Polycrystalline iron phosphide coated iron oxide and hollow iron phosphide nanoparticles were synthesized by sonichemistry. Structure analysis indicated that the diameters of these nanoparticles were less than 14 nm, and the core-shell and. hollow nanoparticles are Fe3O4-FeP and FeP, respectively. Magnetic measurement demonstrated that both the core-shell Fe3O4-FeP and hollow FeP nanoparticles exhibited ferromagnetic behaviors. (c) 2006 Elsevier B.V. All rights reserved.