Weak ferromagnetism in CoO and exchange biasing of CoPt in core-shell nanoparticles

Aphrodite Tomou 1Dimitrios Gournis 1B Kooi 2Ioannis Panagiotopoulos 1

1. Department of Materials Science and technology, University of Ioannina, Ioannina 45110, Greece
2. Materials Science Centre, University of Groningen,, Nijenborgh 4, Groningen 9747 AG, Netherlands

Abstract

CoO and CoPt/CoO core-shell nanoparticles with size around 9 nm were prepared by solution phase chemical synthesis involving simultaneous reduction of cobalt acetate and platinum acetylacetonate. This method is known to lead to a narrow particle size distribution due to the use of stabilizing agents as oleic acid and oleyl amine creating an organic residue that surrounds and separates the nanoparticles. The microstructure has been studied by Transmission Electron Microscopy (TEM). The blocking temperature of the particles is considerably lower than the Néel temperature of CoO. Magnetic measurements show appearance of coercivity and significant exchange biasing (loop shift of Heb=1125 Oe) in the CoPt/CoO composites at temperatures below 50 K, as a result of exchange coupling between CoO and CoPt. The antiferromagnetic CoO nanoparticles on the other hand, when field cooled to temperatures below 200K, show displacement of the magnetization vs field curves (along the magnetization axis) characteristic of weak ferromagnetism phenomena, that are attributed to the uncompensated surface magnetic moments. The temperature dependencies of the two phenomena are compared and discussed within recent models that stress on the correlation of unidirectional anisotropy with the pinned surface moments as opposed to the total uncompensated surface moment of CoO.

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Presentation: poster at E-MRS Fall Meeting 2005, Symposium F, by Ioannis Panagiotopoulos
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-20 07:06
Revised:   2009-06-07 00:44
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