NiFe2O4 nanoparticles were prepared by the sol-gel method using polyacrylic acid (PAA) as a chelating agent. It was shown that pure spinel NiFe2O4 nanoparticles with a diameter of 5∼30 nm and a specific surface area of 20.0∼55.2 m2/g could be produced by calcining the gel precursors with various molar ratios of PAA to total metal ions (0.5∼2.0) in air at 300°C for 2h. As the molar ratio of PAA to total metal ions increased particle sizes decreased but the crystallinity increased revealing PAA not only worked as a chelating agent to disperse the cations homogeneously but also contributed its combustion heat to increase the crystallinity of NiFe2O4 nanoparticles. Moreover the magnetic analysis showed the resultant NiFe2O4 nanoparticles might be superparamagnetic. Their saturation magnetization (0.19 emu/mol at 298 K) was slightly lower than that for bulk materials due to small size and increased slightly with decreasing the temperature below 360 K.
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