Role of the oxygen partial pressure in the formation of composite Co-CoO nanoparticles by reactive aggregation
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The magnetic properties of diluted films composed of nanocomposite Co-CoO nanoparticles (of ~8 nm diameter) dispersed in a Cu matrix have been investigated. The nanoparticles were formed in an aggregation chamber by sputtering at different Ar/O2 partial pressures (0–0.015). The exchange-bias properties appear to be insensitive to the amount of O2 during their formation. However, the temperature dependence of the magnetization, M(T), exhibits two different contributions with relative intensities that correlate with the amount of O2. The magnetic results imply that two types of particles are formed, nanocomposite Co-CoO (determining the exchange-bias) and pure CoO, as confirmed by transmission electron microscopy observations. Importantly, as the O2 partial pressure during the sputtering is raised the number of nanocomposite Co-CoO nanoparticles (exhibiting exchange-bias properties) is reduced and, consequently, there is an increase in the relative amount of pure, antiferromagnetic CoO particles.
KeywordsExchange bias Magnetic nanoparticles Cluster source Nanocomposite
We thank M. Rivera and E. Prado for their assistance in the synthesis of the samples, and O. Crisan for his help with the TEM characterization. We acknowledge financial support from the Consejería de Educación y Ciencia de Castilla-La Mancha (PAI08-0203-1207), the Generalitat de Catalunya (2009SGR1292), and the Consejo Interministerial de Ciencia y Tecnología (CICYT—MAT2008-01158/NAN and MAT2010-20616-C02).
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