The use of a ferromagnetic fluid for cooling applications represents an encouraging alternative to traditional methods; the fact that the fluid can be pumped with no moving mechanical parts, using the magnetocaloric effect, can be a great advantage for many applications where high maintenance costs and power consumption are undesirable. The nanocrystalline material suitable for this specific application must exhibit certain specific properties, such as tunable Curie temperature (TC) and high saturation magnetization (Ms). The present work is focused on the aqueous synthesis and characterization of Mn-Zn ferrite nanocrystals and their subsequent doping with rare-earth elements (Gd and Eu ions), as an attempt to modify the TC. Magnetic characterization of ferrite nanocrystals showed that room temperature Ms and the corresponding TC values were strongly dependent on the type and amount of the dopant species. © 2008 Elsevier Ltd. All rights reserved.