TRANSPORT-PROPERTIES OF STAGE-1 MAGNETIC RANDOM-MIXTURE GRAPHITE-INTERCALATION COMPOUNDS

We have measured the temperature dependence of the in-plane electrical resistivity for stage-1 CocMn1-cCl2 GICs (0.70 less than or equal to c less than or equal to 1) and stage-1 CocMg1-cCl2 GICs (0.85 less tha n or equal to c less than or equal to 1) in the temperature range betw een 2.6 and 300 K. The resistivity shows a drastic increase with decre asing temperature below the critical temperature T-c for stage-1 CocMn 1-cCl2 GICs (c greater than or equal to 0.85) and stage-1 CocMg1-cCl2 GICs (c greater than or equal to 0.90). The temperature dependence of this resistivity anomaly is described by a smeared power law with an e xponent 2 beta, where beta is the critical exponent of the spontaneous magnetization. This anomaly is explained in terms of a model based on the pi-d exchange interaction between pi-electrons in the graphite la yers and spins in the intercalate layers. Below T-c, two-dimensional ( 2D) ferromagnetic intercalate layers are antiferromagnetically stacked along the c-axis. The pi-electrons are scattered by spin fluctuations of a virtual antiferromagnetic in-plane spin configuration arising fr om the superposition of two ferromagnetic in-plane structures with spi n directions antiparallel to each other. The Co concentration dependen ce of T-c for stage-1 CocMg1-cCl2 GICs is also discussed in the light of the 2D percolation problem.