Magnetite (Fe3O4) nanoparticles coated with organic material are of considerable importance in various areas of engineering, as well as in biomedicine. Several papers show drastic changes in the magnetic properties related to the surface effects and the particle-particle interactions strength. However, there is no consensus about the origin or mechanisms that produce these changes, which could be different depending on the particle size and shape, coating efficiency and particle-particle interaction strength. Aiming to shed light on these issues, oleic acid (OA) coated Fe3O4 nanoparticles with different sizes were synthesized by a thermal decomposition method. Structural and microscopic results revealed Fe3O4 nanoparticles with good crystallinity and almost-spherical or polyhedral shapes depending on the solvent used for the synthesis. Infrared (FTIR) spectroscopy indicated OA molecules ...