Positron annihilation and magnetic studies of gamma irradiated nickel ferrite nanoparticles sintered at various temperature

Abstract

The effects of gamma irradiation on structure and magnetic properties of nickel ferrite nanoparticles, prepared by the sol-gel method and sintered at 300, 500 and 900 °C, are studied through X-ray diffraction, positron annihilation lifetime spectroscopy, coincidence Doppler broadening spectroscopy and vibrating sample magnetometer. The prepared samples were irradiated with gamma rays from 60Co, 137Cs and 22Na for 1, 2 and 3 weeks that received 0.47, 0.94 and 1.41 rad dose of radiation. The XRD patterns showed that the prepared and irradiated samples are single phase with cubic spinel structure. The cation distribution, lattice constant, average crystallite size and lattice strain were obtained by the Rietveld refinement of XRD patterns and their variations with irradiation depend on sintering temperature and therefore the particle size. Scanning electron microscopy confirmed formation of nanoparticles. Positron annihilation lifetime and coincidence Doppler broadening measurements showed that for prepared samples with an increase in sintering temperature the size of vacancies increase while their concentrations decrease. It is observed that the effect of gamma radiation on samples depend on sintering temperature or particles size of samples. Also, the type of defects in sample sintered at 300 °C recognized different from samples sintered at 500 and 900 °C. Saturation magnetization and coercivity increased for prepared samples with increase in sintering temperature. The variations of saturation magnetization and coercivity with gamma irradiation for samples sintered at various temperature were different and explained with variation in cation distribution and surrounding environment of vacancies in samples. The measurements showed that samples with more vacancy concentration undergo more cation redistribution due to gamma radiation.