We have measured the temperature dependence of the in-plane electrical resistivity for stage-1 GICs and stage-1 GICs () in the temperature range between 2.6 and 300 K. The resistivity shows a drastic increase with decreasing temperature below the critical temperature for stage-1 GICs () and stage-1 GICs (). The temperature dependence of this resistivity anomaly is described by a smeared power law with an exponent , where is the critical exponent of the spontaneous magnetization. This anomaly is explained in terms of a model based on the - d exchange interaction between -electrons in the graphite layers and spins in the intercalate layers. Below , two-dimensional (2D) ferromagnetic intercalate layers are antiferromagnetically stacked along the c-axis. The -electrons are scattered by spin fluctuations of a virtual antiferromagnetic in-plane spin configuration arising from the superposition of two ferromagnetic in-plane structures with spin directions antiparallel to each other. The Co concentration dependence of for stage-1 GICs is also discussed in the light of the 2D percolation problem.

Stage-1 Cu c Co 1−c Cl 2 graphite intercalation compounds approximate quasi-twodimensional (2D) random spin systems with competing ferromagnetic and antiferromagnetic intraplanar exchange interactions. The temperature dependence of the in-plane electrical resistivity of these compounds has been measured near critical temperatures. The magnetic resistivity ζ mag consists of the long-range spin-order part ζ LS and the spin-fluctuation part ζ SF . For 0 c 0.2 the long-range spin-order part ζ LS is dominant: the temperature dependence of ζ LS is described by a smeared power law with an exponent 2β, where β is the critical exponent of staggered magnetization. For 0.3 c 0.4 the spin-fluctuation part ζ SF becomes larger than ζ LS . For 0.5 c 0.95 no appreciable magnetic resistivity is observed. For c = 1 the derivative −dζ mag /dT shows a small peak at around 67 K due to the growth of short-range spin order which is characteristic of the 2D Heisenberg antiferromagnet. The critical behaviour of the in-plane resistivity can be explained in terms of a model based on π -d exchange interactions between π-electrons in the graphite layers and magnetic spins in the intercalate layers. The πelectrons are scattered by spins of a virtual antiferromagnetic in-plane spin configuration arising from the superposition of two ferromagnetic in-plane structures with spin directions antiparallel to each other. The π-d exchange interactions of these compounds are also discussed.

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