Monodisperse Fe 1− x O nanoparticles (NPs) with a mean size of 21.7 ± 2.1 nm were prepared by the thermal decomposition of iron(III) oleate complex at 380 °C using oleic acid as the solvent. Variation in their composition was monitored using XRD for a period of 120 days under ambient conditions, under which the dominant phase changed from wüstite to a spinel-type iron oxide phase. HR-TEM images and absorption spectra of the 10-day sample further revealed an FeO/spinel-type phase core−shell structure. Exchange-bias coupling on the interfaces between the wüstite and the spinel-type phase accompanied the variation in composition. The dependence of H E on temperature demonstrates that the H E onset temperature is approximately 200 K, which correlates with the T N of bulk FeO.
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Chen et al. (2010) studied this question.