Percolation and spin-glass behavior in stage-2 Co_cMg1-_cCl₂ graphite intercalation compounds

Stage-2 Co_cMg1-_cCl₂ graphite intercalation compounds magnetically behave like quasi-two-dimensional (2D) XY-like site-diluted ferromagnets on the triangular lattice. The percolation and spin-glass behavior of these compounds has been studied using superconducting quantum interference device (SQUID) ac magnetic susceptibility (x_aa and x_aa) and SQUID dc magnetization. The pure system (c=1) undergoes two magnetic phase transitions at T_cu and T_cl. 2D ferromagnetic long range order exists between T_cu and T_cl. The critical temperature T_cl decreases to zero at the percolation threshold c_p=0.5, while Tcu above cp decreases with decreasing Co concentration and may be connected to a crossover temperature T_x below c_p. A spin-glass-like behavior is observed for c=0.3 and 0.46. Spin frustration occurs because of the competition between the ferromagnetic intraplanar exchange interaction and an effective antiferromagnetic interplanar exchange interaction which is not large enough to give rise to three-dimensional antiferromagnetic correlation.