Exchange-coupled α-Fe/Fe3O4 bi-magnetic nanoparticles with non-core/shell morphology formed by eutectoid decomposition of FeO nanoparticles

•α-Fe/Fe3O4 bi-magnetic nanoparticles with non-core/shell morphology have been prepared.•The process is based on the eutectoid decomposition of chemically synthesized FeO nanoparticles.•FeO nanoparticles were fully decomposed after annealed at 673 K for 1 h in an inert atmosphere.•A smooth, single-phase-like magnetization curve was obtained at room temperature.•The smoothness in the curve suggests that the two magnetic phases could be exchange-coupled. A new approach to fabricating ferrite-based bi-magnetic nanoparticles with non-core/shell morphology, generated by a lamella-based phase separation process induced by the eutectoid reaction, is presented. The fabrication process is based on the eutectoid decomposition of chemically synthesized FeO nanoparticles into a nanoscale mixture consisting of a ferromagnetic metal (α-Fe) and a ferrimagnetic spinel ferrite (Fe3O4). It was found that decomposition of FeO was almost fully completed after annealed at 673 K for 1 h in an inert atmosphere. The room-temperature magnetization measurements gave rise to smooth curves with single-phase-like behavior despite the coexistence of two magnetic phases, suggesting that the two magnetic phases could be magnetically exchange-coupled. Since the substitution of constituent metal ions can tune the magnetic properties of the spinel ferrites, the fabrication process described here is expected to provide a potential route to making various kinds of iron-based alloy/ferrite bi-magnetic nanoparticles.