In general, the intrinsic magnetic properties of a single metallic elemental can be increased by forming alloys containing one or two additional metals. In this article, metallic cobalt, cobalt/platinum alloys, and gold-coated cobalt/platinum nanoparticles have been synthesized in reverse micelles of cetyltrimethlyammonium bromide. Magnetic characterization of all samples demonstrate that the particles containing platinum and gold exhibit a higher blocking temperature and larger coercivities relative to pure cobalt nanoparticles of the same size. The dc susceptibility of a sample of 15 nm cobalt nanoparticles exhibit a blocking temperature of 70 K and coercivity, Hc, of 1800 G at 2 K. When equimolar quantities of cobalt and platinum were combined and reduced in the reverse micelle, the blocking temperature increased to 130 K and Hc at 2 K is reported as 2700 G. When additional platinum is added, however, the blocking temperature dropped to 100 K and coercivity at 2 K decreased to 2000 G. Addition of a gold coating to the equimolar CoPt nanoparticles further reduced the blocking temperature to 30 K and coercivity of the sample at 2 K decreased to 1000 G. Above the blocking temperature, all samples exhibit superparamagnetic behavior.

Carpenter, E. E., Seip, C. T., & O’Connor, C. J. (1999). Magnetism of nanophase metal and metal alloy particles formed in ordered phases. Journal of Applied Physics, 85(8), 5184–5186.