Influence of carboxylic acid type on microstructure and magnetic properties of polymeric complex sol–gel driven NiFe₂O₄

Citric, oxalic and tartaric acids were used for synthesis of NiFe₂O₄ using polymeric complex precursor route. The dry precursor gels were calcined at various temperatures (400–1100 °C) for 2 h. All carboxylic acids produce iron-deficient NiFe₂O₄ with considerable amount of α-Fe₂O₃ at 400 °C. Increase in the annealing temperature caused reaction of α-Fe₂O₃ with iron-deficient ferrite phase. The amount of initially formed α-Fe₂O₃ is directly correlated with stability constant and inversely correlated with the decomposition temperature of Fe(III) carboxylate precursors. In case of tartaric acid precursor, single phase of the ferrite was obtained at 450 °C. However, in case of oxalic acid and citric acid precursors, single phase ferrite was obtained at 550 °C and 700 °C, respectively. The lattice parameters were increased with increasing annealing temperature and with decreasing the amount of α-Fe₂O₃. Maximum saturation magnetization (55 emu/g) was achieved using tartaric acid precursor annealed at 1100 °C. - Highlights: • Citric, oxalic and tartaric acids were used for synthesis of NiFe₂O₄. • Carboxylic acid type affects the produced powders. • At low temperatures all carboxylic acids produce iron-deficient NiFe₂O₄ and α-Fe₂O₃. • α-Fe₂O₃ is correlated with the decomposition of Fe(III) carboxylate precursors.