Magnetoresistance of an entangled single-wall carbon-nanotube network

The resistivity ρ(T) and magnetoresistance (MR) (Δρ/ρ) of an entangled single-wall carbon-nanotube network are investigated. The temperature dependence of the resistivity shows a negative dρ/dT from T=4.3--300 K with no resistivity minimum, which is fitted well to the two-dimensional variable-range-hopping (VRH) (ρ(T)=ρ₀exp[(T₀/T)1/3]) formula with T₀=259.2 K. The MR shows a negative H² behavior at low magnetic field. At T<~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.