We present a study on the magnetic behavior of dextran-coated magnetite nanoparticles (DM NPs) with sizes between 3 and 19 nm, synthesized by hydrothermal-assisted co-precipitation method. The decrease of saturation magnetization (Ms) with decreasing particle size has been modeled by assuming the existence of a spin-disordered layer at the particle surface, which is magnetically dead. Based on this core–shell model and taking into account the weight contribution of non-magnetic coating layer (dextran) to the whole magnetization, the dead layer thickness (t) and saturation magnetization Ms of the magnetic cores in our samples were estimated to be t = 6.8 Å and Ms = 98.8 emu∕g, respectively. The data of Ms were analyzed using a law of approach to saturation, indicating an increase in effective magnetic anisotropy (Keff) with decreasing the particle size as expected from the increased surface/volume ratio in small MNPs. The obtained Keff values were successfully modeled by including an extra contribution of dipolar interactions due to the formation of chain-like clusters of MNPs. The surface magnetic anisotropy (Ks) was estimated to be about Ks = 1.04 ×105J∕m3. Our method provides a simple and accurate way to obtain the M s core values in surface-disordered MNPs, a relevant parameter required for magnetic modeling in many applications
