Sonochemical decomposition of Fe(CO)5 was carried out in the presence of different surfactants. The reactions give stable colloids of undecenoate, dodecyl sulfonate, and octyl phosphonate coated Fe2O3 nanoparticles of 5-16 nm in diameter. The ionic binding of the surfactants to the nanoparticle surfaces was confirmed by FTIR spectroscopy. Electron paramagnetic resonance measurements, magnetization curves, and zero-field cooled and field cooled studies indicate that the as-prepared amorphous nanoparticles are superparamagnetic. These studies show that the phosphonate-coated nanoparticles behave in a strikingly different manner from the other particles. It is proposed that the extra negative charge on the phosphonate, as compared to the carboxylate and sulfonate groups, makes it a strong bridging bidentate ligand, resulting in the formation of strong ionic bonds to the surface Fe 3+ ions, which decreases the number of unpaired spins, possibly through a double superexchange mechanism through a Fe 3+ -O-P-O-Fe 3+ pathway.

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We show that sonochemistry is an efficient and facile route for quantitative coating of γ-Fe2O3 nanoparticles with octadecyltrihydrosilane (OTHS, CH3(CH2)17SiH3). The presence of C−H stretching (2950−2850 cm-1) and bending (1475−1375 cm-1) bands, and the absence of peaks at 2150 and 925 cm-1 (Si−H stretching and bending respectively) confirm the presence of grafted hydrocarbon chains in the irradiated sample, hence the reaction of OTHS with γ-Fe2O3 nanoparticles. The coated nanoparticles show increased magnetization compared to the uncoated ones. X-ray diffraction, magnetization studies, Mössbauer spectra, and electron paramagnetic resonance spectra reveal that the magnetic ordering is more prominent in the OTHS-coated γ-Fe2O3 sample because of improved crystallinity.