Structural and magnetic properties of γ- and ε-Fe2O3 nanoparticles dispersed in silica matrix

Fabrication of composite materials by in situ generation of γ- and ε-Fe2O3 nanoparticles in a SiO2 matrix through sol–gel process is reported. The process involves the hydrolysis and condensation of 1:3:10:x (x = 0.05, 0.1 and 0.2) molar ratios of tetraethoxysilane, absolute ethanol, nitric acid (0.16 N) and ferric nitrate, respectively, and subsequent thermal-treatment at temperatures ranging from 110 to 1000 °C. The in situ generation and growth of γ- and ε-Fe2O3 nanoparticles, and their distribution in SiO2 matrix strongly depend on the concentration of Fe3+ ions and thermal-treatment temperatures. The restricted growth of Fe2O3 in SiO2 matrix seems to stabilize the metastable ε-Fe2O3 phase and prevent the formation of α-Fe2O3 even at 1000 °C. Further, the presence of Fe2O3 nanoparticles in SiO2 matrix modified the gel morphology on thermal-treatment, leading to strong structural and chemical changes which influence the magnetic properties to a large extent. The concentration of individual magnetic phase (γ- and ε-Fe2O3) in the samples, the particle size and distribution, and thermal-treatment temperature determine the net magnetic moment, shape of the hysteresis loop (symmetric or concentric), coercivity and magnetic phase transition.