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

Fabrication of composite materials by in situ generation of gamma- and epsilon-Fe(2)O(3) nanoparticles in a SiO(2) 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 degrees c. The in situ generation and growth of gamma- and epsilon-Fe(2)O(3) nanoparticles, and their distribution in SiO(2) matrix strongly depend on the concentration of Fe(3+) ions and thermal-treatment temperatures. The restricted growth of Fe(2)O(3) in SiO(2) matrix seems to stabilize the metastable epsilon-Fe(2)O(3) phase and prevent the formation of alpha-Fe(2)O(3) even at 1000 degrees C. Further, the presence of Fe(2)O(3) nanoparticles in SiO(2) 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 (gamma- and epsilon-Fe(2)O(3)) 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. (C) 2009