Novel synthesis of silica-coated ferrite nanoparticles prepared using water-in-oil microemulsion

Ferrite nanoparticles (magnetite (Fe₃O₄) and cobalt-ferrite (Co_xFe_3-xO₄)) coated with silica (SiO₂) were prepared using water-in-oil microemulsion of a polyoxyethylen(15)cetylether/cyclohexene system. Observation via transmission electron microscopy revealed that the ferrite nanoparticles were located nearly at the center of spherical SiO₂ particles. The sizes of Fe₃O₄ and Co_xFe_3-xO₄ nanoparticles were in the range of 8-12 nm and 10-14 nm, respectively, and the thickness of the SiO₂ layer was ∼ 14.0 nm. The solid solution of cobalt ions into Fe₃O₄ to form Co_xFe_3-xO₄ nanoparticles led to an increase in the coercivity of the SiO₂-coated ferrite nanoparticles. The coercivity increased with increasing amounts of added cobalt, obtaining a maximum value of 780 Oe around a Co/Fe ratio of 0.3-0.4.