An "in situ" method for the production of akaganeite polymer nanocomposites is described. A controlled precipitation is achieved by using a polymer matrix, polyvinyl pyridine, containing N-base functional groups that form coordination bonds with iron ions. The resulting materials have permitted the observation of two sources of magnetic moment in akaganeite nanoparticles: (1) finite size effects with a characteristic blocking temperature below 2 K; and (2) a deficient Cl- occupancy, with a characteristic blocking temperature around 18 K. Moreover, the nanocomposites can be dissolved in slightly acidic media to obtain stable aqueous nanoparticle dispersions that could be useful in biomedical applications. (c) 2009 Elsevier Ltd. All rights reserved.