Magnetism of nanophase metal and metal alloy particles formed in ordered phases

In general, the intrinsic magnetic properties of a single metallic elementa l can be increased by forming alloys containing one or two additional metal s. In this article, metallic cobalt, cobalt/platinum alloys, and gold-coate d cobalt/platinum nanoparticles have been synthesized in reverse micelles o f cetyltrimethlyammonium bromide. Magnetic characterization of all samples demonstrate that the particles containing platinum and gold exhibit a highe r blocking temperature and larger coercivities relative to pure cobalt nano particles of the same size. The dc susceptibility of a sample of 15 nm coba lt nanoparticles exhibit a blocking temperature of 70 K and coercivity, H-c , of 1800 G at 2 K. When equimolar quantities of cobalt and platinum were c ombined and reduced in the reverse micelle, the blocking temperature increa sed to 130 K and H-c at 2 K is reported as 2700 G. When additional platinum is added, however, the blocking temperature dropped to 100 K and coercivit y at 2 K decreased to 2000 G. Addition of a gold coating to the equimolar C oPt nanoparticles further reduced the blocking temperature to 30 K and coer civity of the sample at 2 K decreased to 1000 G. Above the blocking tempera ture, all samples exhibit superparamagnetic behavior. (C) 1999 American Ins titute of Physics. [S0021-8979(99)17808-3].