Superparamagnetic Ni0.5Zn0.5Fe2O4 nanoparticles prepared by ball-milling

Ni0.5Zn0.5Fe2O4 ferrite nanoparticles were synthesized by high-energy ball- milling using binary oxide precursors, followed by a subsequent heat treatment at comparatively lower temperature (500 degrees C for 1 h) to promote the redistribution of the cations to their equilibrium positions. Although, several articles are available on Ni-Zn ferrite nanoparticles, the magnetic data of ball-milled sample has hardly been analysed based on existing theoretical models to extract useful information. The samples were characterized by X-ray diffraction, scanning electron microscopy and dc magnetization study. The average crystallite sizes estimated from the structural data (using Scherrer equation) are found to be around 14.4 nm. The temperature and field variation of magnetic data suggests a superparamagnetic nature with a distribution of particle sizes. The zero field cooled and field cooled magnetization data are analysed based on a theoretical model of superparamagnetic particles with a log-normal distribution, resulting in an average particle size of 24.4 nm (width parameter of 0.23). The Langevin function adequately describes the field variation of magnetization (M-H) data at room temperature, in addition to a paramagnetic contribution, suggesting the existence of significant spin disorder. The above fit yields an average magnetic moment of 7860 mu B. The results of the magnetic study corroborate well with the structural and morphological characteristics.