Phase diagram of Fe(1−x)CoxCl2; a random mixture of two antiferromagnets with competing spin anisotropies

Concentration vs transition temperature phase diagram of a random mixture of two anisotropic antiferromagnets, FeCl₂ and CoCl₂, is obtained from the measurement of the susceptibility on the single crystals. Two distinct critical points, one along the c-axis and the other in the c-plane, are observed for the respective concentrations between x = 0.264 and x = 0.481. Three kinds of ordered phases, namely, Fe-rich and Co-rich antiferromagnetic phases and a new phase are found. The phase diagram shows a tetracritical point as well.A hump is observed in the temperature dependence of the susceptibility along the c-axis (c-plane for x ? 0.3) near the ordering temperature occuring in the c-plane (c-axis for x ? 0.3).