Magnetoresistance in arrays of fine graphite powders with nearest-neighbor tunneling conduction

Resistance of samples prepared by compressing submicron graphite powders was measured in the temperature range 1.8-300 K and magnetic field up to 4.5 T. The temperature dependence of resistance for some samples is satisfactorily described by a fluctuation-induced tunneling model R∝exp[T₁/(T+T₂)]. In others resistance follows a power law R∝1/T^α with α close to 1, and we propose a phonon-assisted bunch hopping (or tunneling) model for this case. The dominating mechanism is determined by the effective tunneling distance. Magnetoresistance (MR) is dominantly positive, with mesoscopic oscillations at low fields, in the fluctuation-induced tunneling regime, and has a negative component in the phonon-assisted bunch hopping regime. Our results confirm the existence of negative MR in systems with nearest-neighbor tunneling conduction and point the conditions for this effect. We interpret NMR as due to shifting of discrete electron levels towards the Fermi level in the magnetic field, which enhances the phonon-assisted bunch hopping.