Effect of Digestion Time on Size and Magnetic Properties of Spinel CoFe2O4 Nanoparticles

We investigate the effect of digestion time on size, distribution, and magnetic properties of CoFe2O4 nanoparticles prepared by the precipitation approach. The average crystalline size increases from 14 to 19 nm as the digestion time is increased from 1 to 120 min. The observed dependence of average particle size with digestion time is in good agreement with Lifshitz, Slyozov, and Wagner theory. Our results suggest that the dissolution of Co2+-Fe3+ complexes form the surface followed by crystallization of spinel oxide through Oswald ripening process. This observation is in sharp contrast to the growth process in Fe3O4 nanoparticles where Oswald ripening is insignificant beyond 5 min due to the electron hopping between Fe2+ and Fe3+ ions. Magnetization measurements show that the saturation magnetization value decreases from 60 to 49 emu/g as the digestion time is increased from 1 to 120 min. Raman and Mossbauer spectroscopic investigations reveal that reduction of magnetization with increasing particle size is due to the formation of nonmagnetic ferrihydrite at higher digestion times.