Experimental study of Hopkinson effect in single domain CoFe2O4 particles

Nanosize particles of CoFe2O4 have been synthesized by the citrate precursor technique. Considerably higher coercive force (1.68 kOe) than that obtained by the conventional technique (1.00 kOe) is associated with the nanostructure of CoFe2O4. These nanosize ferrimagnetic CoFe2O4 particles exhibit chainlike clusters indicating strong interparticle interactions and reduced magnetic moment, which is attributed to anisotropy and canted spin structure at the surface of the particle. The magnetization shows a peak just below the Curie temperature T-c during heating in the presence of a small magnetic field (the Hopkinson effect). On the other hand, the magnetization increases monotonically when the system is cooled from T-c. This peak is associated with the single domain behavior of nanocrystalline CoFe2O4 particles and explained within the mathematical formalism given by Stoner and Wohlfarth in conjunction with other explanations of Hopkinson effect.