Characterization of Ferromagnetic Nanoparticles Produced by a Carbon ARC
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ABSTRACT The effect of increasing metal fraction on carbon arc nanoparticle production is examined for 10-50 weight percent cobalt starting materials. With 500 Torr of helium buffer gas, the carbon arc process yields carbon-coated FCC Co nanoparticles of similar sizes throughout this range. The saturation magnetization is believed to scale linearly with the relative abundance of Co. The variation in the coercivity with abundance is small compared to the dramatic changes which can arise from changes in the size of fine particles. The approach to magnetic saturation is more rapid in high abundance samples. This is attributed to interparticle interactions which align the easy axes when the nanoparticles crystallize within interconnected carbon shells. The switching field distribution as a function of Co abundance arises from both rotational barriers and from barriers due to the particle size distribution.
INTRODUCTION 2 The carbon arc process has been used to prepare fullerenes, l endohedral fullerenes, carbon 4 nanotubes, 3 and giant fullerenes now called carbon-coated nanocrystals or nanoparticles. Our group has focussed on this last group of carbon arc products, particularly those made of magnetic 5 materials. Paramagnetism has been observed in rare-earth (RE) carbide nanocrystals. Superparamagnetism was seen in elemental ferromagnet nanocrystals at room temperature, but are hysteretic below 160 K. 6 In all previous studies of endohedral fullerenes and carbon coated nanoparticles, the abundance of metal relative to that of carbon is below ten weight percent in the starting materials. Here we examine the effect of increasing the relative metal abundance. A process is developed enabling the abundance of cobalt metal precursor in the anode of the carbon arc to range from zero to fifty weight percent. Nanoparticles are generated as before, and then are characterized structurally to determine the effect on particle size, phase, and spacing. The magnetic properties of these particles are compared with those of bulk cobalt and those of isolated cobalt nanoparticles. The properties were then compared to the bulk Co values.
EXPERIMENTAL Composite rods were used to study the effect of concentration on saturation magnetization in Co nanoparticles. The composite rods were made by first mixing dextrin and graphite in a 3:4 ratio by weight, and then combined with varying amounts of Co metal powder. A series of samples was prepared with Co concentrations of 10, 20, 30, 40, and 50 weight percent. The powder mixtures were individually packed into a mold and baked overnight at 300°C to drive off water vapor and harden the composite rods. They were then pressed out of the mold and baked a 35 Mat. Res. Soc. Symp. Proc. Vol. 359 0 1995 Materials Research Society
second time at 200"C immediately prior to use. To create a conductive pathway in the rod, current was passed briefly with the electrodes touching prior to the standard Huffman-Kratschmer carbon arc with a 1 mm gap. The 1/4 in. diameter composite rods were used as the anode in
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