Characterization of Superparamagnetic “Core−Shell” Nanoparticles and Monitoring Their Anisotropic Phase Transition to Ferromagnetic “Solid Solution” Nanoalloys

• Published in: Journal of the American Chemical Society 2004-07, Vol.126 (29), p.9072-9078
• Main Authors: Park, Jong-Il, Kim, Min Gyu, Jun, Young-wook, Lee, Jae Sung, Lee, Woo-ram, Cheon, Jinwoo
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Fine-particle systems; Magnetic properties and materials; Physics; Small particles and nanoscale materials; Studies of specific magnetic materials
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja049649k

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.