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Castellano-Soria, Alberto; López-Sánchez, Jesús; Serrano, Aida; Gorni, Giulio; Varela, María; Sardinero, Ignacio; Carmona, Noemí; Hernando, Antonio; Marín, Pilar; Navarro, Elena
Journal of alloys and compounds, 10/2023, Volume: 959Journal Article
We explore a simple and optimized approach for obtaining magnetic nanoparticle-carbon composites by a novel one-pot sol-gel method. In them, various metal cations ratios of Fe3+ and Co2+ are evaluated. The ratio of Fe:Co is a critical parameter that governs the presence of different crystalline phases with tailored magnetic properties. Interestingly, the smallest introduction of Co into the synthesis (19Fe:1Co) provides an abrupt emergence of the body-centered cubic (bcc) Fe-Co alloy, instead of the orthorhombic Pnma structure characteristic of the Fe3C intermetallic compound (1Fe:0Co). Advanced structural and electronic characterizations reveal the formation of Fe-Co/Co-ferrite core/shell nanoparticle structures embedded in a carbon matrix. The sphere-like nanoparticles range from 10 to 45 nm and the shells show a spinel structure with a thickness of 2–3 nm. In addition, X-ray absorption spectroscopy unveils that the oxidation state of Fe and Co cations is close to zero, demonstrating their predominant metallic character. The magnetic properties can be modulated by a precise control of the alloy composition varying the Co content, displaying saturation magnetization values close to ∼137 emu/g. The nanoparticles are mainly single magnetic domain with a considerable coercive field (∼450 Oe), higher than those reported in the literature for Fe-Co nanoparticles. This semi-hard character is due to a notable spring exchange effect emerged by passivating the surface of Fe-Co-bcc cores with a thin Co-ferrite-like shell. •Sol-gel route for the synthesis of Fe-Co alloy/Co-ferrite core/shell nanoparticles.•Magnetic composite of nanoparticles supported on a carbon matrix.•Semihard magnetic properties by an effective spring-exchange interaction.•Low thickness of the Co-ferrite shell and elevated saturation magnetization.
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