Experimental phase diagram of a random mixture of two anisotropic antiferromagnets

Specific heat, susceptibility, and magnetization measurements were performed on a random mixture of two antiferromagnets with competing spin anisotropies, Fe${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O and Co${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O. From these experiments, the concentration versus temperature phase diagram is obtained, which shows clearly the existence of a tetracritical point. The phase diagram is compared with those obtained by recent renormalization-group analysis and mean-field theories.