Observations of oxidation mechanisms and kinetics in faceted FeCo magnetic nanoparticles

(Fe₅₀Co₅₀)₉₇V₂Nb₁nanoparticles were synthesized in an induction plasma torch and oxidized isochronally at temperatures between room temperature (RT) and 900 °C. The particles exhibited three stages of oxidation present at different temperatures. The initial oxide layer was nearly Co-free, beginning at 3 nm at RT, and Co appeared to oxidize separately from the iron.Iron cations were determined to be the mobile species during oxidation, yielding a progressively more Co-rich core as oxidation progressed, decreasing from an average diameter of 42 to 25 nm, while the oxide tripled in size. At 350 °C, the particles exhibited a less dense core, and at 900 °C the particles were observed to have changed morphology completely with some sintering, forming completely oxidized particles with an average diameter three times the as received size.