Investigation of mixed spinel structure of nanostructured nickel ferrite

Single crystalline nickel ferrite (NiFe2O4) which has an inverse spinel structure is reported to exhibit a mixed spinel structure when its grain size is reduced to nanometer range. It is known that structural transformations in the nanosize regime are not akin to those of bulk crystals. Additionally, magnetic properties like superparamagnetism and contribution to it by surface layers are critically influenced by the synthesis route of the samples. In this article, we present an investigation on the structural transformation of nanostructured NiFe2O4 using x-ray diffraction (XRD), Mössbauer and magnetization measurements, and Fourier transform infrared (FTIR) and micro-Raman spectroscopy. Nanostructured NiFe2O4 samples were synthesized by a coprecipitation technique followed by thermal processing. Four samples having average grain sizes 13, 20, 26, and 51 nm were synthesized. XRD results confirmed the samples to be nanostructured NiFe2O4 and gave evidence for the mixed spinel structure of the samples of lower grain sizes. Mössbauer study of the smallest and largest grain-sized samples revealed surface spin canting and change in coordination of the iron ions at tetrahedral and octahedral sites with reduction in grain size. The spin canting angle and the coordination factor were determined from the Mössbauer spectra. Vibrating sample magnetometer measurements gave a lower value of magnetization for the samples of lowest grain size and this observation could be explained on the basis of a structural transformation of the sample from inverse to mixed spinel. FTIR and micro-Raman spectroscopic studies also yielded convincing evidence for a transformation of the structure. The results of the present study lead to the inference that the properties of nanosized NiFe2O4 particle emerged from a transformation of their structure from inverse spinel structure to mixed spinel.