Susceptibility of [Fe${\left({\mathrm{C}}_5{\mathrm{H}}_5\mathrm{N}\mathrm{O}\right)}_6$]${\left(\mathrm{C}\mathrm{I}{\mathrm{O}}_4\right)}_2$, an $S=\frac{1}{2}$, simple-cubic Ising antiferromagnet

The susceptibility of polycrystalline [Fe${\left({\mathrm{C}}_5{\mathrm{H}}_5\mathrm{N}\mathrm{O}\right)}_6$]${\left(\mathrm{C}\mathrm{I}{\mathrm{O}}_4\right)}_2$ is reported over the temperature interval 40 mK-4.2 K. The observed transition temperature is 700 ± 10 mK, and the data have been analyzed above 720 mK according to the high-temperature series expansion for the $S=\frac{1}{2}$, simple-cubic Ising antiferromagnet. The fit is excellent, and the parameters resulting are $〈g〉=5.38\mathrm{}\pm 0.05$ and $\frac{J}{k}=-0.34\mathrm{}\pm 0.02$ K. The experimental procedures are also described in detail.