Magnetic phase transition in two-dimensional XY random spin system: Stage-2 Cu_cCo_1-cCl₂ graphite intercalation compounds

The magnetic phase transition of a two-dimensional (2D) random spin system with competing exchange interactions, stage-2 Cu_cCo_1-cCl₂ GICs, has been studied using SQUID AC magnetic susceptibility and SQUID DC magnetization. The magnetic phase diagram of critical temperature versus Cu concentration is determined. For 0 ≤ c ≤ 0.3 the system undergoes two phase transitions at T_cu and T_cl (T_cu > T_cl). Below T_cu a 2D ferromagnetic order is established in each intercalate layer. Below T_cl there appears a 3D antiferromagnetic phase with the 2D ferromagnetic layers being antiferromagnetically coupled along the c-axis. For 0.4 ≤ c ≤ 0.9 the system undergoes a phase transition at T_c (= T_cl) and a reentrant spin glass transition at T_RSG (< T_cl). For 0.9 < c < 0.93 the system undergoes a spin glass transition at T_SG from the paramagnetic phase to the spin glass phase. No phase transition is observed for c = 1 at least above 0.3 K because of the fully frustrated nature of the 2D antiferromagnet on the triangular lattice. The enhancement of T_c in the intermediate concentration and the positive sign of the Curie-Weiss temperature for c < 0.83 indicates that the intraplanar interaction between Cu²⁺ and Co²⁺ spins is ferromagnetic, The nature of the ferromagnetic phase, reentrant spin glass phase and spin glass phase is examined from the frequency dependence of absorption χ".