Disproportionation of cobalt ferrite nanoparticles upon annealing

Abstract

Size control of cobalt ferrite (CoFe2O4) nanoparticles is often achieved through annealing. To better understand the annealing process we have synthesized 5 nm CoFe2O4 nanoparticles using a co-precipitation technique. Characterization of the intrinsic magnetism of these nanoparticles using magnetometry and susceptometry shows typical CoFe2O4 nanoparticle behaviour [1]. The temperature evolution of the annealing process was examined using transmission Mössbauer spectroscopy. The Mössbauer data provide clear evidence for a disproportionation of the CoFe2O4 phase, beginning at around 500 K and continuing to evolve up to 800 K. Using a combination of Mössbauer spectroscopy and x-ray powder diffraction we show that annealing the CoFe2O4 nanoparticles not only results in a larger nanocrystallite size, 17 nm, but also a significant disproportionation into a mixture of residual CoFe2O4, CoO and α-Fe(Co) (the majority phase). Upon cooling back to room temperature from 825 K a magnetite-like component develops and by room temperature this phase accounts for 20% of the Mössbauer spectral area. © 2010 IOP Publishing Ltd.