MAGNETIC-PROPERTIES OF NANOPHASE COBALT PARTICLES SYNTHESIZED IN INVERSED MICELLES

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.