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

Ferrite nanoparticles (magnetite (Fe 3 O 4 ) and cobalt‐ferrite (Co x Fe 3− x O 4 )) coated with silica (SiO 2 ) 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 SiO 2 particles. The sizes of Fe 3 O 4 and Co x Fe 3− x O 4 nanoparticles were in the range of 8–12 nm and 10–14 nm, respectively, and the thickness of the SiO 2 layer was ∼14.0 nm. The solid solution of cobalt ions into Fe 3 O 4 to form Co x Fe 3− x O 4 nanoparticles led to an increase in the coercivity of the SiO 2 ‐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.