INIFTA   05425
INSTITUTO DE INVESTIGACIONES FISICO-QUIMICAS TEORICAS Y APLICADAS
Unidad Ejecutora - UE
artículos
Título:
Spontaneous oxidation of disordered fcc FePt nanoparticles.
Autor/es:
P. DE LA PRESA; T. RUEDA; A. HERNANDO; J. M. RAMALLO-LÓPEZ; L. J. GIOVANETTI; F. G. REQUEJO
Revista:
JOURNAL OF APPLIED PHYSICS
Editorial:
American Institute of Physics
Referencias:
Lugar: Melville, NY 11747-4502, USA; Año: 2008 vol. 103 p. 1 - 8
ISSN:
0021-8979
Resumen:
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In this work we present new results on spontaneous oxidation of
disordered fcc FePt nanoparticles.
The as-made oleic acid and oleylamine coated FePt nanoparticles
of average size 4 nm synthesized by a high-boiling coordinating solvent method
were exposed to air over a period of days and characterized structurally and
magnetically by means of different techniques such as XANES, XPS, EXAFS, and
SQUID magnetometry. The as-made FePt nanoparticles stabilize in the disordered
fcc structure and have a very low magnetic saturation Ms=11 emu/g and a huge coercive
field Hc=1800
Oe compared
to the low temperature bulk values of the disordered fcc FePt. We observed that
the coercive field and the magnetic saturation change with the time the sample
is exposed to air and these changes are associated with the oxidation or
passivation of the nanoparticle surface that gives place to a core-shell
structure. Indeed, the study on the electronic properties of the nanoparticles
confirms the magnetic results and indicates that when the nanoparticles are
exposed to air, changes in the oxidation state of both Fe and Pt occur, the
oxidation state of Fe coming close to hematite. The formation of hematite tends
to soften the as-made FePt nanoparticles as observed by the reduction of the
coercive field to almost one third of the original value. Although the hematite
softens the FePt nanoparticles, there is an exchange coupling at the interface
of the core-shell characterized by the increase of the coercive field from 300
to 900 Oe when the sample is cooled in an applied field of 50 kOe.