Controlled oxidation of FeCo magnetic nanoparticles to produce faceted FeCo/ferrite nanocomposites for rf heating applications

We report the oxidation products and qualitative rates for polydisperse FeCo magnetic nanoparticles (MNPs) synthesized using an induction plasma torch. X-ray diffraction (XRD) and TEM showed MNPs to have a thin ferrite shell. Nanopowders were isochronally annealed to promote oxidation and XRD was used to follow the evolution of the FeCo core and the Fe3O4 and FeO oxide shells. Isothermal anneals were used to follow oxidation kinetics at 350 and 500 C. High resolution transmission electron microscopy (HRTEM) revealed faceted morphologies terminated at (100) and (110) FeCo faces with (110)(FeCo)parallel to (111)(oxide) and (100)(FeCo)parallel to (100)(oxide), and [010](FeCo)parallel to [011](oxide) orientation relationships between the FeCo core and oxide shell. We show HRTEM images of MNP chaining and compare the rf heating of samples of aqueous ferrofluids similarly loaded with as synthesized and oxidized MNPs. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3054376]