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Summary Air-stable Co, Fe, and Fe/Co nanoparticles are accessible by thermolysis of the metal carbonyl precursors in the presence of aluminium alkyls and subsequent “smooth oxidation”. The structure of the particles was investigated by transmission electron microscopy (TEM, HRTEM), X-ray absorption spectroscopy (XAS), X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), metastable impact electron spectroscopy (MIES), and small-angle neutron scattering (SANS). The peptization of the nanoparticles with suitable surfactants (oleic and lauric acid, sodium dioctylsulfosuccinate (AOT), LP-4 (a fatty acid condensation polymer), and KorantinSH (N-oleyl sarcosine)) yields magnetic fluids dispersed in carrier liquids such as toluene, kerosene, vacuum and mineral oils which are remarkably stable in air under ambient conditions. The resulting magnetic fluids show good magnetic properties. Several methods for the preparation of water-based MF are presented, e.g., formation of surfactant bilayers, using phase transfer agents, or surface modification with L-cysteine ethyl ester. Water-based metallic magnetic fluids have a high potential for a number of technical and biomedical applications. Technical applications of the Co-based ferrofluids in the field of positioning systems and magnetohydrostatic bearings were investigated. The results emphasize the scope of nanoparticulate ferrofluids having a metallic core.

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