Magnetoresistance of an entangled single-wall carbon-nanotube network

The resistivity rho(T) and magnetoresistance (MR) (Delta rho/rho) of an entangled single-wall carbon-nanotube network are investigated. The temperature dependence of the resistivity shows a negative d rho/dT from T =4.3-300 K with no resistivity minimum, which is fitted well to the two-dimensional variable-range-hopping (VRH) (rho(T)= rho(0)exp[(T-0/T)(1/3)]) formula with T-0 = 259.2 K. The MR shows a negative H-2 behavior at low magnetic field. At T less than or equal to 3.8 K and high magnetic field, the negative MR becomes positive. The positive MR tends to be saturated for H> 10 T. The negative MR with a positive upturn can be decomposed into a positive contribution from the two-dimensional spin-dependent VRH and a negative contribution from the two-dimensional weak localization, with some contribution of the Ni impurities in the sample found with the transmission electron microscope and by energy dispersive spectrometer analysis. [S0163-1829(98)07848-5].