Synthesis and controllable oxidation of monodisperse cobalt-doped wüstite nanoparticles and their core–shell stability and exchange-bias stabilization

Abstract

Cobalt-doped wüstite (CWT), Co_0.33Fe_0.67O, nanoparticles were prepared via the thermal decomposition of CoFe₂–oleate complexes in organic solvents. A controllable oxidation process was then performed to obtain Co_0.33Fe_0.67O/CoFe₂O₄ core–shell structures with different core-to-shell volume ratios and exchange bias properties. The oxidized core–shell samples with a ∼4 nm CoFe₂O₄ shell showed good resistance to oxygen transmission. Thus, it is inferred that the cobalt ferrite shell provides a better oxidation barrier performance than magnetite in the un-doped case. The hysteresis loops of the oxidized 19 nm samples exhibited a high exchange bias field (H_E), an enhanced coercivity field (H_C), and a pronounced vertical shift, thus indicating the presence of a strong exchange bias coupling effect. More importantly, the onset temperature of H_E was found to be higher than 200 K, which suggests that cobalt doping increases the Néel temperature (T_N) of the CWT core. In general, the results show that the homogeneous dispersion of Co in iron precursors improves the stability of the final CWT nanoparticles. Moreover, the CoFe₂O₄ shells formed following oxidation increase the oxidation resistance of the CWT cores and enhance their anisotropy energy.