The magnetic nanoparticles were prepared by chemically induced transition using an FeOOH/Mg(OH)(2) precursor and an FeCl2 or FeCl2/NaOH treating solution. The morphology, bulk and surface chemical compositions, crystal properties, and specific magnetization of the magnetic nanoparticles were characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, X-ray diffraction, and vibrating sample magnetometer. A self-consistent explanation of these characterization results was proposed. As a consequence, the particles' physical structure, including the chemical phases, microstructure, average density, and sizes (crystal, physical, and magnetic sizes), were determined. The apparent magnetization of magnetic nanoparticles system having magnetic interaction between particles can be described by a modified Langevin function. It is confirmed that as-prepared nanoparticles were composed of a gamma-Fe2O3 core and a FeCl3 center dot 6H(2)O coating. The spins in the surface layer of the gamma-Fe2O3 core could be disordered to weaken the apparent magnetization of the particle system. The difference in the magnetization of the nanoparticles prepared using different treating solutions is explained, and it is revealed that additional NaOH can stimulate the epitaxial growth of gamma-Fe2O3 during the chemically induced transition. In addition, it was ascertained that the crystallite size determined by X-ray diffraction was the most probable crystallite size rather than the average crystallite size. (C) 2017 Published by Elsevier B.V.