Synthesis of nanoparticles under restricted environments offered by water-in-oil microemulsions provides excellent control over particle size and shape and interparticle spacing. These environments have been used in the synthesis of silica nanoparticles with a ZnFe2O4 magnetic core. First, aqueous magnetic fluids constituted of zinc ferrite nanoparticles with a size ranging between 4 and 6 nm have been synthesized using a soft chemical approach. Chemical analysis has shown that the zinc ferrite nanoparticles are nonstoichiometric with the estimated formula Zn0.87Fe2.09X0.04O4 (X represents vacancies). The obtained silica nanoparticles (40-60 nm) with a zinc ferrite magnetic core (4-6 nm) have been characterized by X-ray diffraction, electron microscopy, and magnetization measurements. Preliminary magnetic measurements have inferred that the magnetic properties of these nanoparticles at low temperature are essentially governed by the interface particle-habitat.