Mossbauer, Raman and X-ray diffraction studies of superparamagnetic NiFe2O4 nanoparticles prepared by sol–gel auto-combustion method

Superparamagnetic nickel ferrite single phase nanoparticles with the average crystallite size of ∼9 nm have been synthesized at a low temperature (220 °C) by the sol–gel auto-combustion method. In the present study the as prepared powder was further calcined at different temperatures for 4 h, resulting in nanoparticles of larger size. The nanoparticles exhibited superparamagnetic behavior and changes in cation distribution as revealed by the Mossbauer, Raman and X-ray diffraction studies. The Mossbauer spectra collected at 5 K and under 5 T applied magnetic field showed mixed spinel structure and canted spin order for the nanoparticles, whereas there is collinear spin order with inverse spinel structure for larger particles. The vibrational spectra of the nanoparticles showed a redshift and broadening in the Raman line shape due to confinement effects.

► Mossbauer spectra show a canting angle of 48° for the nanoparticle samples measured at 5 K and 5 T applied magnetic field, the highest canting angle obtained so far in NiFe2O4 nanoparticles. Site inversion in nanoparticles, thus converting it from inverse spinel to mixed spinel structure. ► X-ray diffraction results showed a change in sign for the strain of the nanoparticle sample that showed mixed spinel structure. ► Our Raman measurements showed a redshift and broadening for nanoparticle samples that is generally interpreted as a signature of quantum confinement.