The CoFe 2 O 4 nanoparticles have been synthesized by using a stable ferric salt of FeCl 3 with a micellar microemulsion method. The normal micelles are formed by sodium dodecyl sulfate (NaDS) in aqueous solutions. The mean size of the nanoparticles can be controlled from less than 4 nm to about 10 nm through controlling the concentrations of the reagents. The neutron diffraction in combination with the Rietveld refinement shows that these CoFe 2 O 4 nanoparticles have a high degree of inversion with 66% of the tetrahedral sublattice occupied by Fe 3+ . Magnetic measurements and neutron diffraction studies demonstrate the superparamagnetic nature of these CoFe 2 O 4 nanoparticles. The size-dependent superparamagnetic properties of CoFe 2 O 4 nanoparticles have also been systematically studied. The blocking temperature and coercive field of the nanoparticles increase with increasing size of the nanoparticles. The superparamagnetic behaviors of CoFe 2 O 4 nanoparticles are consistent with the Stoner-Wohlfarth theory of single domain particles.
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Liu et al. (2000) studied this question.