Annealing temperature dependent structural and magnetic properties of MnFe₂O₄ nanoparticles grown by sol-gel auto-combustion method

Highlights: • Phase pure MnFe₂O₄ samples were prepared by sol-gel auto-combustion method. • Annealing MnFe₂O₄ below ∼500 °C, two spinel phases were observed indicating partial oxidation of Mn²⁺ to Mn³⁺. • Oxidation of Mn²⁺ to Mn³⁺ results in decrease in lattice parameter of the spinel lattice. • Annealing at ≥ 600 °C, MnFe₂O₄ decomposes into crystalline α-Mn₂O₃ and α-Fe₂O₃ along with amorphous-FeO phase. - Abstract: Manganese ferrite (MnFe₂O₄) nanoparticles were synthesized by sol-gel auto-combustion method using manganese nitrate and ferric nitrate as precursors and citric acid as a fuel. Scanning electron micrographs show irregularly shaped morphology of the particles. The as-prepared samples were annealed at 400, 500, 600 and 800 °C for 2 h in air. The phase identification and structural characterizations were performed using powder X-ray diffraction technique along with Mössbauer spectroscopy. Magnetization loops and ⁵⁷Fe Mössbauer spectra were measured at RT. After annealing the sample at or below ∼ 500 °C, we observed two different spinel phases corresponding to two different lattice parameters. This is originating due to the partial oxidation of Mn²⁺ to Mn³⁺. At high annealing temperatures (∼ 600 °C or above) the spinel MnFe₂O₄ phase decomposes into crystalline α-Mn₂O₃ and α-Fe₂O₃ phases, and amorphous FeO phase.