Large anisotropy and stage dependence of the magnetic susceptibility of alkali-graphite intercalation compounds

We have measured the magnetic susceptibility of alkali-graphite intercalation compounds for $\overrightarrow{\mathrm{H}}$ parallel to the hexagonal $\overrightarrow{\mathrm{c}}$ axis (${\chi }^c$) and for $\overrightarrow{\mathrm{H}}\perp \overrightarrow{\mathrm{c}}\left({\chi }^a\right)$ from 4.2 to 300 K. We find (1) ${\chi }^c$ is large and paramagnetic and (2) the ratio of the molar values of ${\chi }^c$ and the Pauli contribution to the susceptibility (${\chi }_P$) as inferred from the specific heat increases rapidly as a function of stage, varying from about 2.4 in stage 1 to 17 in stage 4. The ratio $\frac{{\chi }^c}{{\chi }_P}$ is essentially independent of which alkali metal is intercalated (at least at stage 1). The stage dependence of $\frac{{\chi }^c}{{\chi }_P}$ is in qualitative agreement with the results of a calculation of the $\pi$-electron orbital contribution to the susceptibility of graphite intercalation compounds. The orbital part of the susceptibility is shown to be sensitive to the conduction-electron charge distribution from layer to layer in stage 3 and higher compunds, while the spin susceptibility and specific-heat coefficient are less sensitive to this distribution.