Growth of ultra-fine cobalt ferrite particles by a sol-gel method and their magnetic properties

Ultra-fine CoFe2O4 particles are fabricated by a sol-gel method and magnetic and structural properties of powders a reinvestigated. Cobalt ferrite powders fired at and above 450 degrees C have only a single-phase spinel structure and behave ferrimagnetically. Powders annealed at 350 degrees C have a typical spinel structure and are of the paramagnetic and ferrimagnetic nature, simultaneously. With X-ray diffraction and Mossbauer spectroscopy measurements, the formation of nano-crystallized particles is confirmed when cobalt ferrite is annealed at 200 degrees C. In addition, the transition from the paramagnetic to the ferrimagnetic state is observed in samples fired at 200 degrees C as the measuring temperature decreases from the room to liquid nitrogen temperature. The magnetic behaviour of CoFe2O4 powders fired at and above 350 degrees C shows that an increase of the annealing temperature yields a decrease in the coercivity and, in contrast, an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization of cobalt ferrite powders prepared by the sol-gel method are 2020 Oe and 76.5 e.m.u. g(-1), respectively. (C) 1998 Kluwer Academic Publishers.