MAGNETIC-PROPERTIES OF CARBON-COATED RARE-EARTH CARBIDE NANOCRYSTALLITES PRODUCED BY A CARBON-ARC METHOD

Carbon-coated gadolinum and holmium carbide nanocrystallites have been generated using a modification of the Huffman-Kratschmer carbon arc p rocess. Bulk amounts of these particles were isolated from the other b y-products using a magnetic field gradient. Transmission electron micr oscopy revealed the presence of 10-50 nm diameter crystallites coated with numerous graphitic layers. The nanocrystallite phases were identi fied as Gd2C3 and Ho2C3, respectively, by x-ray and electron diffracti on. Magnetization measurements were performed using a superconducting quantum interference device magnetometer between +/- 5 T at temperatur es ranging from 4 to 200 K. The magnetization curves were shown to sca le as a function of H/T. The RE3+ sites in RE2C3 have C3 site symmetry . For Gd2C3 the universal curve was fit with a Brillouin function cons istent with the Gd3+ free-ion ground-state values of J = 7/2 and g = 2 . The I-5(8) Ho3+ free-ion ground state is split, presumably due to a C3 symmetry crystal field. Consequently, for Ho2C3 the Ho3+ free-ion p arameters could not be used to fit the experimental data. Empirical fi ts to the Brillouin function yield a reduced moment of 7.5mu(B), compa red to the free-ion value of 10.6mu(B). A similarly reduced moment was observed in holmium-containing endohedral fullerenes.