Magnetic susceptibility of stage-2 ${\mathrm{Co}}_{\mathrm{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathrm{c}}$${\mathrm{Cl}}_2$ graphite intercalation compounds

dc magnetic susceptibility measurements were carried out on stage-2 ${\mathrm{Co}}_{\mathrm{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathrm{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds which approximate two-dimensional randomly mixed ferromagnets with XY spin symmetry. The average effective magnetic moment and the Curie-Weiss temperature are determined as a function of cobalt concentration and discussed in terms of the molecular field approximation. The results indicate that (i) the ${\mathrm{Co}}^{2+}$ and ${\mathrm{Ni}}^{2+}$ spins are randomly distributed on the triangular lattice sites inside the intercalate layer and (ii) the intraplanar interaction between ${\mathrm{Co}}^{2+}$ and ${\mathrm{Ni}}^{2+}$ is enhanced and larger than that between ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{2+}$ and that between ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Ni}}^{2+}$:J(Co-Ni)=1.28√J(Co-Co)J(Ni-Ni).