Magnetic phase transition of stage-2 ${\mathrm{Cu}}_{\mathit{c}}$${\mathrm{Co}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds

Stage-2 ${\mathrm{Cu}}_{\mathit{c}}$${\mathrm{Co}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds (GIC’s) (0≤c≤1) provide ideal two-dimensional random spin systems with a spin frustration effect arising from competing ferromagnetic and antiferromagnetic interactions. The magnetic properties of these compounds have been studied by dc and ac magnetic susceptibility, and superconducting quantum interference device magnetization. The sign of the Curie-Weiss temperature changes from positive to negative with increasing concentration around c=0.80 to 0.85. The intraplanar exchange interaction J(Cu-Co) between ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Co}}^{2+}$ spins is ferromagnetic and depends on the Cu concentration. These systems with c<0.9 undergo a ferromagnetic phase transition at the critical temperature ${\mathit{T}}_{\mathit{c}}$. The irreversible effect of magnetization is observed below ${\mathit{T}}_{\mathit{c}}$. The low-temperature phase below ${\mathit{T}}_{\mathit{c}}$ may correspond to a cluster glass phase where the spin direction of ferromagnetic clusters is frozen because of frustrated interisland interactions which include a dipole-dipole interaction and an interplanar antiferromagnetic interaction. The critical temperature ${\mathit{T}}_{\mathit{c}}$ increases as c increases and exhibits a broad maximum around c=0.5. This enhancement of ${\mathit{T}}_{\mathit{c}}$ is partly due to the ferromagnetic interaction J(Cu-Co). No magnetic phase transition is observed for 0.9<c<1 partly because of the spin frustration effects arising from (i) the competition between ferromagnetic J(Cu-Co) and antiferromagnetic J(Cu-Cu) interactions, and (ii) the fully frustrated nature of the antiferromagnet on the triangular lattice.