Sonochemical preparation of nanosized amorphous NiFe₂O₄ particles

Nanosized amorphous NiFe₂O₄ powder was prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO)₅ and Ni(CO)₄, in decalin at 273 K, under an oxygen pressure of 100-150 kPa. The amorphous nature of these particles was confirmed by various techniques, such as SEM, TEM, electron microdiffraction, and X-ray diffractograms. Magnetic measurements, Mössbauer, and EPR spectral studies indicated that the as-prepared NiFe₂O₄ ferrite particles were superparamagnetic. The Mössbauer spectrum of the crystallized sample showed a clear sextet pattern, with hyperfine field values of 500 and 508 kOe for A (tetrahedral) and B (octahedral) sublattices, respectively, of the inverse spinel NiFe₂O₄. Saturation magnetization of the annealed sample (25 emu/g) was significantly lower than that for the reported multidomain bulk prticles (55 emu/g), reflecting the ultrafine nature of the sample. Thermogravimetric measurements with a permanent magnet gave Curie temperatures of 440°C for amorphous and 560°C for the crystallized forms.