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

Stage-2 CocMn1-cCl2-graphite intercalation compounds (0c1) are two-dimensional random-spin systems with competing ferromagnetic and antiferromagnetic exchange interactions. The structural and magnetic properties of these compounds for 0c1 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 CocMn1-cCl2 intercalate layers. The phase diagram determined from the dc magnetic susceptibility exhibits a ferromagnetic phase for 0.45c1.0. The Curie-Weiss temperature increases monotonically with increasing Co concentration. Its sign changes from negative to positive around c0.2. 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 c0.3, suggesting a partial replacement of Mn2+ ions by Mn+ or Mn3+ ions because of a charge transfer.