Magnetic susceptibility of CoCl_2-cBr_c

A bond-random spin system CoCl_2-cBr_c(0≤c≤2) undergoes an antiferromagnetic phase transition at a Néel temperature T_N, below which the 2D ferromagnetic Co²⁺ layers are antiferromagnetically stacked along the c-axis. The d.c. magnetic susceptibility shows a sharp peak just above T_N. The value of T_N linearly decreases with increasing Br concentration c from 24.5 K (c=0) to 18.9 K (c=2). The susceptibility obeys the Curie-Weiss law in the temperature range between 150 and 300 K. The Curie-Weiss temperature θ linearly decreases as c increases from 40.63 K (c=0) to 8.37 K (c=2). The ratio T_N/θ monotonically increases with increasing c, and becomes larger than 1 for c>1, contrary to the prediction from the molecular field theory. The cause for T_N/θ> 1 for c>1 is partly due to antiferromagnetic interplanar exchange interaction whose magnitude increases with increasing c as a result of a possible change in structure from CdCl₂ type to CdI₂ type.