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Jong-Il Park, Min Gyu Kim, Young-wook Jun, Jae Sung Lee, Woo-ram Lee, and Jinwoo Cheon*
Characterization of Superparamagnetic “Core−Shell” Nanoparticles and Monitoring Their Anisotropic Phase Transition to Ferromagnetic “Solid Solution” Nanoalloys
J. Am. Chem. Soc., 126 (29), 9072-9078
Date: Jul 1, 2004

The structure, magnetism, and phase transition of core−shell type CoPt nanoparticles en route to solid solution alloy nanostructures are systematically investigated. The characterization of CocorePtshell nanoparticles obtained by a “redox transmetalation” process by transmission electron microscopy (TEM) and, in particular, X-ray absorption spectroscopy (XAS) provides clear evidence for the existence of a core−shell type bimetallic interfacial structure. Nanoscale phase transitions of the CocorePtshell structures toward c-axis compressed face-centered tetragonal (fct) solid solution alloy CoPt nanoparticles are monitored at various stages of a thermally induced annealing process and the obtained fct nanoalloys show a large enhancement of their magnetic properties with ferromagnetism. The relationship between the nanostructures and their magnetic properties is in part elucidated through the use of XAS as a critical analytical tool.

 

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