Magnetoresistance in carbon micro-coils annealed at various temperatures

Temperature dependence of the electric resistance and the magnetoresistance for both the as-grown carbon micro-coil obtained by chemical vapor deposition and those annealed at various temperatures up to 3000°C were measured in the range from 5 to 300 K. The resistivity and the activation energy decreases with increase in the annealing temperature, which seems to show that coils annealed at higher temperature are more highly graphitized. All annealed coils show negative magnetoresistance at least at low temperature and under low magnetic field, and the positive part of the magnetoresistance becomes predominant under high magnetic field and at high temperatures in highly graphitized coils annealed at temperatures higher than 2500°C. This annealing temperature dependence of the magnetoresistance is assumed to be due to the difference of the carrier concentration by graphitization. The as-grown coil shows no magnetoresistance. The field direction dependence of the magnetoresistance suggests that the current flows helically along the fiber constituting the coil.