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

<p>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 Fe₃O₄and 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∥(111)_oxide and (100)_FeCo∥(100)_oxide, and [010]_FeCo∥[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.</p>