Magnetic Susceptibility of FeCl2·4H2O from 0.35 to 4.2°K

The magnetic susceptibility of Fe${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O in powder and single-crystal form has been measured between 0.35 and 4.2°K by an audio-frequency mutual-inductance technique. Temperatures below 2°K were achieved with a liquid ${\mathrm{He}}^3$ bath. These measurements show Fe${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O to be an antiferromagnet with $T_N\sim 1°$K. With the aid of crystallographic evidence, it has been found possible to fit the data in the paramagnetic region with a detailed picture of the lowest spin quintet of the ${\mathrm{Fe}}^{++}$ ion in a monoclinic crystalline field. The same level scheme describes adequately the measured specific heat of the salt in this region. The analysis performed for $T>\mathrm{}{T}_N$ combined with the data for $T<\mathrm{}{T}_N$ suggests that a four-sublattice model may be necessary to interpret in detail the measurements in the ordered state.