Mössbauer spectroscopic studies of Al³⁺ ions substitution effects in superparamagnetic Mg_0.2Mn_0.5Ni_0.3Al_yFe_2−yO₄ compositions

Nanoparticles of Al³⁺ ions substituted Mg−Mn−Ni materials with compositions Mg_0.2Mn_0.5Ni_0.3Al_yFe_2−yO₄ (y = 0.15–0.25) were synthesized by citrate precursor technique. Samples were characterized by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer and room temperature ⁵⁷Fe Mössbauer spectroscopy. Saturation magnetization decreases with increasing Al³⁺ ions concentration because replacement of Fe³⁺ ions by Al³⁺ ions at octahedral B-site weaken sublattice interaction and lowers magnetic moments. Mössbauer spectral studies show that as-prepared nano-sized samples are superparamagnetic at room temperature. Superparamagnetic relaxation was observed for small crystallite in samples with higher Al content, which is attributed to weakening of A–B exchange interaction. Mössbauer spectra at 300 K show a gradual collapse of magnetic hyperfine splitting typical for superparamagnetic relaxation. An increase in inversion parameter is observed with increasing Al³⁺ ions substitution, which is attributed to decrease in crystallite size. - Highlights: • Single phase nanocrystalline samples were synthesized by citrate precursor method. • Particle size decreases as non-magnetic Al³⁺ ions concentration increase. • Presence of doublet in Mössbauer spectra was due to superparamagnetic relaxation. • Study shows collapse of long range magnetic order and quenching of magnetic moment.