Novel Synthesis of Silica-Coated Ferrite Nanoparticles Prepared Using Water-in-Oil Microemulsion

Ferrite nanoparticles (magnetite (Fe3O4) and cobalt‐ferrite (CoxFe3−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 CoxFe3−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 CoxFe3−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.