In the chemical-induced transition of FeCl2 solution, the FeOOH/Mg(OH)(2) precursor was transformed into spinel structured gamma-Fe2O3 crystallites, coated with a FeCl3 center dot 6H(2)O layer. CoCl2 surface modified gamma-Fe2O3 nanoparticles were prepared by adding Co(NO3)(2) during the synthesis. CoFe2O4 modified gamma-Fe2O3 nanoparticles were prepared by adding NaOH during the surface modification with Co(NO3)(2). The CoFe2O4 layer grew epitaxially on the gamma-Fe2O3 crystallite to form a composite crystallite, which was coated by CoCl2 center dot 6H(2)O. The composite could not be distinguished using X-ray diffraction or transmission electron microscopy, since CoFe2O4 and gamma-Fe2O3 possess similar spinel structures and lattice constants. Xray photoelectron spectroscopy was used to distinguish them. The saturation magnetization and coercivity of the spinel structured gamma-Fe2O3-based nanoparticles were related to the grain size. (C) 2014 Elsevier B.V. All rights reserved.