm) has been investigated. The particles were prepared by hydrolysis and polymerization in an aqueous solution of FeCl₃. Particles in the as-prepared sample are constituted by sub-units of 3–5 nm. The results indicate that the changes in the magnetic properties of the samples under thermal treatments (e.g. Morin transition, superparamagnetic behaviour) are mainly caused by the coalesce of the sub-units and by the variation of the crystallinity of nanoparticles.