Spontaneous oxidation of disordered fcc FePt nanoparticles



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Presa Muñoz del Toro, Patricia de la and Rueda, T. and Hernando Grande, Antonio and Ramallo López, J. M. and Giovanetti, L. J. and Requejo, F. G. (2008) Spontaneous oxidation of disordered fcc FePt nanoparticles. Journal of applied physics, 103 (10). ISSN 0021-8979

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Official URL: http://dx.doi.org/10.1063/1.2931947


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 (M(s)=11 emu/g) and a huge coercive field (H_(c)=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.

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©2008 American Institute of Physics.
This work was financially supported by CM (Grant No. S-0505/MAT/0194) and MCyT (Grant Nos. MAT2005- 06119 and NAN2004-09125-C07-05), Spain; ANPCYT (Grant Nos. PICT 06-17492 and 25515) and CONICET (PIP 6075), Argentina; and LNLS (Project No. D04B-XAFS1- 6609), Campinas, Brazil. One of the authors (P.d.l.P.) gratefully acknowledges S. J. Stewart for the critical reading of the manuscript.

Uncontrolled Keywords:Magnetic-properties; X-ray; Gamma-Fe₂O₃ nanoparticles; Chemical-synthesis; Exchange bias; Shell; Iron; Anisotropy; Boundary
Subjects:Sciences > Physics > Materials
Sciences > Physics > Solid state physics
ID Code:45529
Deposited On:30 Nov 2017 19:24
Last Modified:14 Feb 2018 15:10

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