Cobalt ferrite nanoparticles (CoFe2O4 NPs) were synthesized by coprecipitation followed by treatments with diluted nitric acid and sodium citrate. Transmission electron microscope (TEM) and photon correlation spectroscopy (PCS) characterization showed that the size distributions of these nanoparticles were monodisperse and that no aggregation occurred. This colloid showed a long-term stability. Through adjustment of the concentrations of reactants and reaction temperature, the size of the NPs can be tuned from 6 to 80？nm. The size-control mechanism is explained by a nucleation-growth model, where the local concentration of monomers is assumed to decide the size of nuclei, and reaction temperatures influence the growth of nuclei. Magnetization and relaxivity measurements showed that the NPs revealed size-dependent magnetization and relaxivity properties, which are explained via a “dead magnetic layer” theory where reductions of saturation magnetization ( ) and are assumed to be caused by the demagnetization of surface spins. 1. Introduction The development of uniform magnetic nanoparticles (MNPs) has been intensively pursued for their scientific and technological importance [1–3]. The synthesis of MNPs with average sizes from 2 to 50？nm is of significant importance because of their applications in several fields, especially in biomedicine for magnetic resonance imaging (MRI) [4, 5], cell labelling [6, 7], and drug delivery [8–11]. Of special interest are their magnetic properties in which the differences between a massive or bulk material and a nanoscaled one are especially pronounced. The magnetic properties are particularly sensitive to the particle size, which is determined by the finite size effects (related to the reduced number of spins cooperatively linked within the particle), and by surface effects (greater as the particle size decreases) [12–15]. The water solubility of MNPs is necessary for medical applications, and their aggregation, caused by the huge specific surface area and magnetic interactions, must be avoided. It is thus necessary to adopt methods to stabilize the MNPs, either by using surfactants or by changing their surface potential. Oleic acid and oleylamine are the surfactants most used for the synthesis of MNPs in organic solvent [16–19] and citric acid for water phase synthesis [20–23], as these capping agents tend to be absorbed on the particular high-energy facets. Their overall specific surface energy is more or less reduced, so the tendency towards aggregation is decreased. The ratio between growth rates in different
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