Influence of fuel-nitrate ratio on the structural and magnetic properties of Fe and Cr based spinels prepared by solution self combustion method

Highlights: • CoCrFeO₄ and NiCrFeO₄ nano particles Prepared by Solution Self Combustion Method by three different fuel-to nitrate ratios (F/N), i.e., citric acid to metal-nitrate ratios, of 0.65, 1.0 and 1.35 for fuel-lean, stoichiometric and fuel-rich samples, respectively. • The formation of nanoparticles (NPs) with the pure spinel structure was confirmed for the fuel-rich and stoichiometric cases, but α-Ni and α-Co secondary phases were found in case of fuel- lean samples. • The magnetic properties (Mr, Ms and Hc) were found to depend on the F/N ratio. The relatively large values of Mr and Ms observed for the fuel-lean samples originate from the Ni and Co metallic secondary phases. • CoCrFeO₄ samples show superparamagnetism in the stoichiometric and fuel-rich cases. • The ⁵⁷Fe RT Moessbauer spectra of samples have two quadrupole doublets. In the fuel-lean cases, fitting was performed with one additional doublet for FeO phase. - Abstract: In this study, we report the synthesis of nano-sized CoCrFeO₄ and NiCrFeO₄ using the solution self combustion method and the variation in the magnetic and structural properties with different fuel to nitrate ratios-fuel lean, fuel rich and stoichiometric. Citric acid is used as the fuel. XRD analysis of the samples confirms the formation of pure spinel phased nanoparticles in fuel rich and stoichiometric cases. But CoCrFeO₄ and NiCrFeO₄ samples prepared under the fuel lean condition show the presence of a small amount of impurity phases: α-Ni in fuel lean NiCrFeO₄ and α-Co in fuel lean CoCrFeO₄. Fuel lean samples possess high magnetic saturation. The stoichiometric ratio results in finest nano-particles and structural and magnetic properties are very critically dependent on fuel to nitrate ratio.