Ferromagnetic and spin-glass behaviour of nanosized oriented pyrolytic graphite in Pb-C nanocomposites

Pb-C nanocomposites consisting of pyrolytic carbon nanospheres and Pb nanoparticles were produced by discharging a mixture of Pb and graphite powders in an ethanol and Ar atmosphere. Raman spectrum and x-ray photoelectron spectroscopy indicated that the carbons in the Pb-C nanocomposites are in both ordered and disordered forms. The Pb atoms in the Pb-C nanocomposites were removed by the treatment of a 65% HNO3 solution to form carbon nanocomposites in order to have a comparison. X-ray diffraction data and magnetic measurements revealed the magnetism of the nanosized pyrolytic carbon with different degrees of crystallization, in which disordered carbon with a (0 0 2) interlayer spacing of 0.342 nm is diamagnetic, while the oriented pyrolytic graphite with a spacing of 0.337 nm shows ferromagnetism. The saturation magnetization of highly oriented pyrolytic graphite (HOPG) nanospheres is enhanced by about two orders of magnitude, compared with that of bulk HOPG. Spin-glass behaviour of HOPG nanospheres may be caused by complex competing interactions between magnetic moments in nanosized HOPG.