Nanocrystalline Mn–Zn ferrites from mixed oxalates: Synthesis, stability and magnetic properties

Nanocrystalline Mn–Zn ferrite spinel powders were synthesized by thermal decomposition of mixed β-(Mn, Zn, Fe)(C2O4)·2H2O oxalate. Thermolysis of the oxalate for 2 h at 300 °C in air yields an amorphous product, whereas at 400 and 500 °C ferrite spinels with unit cell parameters of 8.394(7) and 8.495(3) Å, respectively, are formed. Decomposition of the oxalate at 500 °C for 24 h results in a mixture of haematite and a Fe-poor spinel revealing the metastability of the nanocrystalline spinel at that temperature. The crystallite sizes are 12 and 45 nm for ferrite spinel powders synthesized at 400 and 500 °C, respectively. Zero ferrous ion concentrations were measured by redox titration, suggesting that the ferrites are defect spinels containing Fe3+ and cation vacancies. Thermal analysis and XRD demonstrate that the ferrites undergo several phase transitions during annealing in air.Magnetic measurements reveal that the saturation magnetization at 5 K of the ferrites increases with crystallite size. Hysteresis loops measurements demonstrate superparamagnetic behavior at room temperature of the nanocrystalline powder obtained at 300 °C. Mössbauer spectroscopy at room temperature confirms the superparamagnetic character of that sample. The ferrites obtained at 400 and 500 °C, however, represent a mixture of small superparamagnetic and larger ferrimagnetic particles.

Research highlights▶ The structural and morphologic properties are reported. ▶ The fact that a spinel is formed as metastable ferrite phase at low-temperature is highlighted. ▶ The phase transformation series of this defective low-temperature phase to a homogeneous high-temperature Mn–Zn ferrite is studied. ▶ The magnetic properties are correlated with the nanocrystalline nature of the ferrites. ▶ Mössbauer spectrometry reveals superparamagnetic and ferromagnetic particles.