Cobalt particles were prepared with the microemulsion method in the binary system of DDAB (didodecyldimethylammonium bromide)/toluene by reduction of CoCl2 with NaBH4. The average particle size of the as-prepared samples could be varied from 1.8 to 4.4 nm by controlling the concentration of CoCl2 in the solution of DDAB in toluene. TEM studies showed that the particles were quite uniform and well isolated. The particle sizes determined from magnetic data were consistent with those measured by TEM. The coercivity of the particles at 10 K increased from 640 to 1250 Oe as particle size increased from 1.8 to 4.4 nm. The blocking temperature of the particles increased from 19 to 50 K for the same size range. The saturation magnetization σs at 2 K increased with decreasing particle size. The value of σs of the particles with average size of 1.8 nm was about 200 emu/g, which is 20% higher than the bulk value. This implies that the magnetic moment per atom is enhanced in the nanoparticle system.
Two microemulsion systems, the ternary system H20/AOT,%ooctane and the binary system DDAB/ toluene, were used to prepare metallic cobalt particles by borohydride reduction of a cobalt salt. The particles prepared in the AOT system were extremely small and superparamagnetic with magnetic moment per particle of 11.5~s, a blocking temperature of 2 K, and q,=146 emu/g at 1.7 K. The magnetic moment and o, combined imply a particle size of about 5.4 if. However, the particles made in the DDAB system were relatively large,-10 nm. They were ferromagnetic at room temperature but with no coercivity.
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