Magnetite nanoparticles of 40 nm in size have been phosphated in orthophosphoric acid. Large phosphatation rates, equivalent to goethite capacity, have been pointed out, and the possibility of phosphatation−dephosphatation cycles has been proved. Phosphatation occurs rapidly, inhibits the dissolution of magnetite and does not modify the structure and the magnetization of magnetite. IR spectroscopy, X-ray photoelectron spectroscopy (XPS) analysis, and Mössbauer spectrometry have shown that phosphatation occurs by interaction with both positively charged groups and hydroxyl sites at the surface of magnetite and more precisely with Fe3+ in octahedral sites. The main surface species would be a protonated binuclear species and the top layer would be in the (111) plane. The chemical stability of magnetite during cycling and its magnetic macroscopic moment allowing an easy recycling are promising for technological uses.

The effect of surface inorganic-organic interactions on magnetic and structural properties of iron oxide magnetic nanoparticles functionalized by lipophilic stilbene molecules has been investigated. The molecules have been grafted through either phosphonate or carboxylate coupling agents. Mo ¨ssbauer spectra recorded at 300 and 77K suggest a global composition of Fe 2.82 O 4 for the two types of functionalization. Complementary in-field Mo ¨ssbauer and SQUID measurements have demonstrated that the nanoparticles consist in a magnetite core surrounded by an oxidized layer. The oxidized shell exhibits a spin canting in the carboxylate case leading to a decrease of the net magnetization of the oxide nanoparticle. No canting occurs in the phosphonate case, and the magnetic properties are therefore preserved. The magnetic properties thus depend on the coupling agent, e.g., surface interactions. This result is of primary importance to tune the magnetic properties of functionalized nanoparticles for biomedical and high density storage media applications.