Two-dimensional spin-flop transition in CoCl2-graphite intercalation compounds

Two field-dependent susceptibility anomalies are observed in the quasi-two-dimensional spin system of CoCl2-intercalated graphite. These anomalies are explained with use of the Landau free-energy functional applied to the magnetic Hamiltonian of CoCl2-intercalated graphite. In the stage-1 compound, the low-field anomaly at HAS(T)160 Oe is a signature of a two-dimensional antiferromagnetic-spin-flop first-order transition, while the anomaly at a higher field HSF(T)300 Oe is a signature of a spin-flop- ferromagnetic second-order transition. The low-temperature properties of these three phases are analyzed using the transfer matrix method for their c-axis ordering. A comparison of the model calculation and the experimental data indicates that the antiferromagnetic coupling between CoCl2 layers and the sixfold in-plane anisotropy are approximately 160 and 10 Oe, respectively, for the stage-1 compound.