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Nanomaterials 2017, 7(3), 61; doi:10.3390/nano7030061

Exchange Bias Optimization by Controlled Oxidation of Cobalt Nanoparticle Films Prepared by Sputter Gas Aggregation

1
Instituto Regional de Investigación Científica Aplicada (IRICA) and Departamento de Física Aplicada, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
2
Instituto de Energías Renovables, Universidad de Castilla-La Mancha, 02071 Albacete, Spain
*
Authors to whom correspondence should be addressed.
Academic Editor: Thomas Nann
Received: 31 January 2017 / Revised: 6 March 2017 / Accepted: 7 March 2017 / Published: 11 March 2017
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Abstract

Porous films of cobalt nanoparticles have been obtained by sputter gas aggregation and controllably oxidized by air annealing at 100 °C for progressively longer times (up to more than 1400 h). The magnetic properties of the samples were monitored during the process, with a focus on the exchange bias field. Air annealing proves to be a convenient way to control the Co/CoO ratio in the samples, allowing the optimization of the exchange bias field to a value above 6 kOe at 5 K. The occurrence of the maximum in the exchange bias field is understood in terms of the density of CoO uncompensated spins and their degree of pinning, with the former reducing and the latter increasing upon the growth of a progressively thicker CoO shell. Vertical shifts exhibited in the magnetization loops are found to correlate qualitatively with the peak in the exchange bias field, while an increase in vertical shift observed for longer oxidation times may be explained by a growing fraction of almost completely oxidized particles. The presence of a hummingbird-like form in magnetization loops can be understood in terms of a combination of hard (biased) and soft (unbiased) components; however, the precise origin of the soft phase is as yet unresolved. View Full-Text
Keywords: magnetic nanoparticles; core-shell nanoparticles; Co/CoO; exchange bias; magnetic properties magnetic nanoparticles; core-shell nanoparticles; Co/CoO; exchange bias; magnetic properties
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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MDPI and ACS Style

Antón, R.L.; González, J.A.; Andrés, J.P.; Normile, P.S.; Canales-Vázquez, J.; Muñiz, P.; Riveiro, J.M.; De Toro, J.A. Exchange Bias Optimization by Controlled Oxidation of Cobalt Nanoparticle Films Prepared by Sputter Gas Aggregation. Nanomaterials 2017, 7, 61.

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