Magnetic properties of NiFe2O4 nanoparticles in SiO2 matrix

Inverse spinel NiFe2O4 ferrite nanoparticles (2.5 nm to 25 nm) in SiO2 matrix were prepared by sol-gel method. X-ray diffraction method and Transmission Electron Microscope were used to identify the phase and average particle diameter of the samples. The size dependent magnetic properties were investigated using a vibrating sample magnetometer. Superparamagnetic behaviour was observed at room temperature when the average particle size is less than 6 nm. The Mossbauer spectra change from two-peak to six-peak pattern with the increase of particle size and decrease in temperature. Two sets of sextets in the Mossbauer spectra indicate the presence of Fe ion in tetrahedral as well as octahedral sites. The Langevin function used to explain paramagnetism is found to describe well the magnetization behaviour of these samples consisting of weakly-interacting single domain particles in the superparamagnetic state.