Structural and magnetic properties of stage-2 CocMn1-cCl2-graphite intercalation compounds

Stage-2 Co_cMn_1-cCl₂-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 Co_cMn_1-cCl₂ 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 Mn²⁺ ions by Mn⁺ or Mn³⁺ ions because of a charge transfer.