Structural and magnetic properties of sol-gel derived CaFe2O4 nanoparticles

Highlights: • Nanocrystalline CaFe₂O₄ with cubic phase is obtained by sol-gel route. • Cubic CaFe₂O₄ fully transforms to orthorhombic CaFe₂O₄ when annealed at 1100 °C. • Cubic CaFe₂O₄ shows superparamagnetic behavior at room temperature. • Orthorhombic phase exhibits antiferromagnetic to paramagnetic transition at ∼175 K. Calcium ferrite nanoparticles with average crystallite size of ∼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 °C. Single phase orthorhombic CaFe₂O₄ was obtained upon annealing at 1100 °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 °C. As-synthesized nanoparticles heat treated at 1100 °C exhibited mixed characteristics of antiferromagnetic and paramagnetic grains with saturation magnetization of 0.4 emu/g whereas nanoparticles calcined at 400 °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 °C, which was further confirmed by Mössbauer studies carried out at different temperatures across the transition.