Effect of sintering treatment on the microstructure of NiFe2O4 synthesized by the sonochemical method and the conventional method

Nickel ferrite ultrafine nanoparticles were synthesized by the sonochemical method. The sample was sintered at temperatures ranging from 1273 K to 1673 K for 3 h. XRD peaks of as-synthesized NiFe2O4, their Transmission Electron Microscopy (TEM), high resolution TEM, and selective area (electron) diffraction pattern demonstrate that the particles are in the completely crystalline state. The initial particle size of as-synthesized NiFe2O4 was found to be approximately <5 nm. Besides, NiFe2O4 samples were also synthesized by a conventional double sintering technique. The purpose was to compare the effect of sintering treatment on the microstructure quality of the NiFe2O4 prepared by these two techniques. Homogeneous coaxial grains did not form until 1573 K for conventionally prepared samples, while for the sonochemical method, homogeneous grains started to form even as low as 1373 K. Furthermore, other measurements were done only for the sintered samples prepared by the sonochemical method to evaluate the magnetic properties. An abrupt change in B–H loops was found with Ts for a maximum applied field of 1500 A/m. M–H loops with the maximum applied field of 1.6 × 103 KA/m and Mössbauer spectroscopy demonstrate that the samples are all at the ferrimagnetic stage. Curie temperatures Tc, determined from the temperature dependence of the initial permeability μ′, for the same samples almost remained unchanged, which confirms that the cation distribution is almost unchanged with the variation of Ts. A slight variation of cation distribution manifested in the variation of Tc with Ts conforms with the site occupancy of Fe3+ analyzed by Mössbauer spectroscopy.