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 process. Bulk amounts of these particles were isolated from the other by-products using a magnetic field gradient. Transmission electron microscopy revealed the presence of 10-50 nm diameter crystallites coated with numerous graphitic layers. The nanocrystallite phases were identified as Gd2C3 and Ho2C3, respectively, by x-ray and electron diffraction. Magnetization measurements were performed using a superconducting quantum interference device magnetometer between +/- 5 T at temperatures ranging from 4 to 200 K. The magnetization curves were shown to scale 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 consistent 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 parameters could not be used to fit the experimental data. Empirical fits to the Brillouin function yield a reduced moment of 7.5mu(B), compared to the free-ion value of 10.6mu(B). A similarly reduced moment was observed in holmium-containing endohedral fullerenes.