Structural and magnetic properties of stage-2 ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Mn}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds

Stage-2 ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Mn}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$-graphite intercalation compounds (0≤c≤1) are two-dimensional random-spin systems with competing ferromagnetic and antiferromagnetic exchange interactions. The structural and magnetic properties of these compounds for 0≤c≤1 have been studied by x-ray diffraction and dc magnetic susceptibility. An analysis of the (00L) x-ray diffraction using a simple model developed here shows that these compounds are dominantly stage 2 but have a Hendricks-Teller-type staging disorder along the c axis. The c-axis repeat distance is proportional to the Co concentration, suggesting that both Co and Mn ions are randomly distributed on the triangular lattice of the ${\mathrm{Co}}_{\mathit{c}}$${\mathrm{Mn}}_{1\mathrm{-}\mathit{c}}$${\mathrm{Cl}}_2$ intercalate layers. The phase diagram determined from the dc magnetic susceptibility exhibits a ferromagnetic phase for 0.45≤c≤1.0. The Curie-Weiss temperature increases monotonically with increasing Co concentration. Its sign changes from negative to positive around c≃0.2. The exchange interaction between Co and Mn spins is ferromagnetic and is described by J(Co-Mn)=1.2 [‖J(Co-Co) J(Mn-Mn)‖${]}^{1/2}$=1.49 K. The average effective magnetic moment shows a minimum around c≃0.3, suggesting a partial replacement of ${\mathrm{Mn}}^{2+}$ ions by ${\mathrm{Mn}}^{+}$ or ${\mathrm{Mn}}^{3+}$ ions because of a charge transfer.