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 beta-Mn, Zn, Fe)(C2O4).2H(2)O oxalate. Thermolysis of the oxalate for 2 h at 300 degrees C in air yields an amorphous product, whereas at 400 and 500 degrees C ferrite spinels with unit cell parameters of 8.394(7) and 8.495(3) angstrom, respectively, are formed. Decomposition of the oxalate at 500 degrees 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 degrees 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 5K of the ferrites increases with crystallite size. Hysteresis loops measurements demonstrate superparamagnetic behavior at room temperature of the nanocrystalline powder obtained at 300 degrees C. Mossbauer spectroscopy at room temperature confirms the superparamagnetic character of that sample. The ferrites obtained at 400 and 500 degrees C, however, represent a mixture of small superparamagnetic and larger ferrimagnetic particles. (C) 2010 Elsevier B.V. All rights reserved.