Anomalous magnetic behaviour at nano-scale of Mn²⁺-substituted magnesio-ferrite synthesized by auto-combustion technique

The nano-crystalline mixed ferrites with the generic formula Mn_xMg_{1 − x}Fe₂O₄ (x = 0.0–1.0, step: 0.2) were prepared by the citrate-gel auto-combustion technique. Motivated by anomalous magnetic behaviour of coarse-grained MnFe₂O₄, the main aim was to study the influence of Mn²⁺ substitution in MgFe₂O₄ on magnetic structure at nano-regime. The compositional stoichiometry of the final ferrite products was ascertained by EDAX mapping and particle size for each sample was determined by powder X-ray diffraction, TEM, small-angle X-ray and neutron scattering techniques. The lattice constant increases with Mn-content due to larger cation (Mn²⁺) substitution. The distribution of cations in the tetrahedral (A) and octahedral (B) interstitial sites of the spinel lattice is determined by X-ray diffraction and Mossbauer spectral intensity analysis. The Mossbauer spectra at room temperature exhibit two sextets due to A- and B-sites for compositions x = 0.0, 0.2 and 1.0 while spectra showed central paramagnetic doublet superimposed on magnetic sextets for the samples with x = 0.4, 0.6 and 0.8, ascribed to the presence of superparamagnetic clusters. Thermal variation of AC susceptibility showed hump near the Curie temperature due to the presence of superparamagnetic clusters as corroborated by Mossbauer signature. The observed saturation magnetic moment (at temperature 5 K and applied field 9 T) is found lower compared to Neel's moment for the compositions with x > 0.2 which is explained on the basis of the exchange disorder of Fe³⁺ ions in the B-sites in Mn-containing ferrites.