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Dumbbell-like Au−Fe₃O₄ nanoparticles are synthesized using decomposition of Fe(CO)₅ on the surface of the Au nanoparticles followed by oxidation in 1-octadecene solvent. The size of the particles is tuned from 2 to 8 nm for Au and 4 nm to 20 nm for Fe₃O₄. The particles show the characteristic surface plasmon absorption of Au and the magnetic properties of Fe₃O₄ that are affected by the interactions between Au and Fe₃O₄. The dumbbell is formed through epitaxial growth of iron oxide on the Au seeds, and the growth can be affected by the polarity of the solvent, as the use of diphenyl ether results in flower-like Au−Fe₃O₄ nanoparticles.