Magnetic behavior of the three-dimensional Ising ferromagnet Fe${\mathrm{}\left(\mathrm{Cl}\right)\left[{\mathrm{S}}_2\mathrm{CN}{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2\right]}_2$: Single crystals and mixed crystals with a bromide analog

The principal crystal susceptibilities of monoclinic Fe(Cl)${\left[{\mathrm{S}}_2\mathrm{CN}{\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2\right]}_2$ have been measured in the 1.5 to 20 K range, and are found to be along the crystal axes [010], [101], and ($\overline{1}01$) direction. Analysis of the data demonstrates that this system may be described as a three-dimensional Ising ferromagnet, with $T_c=2.457\mathrm{}\pm 0.01$ K and with the preferred axis along [101]. Above 5 K a good fit to the data is obtained based on a spin-Hamiltonian appropriate to $S=\frac{3}{2}$, and including a molecular-field correction for exchange, in which the best parameter values are $\frac{D}{k}=-3.32\mathrm{}\pm 0.05$ K, $\frac{E}{k}=-0.65\mathrm{}\pm 0.05$ K, $g_x=2.09\mathrm{}\pm 0.05$, $g_y=2.10\mathrm{}\pm 0.05$, $g_z=2.12\mathrm{}\pm 0.05$, and $\frac{\mathrm{zJ}}{k}=0.81\mathrm{}\pm 0.05$ K. Analysis of heat-capacity data supports the conclusions drawn from the susceptibilities and leads to a quite similar estimate for the mean-field approximation exchange interaction. Also studied were crystals of isostructural Fe(Br)${\left[{\mathrm{S}}_2{\mathrm{C}\mathrm{N}\left({\mathrm{C}}_2{\mathrm{H}}_5\right)}_2\right]}_2$ mixed with the chloride. These were found to order ferromagnetically, with the same principal axes, for chloride concentrations ranging from 71.2% to 15.7% and to be characterized by transition temperatures ranging from 2.213±0.01 K to 1.613±0.01 K. The susceptibilities in the paramagnetic region were accounted for in terms of pure chloride contributions, including a correction for ferromagnetic exchange, and pure bromide contributions, without any correction for exchange. The comparison suggests that less important effects of exchange between chloride and bromide species are also present. The susceptibilities along the preferred direction, [101], were also studied in the critical regions for both pure chloride and mixed chloride-bromide crystals. Unusually low values of the critical exponent $\gamma$ in the expression for the initial susceptibility were obtained, typically from 1.14 to 1.17. These are interpreted as being due to nondiverging antiferromagnetic contributions to the susceptibility of the pure chloride, arising from a canting (42°) between magnetic moments at inequivalent sites in the crystal lattice, or from randomness in the distribution of exchange and crystalline anisotropies or in site occupation in the mixed crystals.