Pyrolytic material which exhibited a negative magnetoresistance was irradiated with 2MeV electrons at temperatures below 35K. With increasing electron fluence, the zero-field resistivity decreased, while the Hall coefficient and the absolute values of negative magnetoresistance increased. The experimental data were analyzed by assuming that the densities of electrons and holes varied with the magnetic field. The analysis showed that the densities increased with the square of the magnetic field. This result was in good agreement with the Bright theory, in which the 2-dimensional Landau levels were assumed to be broadened due to defect scattering. Both intrinsic defects, and irradiation-induced defects, acted as electron acceptors. The addition of acceptors increased the ratio of hole density to electron density, and resulted in the enhancement of negative magnetoresistance. These results confirmed that the negative magnetoresistance in pyrolytic C was caused by the existence of acceptor defects and 2-dimensional Landau levels which were broadened by the defects. The results also suggested that the intrinsic acceptor defects in pyrolytic C were vacancies.

Negative Magnetoresistance of Pyrolytic Carbon and Effects of Low-Temperature Electron Irradiation. A.Iwase, N.Ishikawa, T.Iwata, Y.Chimi, T.Nihira: Physical Review B, 1999, 60[15], 10811-9