Microstructure and magnetism of nanoparticles with γ-Fe core surrounded by α-Fe and iron oxide shells

Iron-carbon nanocomposites have been elaborated by means of a simple chemical procedure based on in situ synthesis of iron nanoparticles within the nanopores of an activated carbon. The Fe nanoparticles present a broad particle-size distribution (5-40 nm). A combined structural and magnetic study seems to suggest that most of the nanoparticles of mean size ∼15 nm have exotic "onionlike" core-shell morphology of γ-Fe nucleus surrounded by a concentric double shell of α-Fe and maghemitelike oxide. The true nature of Fe-oxide was successfully evidenced through room temperature x-ray absorption spectroscopy. The whole system does not reach a fully superparamagnetic regime even at 750 K, probably due to higher blocking temperatures for the largest nanoparticles. Mössbauer spectrometry indicates that low temperature para-to-antiferromagnetic transition for the γ-Fe phase cannot be discarded. In addition, the external Fe-oxide shell exhibits spin-glass behavior giving rise to the freezing of its magnetic moments at low temperatures. Hence, we propose a competing double magnetic coupling: (i) the oxide shell/ α-Fe interaction and (ii) the possible antiferromagnetic coupling between γ-Fe nucleus and α-Fe layer; as being both responsible for the observed exchange bias effect at T=K (H _ex ≈ 150 Oe).