Magnetic properties of stage-2 ${\mathrm{Ni}}_{\mathit{c}}$${\mathrm{Mn}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds

Stage-2 ${\mathrm{Ni}}_{\mathit{c}}$${\mathrm{Mn}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds (with 0≤c≤1) are two-dimensional random-spin systems with competing ferromagnetic and antiferromagnetic exchange interactions. The magnetic properties of these compounds have been studied by dc and ac magnetic susceptibilities. The Curie-Weiss temperature increases monotonically with increasing Ni concentration, suggesting that both Ni and Mn ions are randomly distributed on the triangular-lattice sites of the ${\mathrm{Ni}}_{\mathit{c}}$${\mathrm{Mn}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-intercalate layers. Its sign changes from negative to positive around c≊0.22. The exchange interaction between Ni-Mn spin pairs is ferromagnetic and is described by J(Ni-Mn)=1.09[‖J(Ni-Ni)J(Mn-Mn)‖${]}^{1/2}$=1.44 K. The critical temperature ${\mathit{T}}_{\mathit{c}}$ decreases rapidly with a dilution of Mn. In spite of ferromagnetic J(Ni-Mn), the ferromagnetic long-range order of ${\mathrm{Ni}}^{2+}$ disappears below c≊0.6. The large initial slope of [dln${\mathit{T}}_{\mathit{c}}$/dc${]}_{\mathit{c}=1\mathrm{}}$=2.38 is ascribed to the two-dimensional Heisenberg-like character of stage-2 ${\mathrm{NiCl}}_2$-graphite intercalation compound. The critical behavior of these compounds shows a two-dimensional XY character. The critical exponent γ appears to be independent of c for c≥0.7. It is determined that γ=2.27±0.02 at c=0.9.