Structural and magnetic properties of sol-gel derived CaFe2O4 nanoparticles

Calcium ferrite nanoparticles with average crystallite size of similar to 11 nm have been synthesized by sol-gel method by mixing calcium and ferric nitrates in stoichiometric ratio in the presence of ethylene glycol. As-synthesized nanoparticles were annealed at different temperatures and their structural and magnetic properties have been evaluated. X-ray diffraction studies showed that unlike most ferrites, as-synthesized cubic calcium ferrite showed a slow transformation to orthorhombic structure when annealed above 400 degrees C. Single phase orthorhombic CaFe2O4 was obtained upon annealing at 1100 degrees C. Divergence of zero field cooled and field cooled magnetization curves at low temperatures indicated superparamagnetic behavior in cubic calcium ferrite particles. Superparamagnetism persisted in cubic samples annealed up to 500 degrees C. As-synthesized nanoparticles heat treated at 1100 degrees C exhibited mixed characteristics of antiferromagnetic and paramagnetic grains with saturation magnetization of 0.4 emu/g whereas nanoparticles calcined at 400 degrees C exhibited superparamagnetic characteristics with saturation magnetization of 22.92 emu/g. An antiferromagnetic to paramagnetic transition was observed between 170 and 190 K in the sample annealed at 1100 degrees C, which was further confirmed by Missbauer studies carried out at different temperatures across the transition. (C) 2017 Elsevier B.V. All rights reserved.