Striking “Nano-onion ” nanoparticles with core (γ-Fe) / double shell (α-Fe / Fe-oxide): synthesis, microstructure and magnetism.

The large variety of magnetic scenarios displayed by nanoparticle (NP) systems is mainly governed by the reduced size and/or miscellaneous morphologies of the particles, because surface, interface or finite-size effects play an important role. Furthermore, the physical-chemical nature of the surrounding medium or matrix (amorphous or crystalline, insulating or conducting, magnetic or non-magnetic…) strongly influences the magnetic properties of the NPs. The complete understanding of the correlation between microstructure, morphology and magnetic behaviour is, at present, an interesting and novel issue that can lead to functionalize NPs for potential applications. We have followed an inexpensive and easy-to-follow synthesis technique to fabricate massive (several grames) samples. The chemical process consists on a pyrolisis process taking place inside the restricted volume formed by the pores of an activated carbon and leads to the encapsulation of Fe-NPs on the nanoporous of the carbon matrix. The Fe-NPs present a broad particle-size distribution (5-40 nm) (Fig. 1). A combined structural and magnetic study seems to suggest that most of the nanoparticles of mean size ~15 nm have exotic “onion-like ” core-shell morphology of γ-Fe nucleus surrounded by a concentric double shell of α-Fe an