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

Autor: Fernández-García, Maria Paz, Gorria, P., Blanco, J. A., Fuertes Arias, Antonio Benito, Sevilla Solís, Marta, Boada, R., Chaboy, Jesús, Schmool, D. S., Grenèche, Jean Marc
Rok vydání: 2010
Zdroj: Digital.CSIC. Repositorio Institucional del CSIC
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Popis: 10 páginas, 8 figuras, 1 tabla.-- PACS number(s): 75.50.Bb, 75.50.Tt, 71.70.Gm, 75.20.-g
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=5 K (Hex≈150 Oe).
Financial support from FEDER and the Spanish MICINN (Grants No. MAT2008-00407 and No. MAT2008-06542-C04) is acknowledged. One of us, M.P.F.G, thanks MICINN for the award of a FPI grant cofinanced by the European Social Fund.
Databáze: OpenAIRE
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