Neutron Experiment of the Tetracritical Phase Transition in a Random Mixture with Competing Spin Anisotropies

A neutron diffraction experiment 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, clearly demonstrates a decoupling of the spin components near the tetracritical point which occurs in the concentration versus transition temperature phase diagram. It is shown that the new phase observed in the intermediate concentration region is the "mixed ordering" phase (or equivalently the "oblique antiferromagnetic" phase).