Temperature dependence of magnetic properties of NiFe2O4 nanoparticles embeded in SiO2 matrix

Spinel ferrite NiFe2O4 nanoparticles (<= 25 nm) in SiO2 matrix were prepared by sol-gel method. The phase and average crystallite size of the samples were determined by X-ray diffraction method and the particle size distributions were studied by a transmission electron microscope. Magnetic properties of the samples were investigated with different ferrite particle sizes and at various temperatures down to 10 K. Superparamagnetic properties were observed at room temperature when the particle size is less than 10 nm. In superparamagnetic state, the field dependence of magnetization follows Langevin function which was originally developed for paramagnetism. The effective anisotropy constant K-eff is found to increase significantly with the decrease in particle volume and an order of magnitude higher than that of the bulk samples when the particle size is below 5 nm due to the dominance of surface anisotropy. In case of nanosized systems, the effect of size reduction on the law of approach to saturation has also been studied in detail. (C) 2006 Elsevier B.V. All rights reserved.