Iron oxide nanoparticles were synthesized by the coprecipitation method. Two varying Fe ₃O ₄/tetraethoxysilane ratios were used for silanization: 1:1.3 and 1:4.5. The samples were investigated using transmission electron microscopy, ferromagnetic resonance, IR spectroscopy, and magnetometry. Magnetic measurements have shown that the magnetite core in nanoparticles has a higher magnetization than stoichiometric magnetite nanoparticles of the same size. The increased magnetization was caused by the deviation of the magnetite stoichiometry due to the interaction with the silicate coating. The blocking temperature distribution was determined from the temperature dependence of the coercive force and from the ZFC/FC dependencies. Nanoparticles with a thicker shell have shown greater efficiency in DNA isolation.