An aqueous magnetic fluid based on iron oxide particles as solid nano-magnetic phase was prepared by applying the chemical precipitation method. Tartaric acid (C4H6O6) was used to functionalize magnetic cores. Physical tests have been performed in order to reveal the microstructural and magnetic features, their optimization being needed for biomedical utilization. The particles’ size was investigated using transmission electron microscopy (TEM), atomic force microscopy (AFM) and magnetization measurements. Fourier transform infrared absorption spectra (FT-IR) have been recorded aiming to get some information on the solid phase structure. 1. Introduction. Recently, the synthesis of magnetic materials on the nano-scale has become a field of increased interest due to the mesoscopic properties shown by nanoparticles of quantum dimensions located in the transition region between atoms and bulk solids. Magnetic fluids (ferrofluids) are two-phase systems, consisting of small ferri- and ferromagnetic nanoparticles dispersed in a liquid [1]. Because the size of the magnetic particles lies in the nanometer range (often around 10 nm in diameter), those suspensions are referred to as magnetic nano-colloids. The colloidal nanoparticles are subject to random displacement due to their bombardment by impulsive forces from the solvent molecules, in which they are dispersed. Colloidal stability is assured by coating the nanoparticles with shells of a nonmagnetic molecular surfactant, which prevents close approach of the magnetic cores, thereby, reduces the possibility of aggregation via Van der Waals or dipolar attractions. When the dipolar interactions are much stronger than the thermal energies, particle chains start growing and forming more complex structures, depending on the particle volume fraction, size distribution, temperature or a magnetic field applied [2]. The colloidal stability of magnetic fluids is a complex issue related to the synthesis procedure followed, including the nature of surfactant(s) and carrier liquids used [3]. The magnetic fluid stability is the main characteristic that determines the possibility to exploit magnetic fluids in different industrial and biomedical applications. Magnetic fluids are said to be superparamagnetic, meaning that they are attracted by a magnetic field but retain no residual magnetism after the field is removed [1]. When the magnetic nanoparticles are dispersed in an ionic solution, the dissociation of ionogenic groups is followed by the differential adsorption on the particle surface generating a particle surface potential. Biophysicists pay more and more attention to the magnetic nanoparticles functionalized with a biocompatible nonmagnetic molecular surfac