Superparamagnetic iron oxide nanoparticles were synthesized by a novel, simple and cost-effective gel-to-crystalline method by alkalizing ferrous chloride with ammonium hydroxide at 80-100 degrees C under refluxing conditions. Average crystallite size is determined as 11 nm from XRD and 11.4 nm from TEM: magnetic domain size is 9.7 nm. The saturation magnetization is 390 emu/cm(3) at 300 K, and TB is 134 K. The reduction of magnetic particle size and M-s is attributed to the presence of non-magnetic (dead) surface layer, compositional variations, superparamagnet relaxation and spin canting because of the ultrafine nature of the material. The maximization in magnetization near the blocking temperature, T-B, is attributed to a large degree of inversion of the Fe3O4 particles. Results also indicated the existence of interparticle interactions in the thermomagnetic characteristics of spinels at lower field strengths. (C) 2008 Elsevier B.V. All rights reserved.