Magnetic behavior and magnetic phase diagram of the insulating mixed ferromagnet Fe[${\mathrm{S}}_{2\mathrm{x}}$${\mathrm{Se}}_{2\left(1\mathrm{-}\mathrm{x}\right)}$CN ($C_2$${\mathrm{H}}_5$${)}_2$${]}_2$Cl

The title system is a mixture of two isomorphous three-dimensional (3D) ferromagnets. One component, the sulfur system or dithiocarbamate, is approximately describable as a 3D Ising ferromagnet, ordering at 2.458 K. The other component, the selenium system or diselenocarbamate, is approximately describable as a 3D XY ferromagnet ordering at 3.463 K. Susceptibilities of polycrystalline samples of the mixed system are Curie-Weiss–like above 20 K. The Weiss constant θ exhibits a pronounced wavelike composition dependence, though always remaining positive. Each mixture is observed to order ferromagnetically. The composition dependence of the ordering temperature, deduced from low-field susceptibility and magnetization measurements, is S-shaped, with a somewhat stronger composition dependence on the Se-rich side of the diagram. This form of $T_c$(x), which seems to be realized for the first time in this mixed system, corresponds to one of the phase-diagram types predicted in earlier theoretical work of Thorpe and McGurn.