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 $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}$ parallel to the hexagonal $\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}$ axis (${\ensuremath{\chi}}^{c}$) and for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{H}}\ensuremath{\perp}\stackrel{\ensuremath{\rightarrow}}{\mathrm{c}}({\ensuremath{\chi}}^{a})$ from 4.2 to 300 K. We find (1) ${\ensuremath{\chi}}^{c}$ is large and paramagnetic and (2) the ratio of the molar values of ${\ensuremath{\chi}}^{c}$ and the Pauli contribution to the susceptibility (${\ensuremath{\chi}}_{P}$) 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 $\frac{{\ensuremath{\chi}}^{c}}{{\ensuremath{\chi}}_{P}}$ is essentially independent of which alkali metal is intercalated (at least at stage 1). The stage dependence of $\frac{{\ensuremath{\chi}}^{c}}{{\ensuremath{\chi}}_{P}}$ is in qualitative agreement with the results of a calculation of the $\ensuremath{\pi}$-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.