Low-temperature heat capacity of magnetic graphite intercalation compounds

The heat capacity Cp of magnetic graphite intercalation compounds Cn(CoCl2)1-q(AlCl3)q (stages n=2,4,5) is measured as a function of temperature (3<T<25 K) at zero and high (up to 14 T) magnetic fields (H applied in plane). We are able to separate Cp into its electronic, lattice, and magnetic contributions by suppressing the magnetic contribution to Cp at the highest magnetic fields. The electronic heat capacity CE is observed to be small (for T>3 K) compared with the lattice contribution CL which exhibits a T2 temperature dependence characteristic of a two-dimensional phonon spectrum. The small value of CE is consistent with the results of our Fermi-surface measurements, which show excellent agreement with the predictions of the rigid-band model and c-axis zone folding. The magnetic heat capacity CM shows a broad peak at 9.1 K. The shape of this peak is consistent with the reported Monte Carlo calculations of CM based on a two-dimensional XY model.