Structural, magnetic and Mössbauer studies of Nd _0.6Sr_0.4Fe_xMn_1-xO₃ perovskites

The effect of iron doping on the structural and magnetic properties of the manganite perovskites Nd_0.6Sr_0.4Fe_xMn_1-xO₃ (0.1 ≤ x ≤ 1.0) was studied. Auto-combustion sol-gel method was used to prepare the compounds. X-ray diffraction refinement indicated that all the compounds crystallized in orthorhombic single-phase with Pnma space group. Scanning electron microscopy micrographs revealed a group of packed grains of about 0.2–1.0 μm whereas the average particle size was found to be in the range of about 40–80 nm via transmission electron microscopy. The zero-field cooled and field cooled magnetization curves, under an applied magnetic field of 0.02 T, revealed that the samples with 0.1 ≤ x ≤ 0.4 exhibit a paramagnetic to ferromagnetic transition with decreasing temperature while the samples with (0.5 ≤ x ≤ 1.0) do not demonstrate such phase transition. At room temperature the dependence of magnetization on the x iron concentration, as revealed by the corresponding M − H curves, can be interpreted as the cumulative result of changes in superparamagnetic, paramagnetic and antiferromagnetic contributions. ⁵⁷Fe transmission Mӧssbauer spectra were recorded at room-temperature (295 K) and liquid nitrogen temperature (78 K) for all the samples. Iron ions were found to be present in these compounds exclusively in a paramagnetic high-spin Fe³⁺ state for 0.1 ≤ x ≤ 0.5 at 295 K and for 0.1 ≤ x ≤ 0.3 at 78 K. At higher iron concentrations an antiferromagnetic state of iron clusters is formed due to the Fe³⁺ – Fe³⁺ superexchange interaction. Room temperature ⁵⁷Fe quadrupole splitting was found to increase near linearly with the x iron concentration up to x = 0.6. For x ≥ 0.6 the formation of paramagnetic Fe⁴⁺ is observed at 295 K, which at lower temperatures undergoes charge disproportionation leading to the appearance of Fe⁵⁺ species as observed at 78 K. Perturbations of the magnetic state of iron ions was found to lead to substantial broadening of the Mössbauer absorption peaks of magnetic components, which is interpreted as the effect of nearest neighbor Fe⁵⁺ and Mn ions at 78 K, and as that of electron-hopping related Fe³⁺ - Fe⁴⁺ valence fluctuations at 295 K.