Tailoring the surface of nanoparticles is essential for biological applications of magnetic nanoparticles. FePt nanoparticles are interesting candidates owing to their high magnetic moment. Established procedures to make FePt nanoparticles use oleic acid and oleylamine as the surfactants, which make them dispersed in nonpolar solvents such as hexane. As a model study to demonstrate the modification of the surface chemistry, stable aqueous dispersions of FePt nanoparticles were synthesized after ligand exchange with mercaptoalkanoic acids. This report focuses on understanding the surface chemistry of FePt upon ligand exchange with mercapto compounds by conducting X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) studies. It was found that the mercapto end displaces oleylamine on the Pt atoms and the carboxylic acid end displaces the oleic acid on the Fe atoms, thus exposing carboxylate and thiolate groups on the surface that provide the necessary electrostatic repulsion to form stable aqueous dispersions of FePt nanoparticles.

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Thin films of the double perovskite La 2 NiMnO 6 ͑LNMO͒ have been grown on various lattice-matched substrates ͑SrTiO 3 , LaAlO 3 , NdGaO 3 , and MgO͒ by pulsed laser deposition under different oxygen background pressure ͑25-800 mTorr͒ conditions. The out-of-plane lattice constant of the LNMO film decreases with increasing pressure, which is likely caused by reduction in the defect concentration and improved structural ordering, before leveling off at higher oxygen concentrations. The scanning transmission electron microscopy results confirm that the films are epitaxial, and the interface is sharp and coherent. While few defects are observed in a film grown at a high oxygen pressure ͑800 mTorr͒, a film grown at a lower pressure ͑100 mTorr͒ clearly shows the formation of defects that extend throughout the thickness except for a very thin layer near the interface. The Raman spectra of the films are dominated by two broad peaks at around 540 and 685 cm −1 , which are assigned to the antisymmetric stretching and symmetric stretching modes of MnO 6 and NiO 6 octahedra, respectively. The Raman peaks of the LNMO thin films grown in the 800 mTorr background O 2 are blueshifted in comparison to those of bulk LNMO, and the shift increases with decreasing film thickness, indicating the increased influence of strain. The critical thickness for strain relaxation, as determined from the Raman spectra, is between 40 and 80 nm. However, the strain is observed to have negligible influence on the magnetic properties of films grown at high oxygen pressures. In contrast, films grown at low pressures exhibit degraded magnetic properties, which can be attributed to a combination of increased B-site cation disorder and the concentration of Mn 3+ and Ni 3+ Jahn-Teller ions caused by oxygen or cation related defects. With increasing oxygen pressure during growth, the paramagnetic-ferromagnetic transition temperature ͑ϳ280 K͒ becomes sharper and the saturation magnetization at low temperatures is enhanced. Based on the electron energy loss spectroscopy studies, the Mn and Ni ions in LNMO thin films are determined to be mixed-valent Mn 3+ / Mn 4+ , and charge transition disproportionation of the Mn 4+ +Ni 2+ → Mn 3+ +Ni 3+ type likely occurs with increasing oxygen deficiency.