Effect of XY spin anisotropy on the critical temperature in stage-2 ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds

Stage-2 ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Ni}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$ graphite intercalation compounds behave magnetically like a quasi-two-dimensional Heisenberg ferromagnet with XY spin anisotropy. The spin symmetry continuously changes from Heisenberg-like at c=0 to XY-like at c=1. The effect of spin anisotropy on the critical behavior of these compounds has been studied by using ac and dc magnetic susceptibility. The ratio of critical temperature ${\mathit{T}}_{\mathit{c}}$ to Curie-Weiss temperature FTHETA as a function of effective XY spin anisotropy parameter ${\mathit{g}}_{\mathrm{eff}}$ is well described by ${\mathit{T}}_{\mathit{c}}$/FTHETA=${\mathrm{Ag}}_{\mathit{e}\mathit{f}\mathit{f}}^{1/\mathrm{\phi }}$+B with A=0.229, B=0.224, and the crossover exponent φ(=1.34). The first and second terms of ${\mathit{T}}_{\mathit{c}}$/FTHETA are due to the spin anisotropy effect in the two-dimensional system and the three-dimensional effect, respectively. The extrapolated value of ${\mathit{Ag}}_{\mathit{e}\mathit{f}\mathit{f}}^{1/\mathrm{\phi }}$ at ${\mathit{g}}_{\mathit{e}\mathit{f}\mathit{f}}$=1 may give an estimate for the ratio of the Kosterlitz-Thouless temperature to the intraplanar exchange interaction.