Transport properties and magnetic interactions in acceptor-type magnetic graphite intercalation compounds

Results on the resistivity of stage-1 and stage-2 CoCl₂ graphite intercalation compounds (GIC's) as a function of temperature (T) and magnetic field (H) are reported. The anomalies observed in the resistivity measurements at the magnetic phase transitions are explained by an interaction based on {π}-d electron coupling. The contrasts in the T and H dependences of the resistivity between the stage-1 and stage-2 compounds for TTcl and HHc2 are attributed to the different correlation lengths in the c-axis antiferromagnetic ordering. The magnitude of the interplanar antiferromagnetic coupling constant (J') in stage-1 CoCl₂ GIC's and the {π}-d exchange coupling constant (J_π-d) are estimated from the transport measurements. Magnetic exchange mechanisms are proposed by considering the relative contributions of the superexchange, dipole-dipole, and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. We conclude that the superexchange interaction is the dominant magnetic interplanar coupling mechanism in pristine CoCl₂ and stage-1 CoCl₂ GIC's, and is of comparable importance to the dipole-dipole interaction in stage-2 compounds. The dipole-dipole interaction is the dominant mechanism in higher stage GIC's (n{≥}3). The RKKY interaction is always found to be negligibly small, due to the quasi-two-dimensional electronic properties of these acceptor GIC's.