Magnetic susceptibility of CoCl 2− cBr c

Abstract

A bond-random spin system CoCl 2− c Br c (0 ≤ c ≤2) undergoes an antiferromagnetic phase transition at a Néel temperature T N , below which the 2D ferromagnetic Co 2+ 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 CdCl 2 type to CdI 2 type.