Mössbauer studies on Mn substituted CoFe2O4/SiO2 nanocomposites synthesized by sol-gel method

Mn-substituted CoFe2O4 nanoparticles dispersed in SiO2 matrix (Co1−xMnxFe2O4/SiO2) were synthesized by a sol-gel method. The effects of Mn2+ content and annealing temperature on their structural and magnetic properties as well as cation distribution were studied in detail by X-ray diffraction, vibrating sample magnetometer and Mössbauer spectroscopy at room temperature. The results show that the Co1−xMnxFe2O4 in the samples annealed at above 700 °C exhibits cubic spinel structure. The lattice constant of Co1−xMnxFe2O4 nanoparticles increases with increasing Mn2+ content because of the substitution of Co2+ ion with a small ionic radius by Mn2+ ion with a relatively large ionic radius and most of Mn2+ ions tend to substitute Co2+ ions at octahedral B sites. The saturation magnetization, coercivity and hyperfine fields for Co1−xMnxFe2O4/SiO2 nanocomposites increase with increasing Mn2+ content when x≤0.2, and then decrease for higher Mn2+ content, these changes are strongly dependent on the cation distribution. Furthermore, with increasing annealing temperature, the crystallite size, saturation magnetization and coercivity of the samples increase, the sample transfers from the mixed state of superparamagnetic and magnetic order to the completely magnetic order and Mn2+ ions migrate from the tetrahedral A sites to octahedral B sites.