Magnetic and Mössbauer spectroscopy studies of NiAl0.2Fe1.8O4/SiO2 nanocomposites synthesized by sol–gel method

Single-phase NiFe2O4 nanoparticles dispersed in SiO2 (30 wt %) matrix were synthesized by sol-gel method. The effects of the annealing temperature on the structural and magnetic properties of the samples were investigated at room temperature by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer, and Mossbauer spectroscopy. The samples annealed at above 800∘C exhibited single-phase cubic spinel structure. The lattice constant of NiFe2O4 increased from 0.8348 to 0.8359 nm, the saturation magnetization of the samples increased from 8.24 to 29.28 emu/g, and their coercivity increased from 23.4 to 179.0 Oe when the average grain size of NiFe2O4 in the nanocomposites increased from 6 to 40 nm with varying annealing temperatures from 800 to 1200∘C. The thickness of the dead layer on the surface of the ferrite grain was obtained to about (0.76 ±0.03) nm. The study of Mossbauer spectroscopy indicated the evolution of magnetic properties of the samples from superparamagnetic to magnetically ordered character with increasing grain size of NiFe2O4.