Metamagnetic Phase Transitions and Hysteresis inFeCl2

Abstract

A reexamination and refinement of magnetization experiments on the prototypal metamagnetic antiferromagnet Fe${\mathrm{Cl}}_2$ is presented. From steady-field single-crystal measurements at 4.2°K, we find the abrupt (metamagnetic) transition at 10.6 kOe, from the low-moment antiferromagnetic state to the saturated configuration whose moment is 4.3±0.1 Bohr magnetons/Fe ion. This transition is of first order up to about 20.4°K, above which it is of higher order. Pulsed-field measurements reveal a marked hysteresis in the metamagnetic transition at 4.2°K. The latter two observations are tested against a critical review of theoretical treatments of the antiferro-paramagnetic transition. It is emphasized that, in order to retain the first-order transition above 0°K and to permit the occurrence of hysteresis, a ferromagnetic intrasublattice exchange is necessary (in addition to a significant anisotropy exceeding the antiferromagnetic intersublattice exchange). The experimental results on Fe${\mathrm{Cl}}_2$, along with those on several other metamagnets (i.e., Fe${\mathrm{Br}}_2$, Co${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O), are successfully compared with predictions both for the upper-limit temperature ($T^{*}$) of the first-order transition (e.g., for Fe${\mathrm{Cl}}_2$; expt. $\frac{T^{*}}{T_N}=0.87\mathrm{}$; calc. $\frac{T^{*}}{T_N}\approx 0.95$) and the variation of hysteresis as a function of $\frac{T}{T_N}$ and of the ratio of intra- to intersublattice exchange. The temperature dependence of the normalized sublattice moment is shown to be reasonably described by an Ising model in the Bethe-Peierls approximation. Although the variation with temperature of the normalized transition field is in close correspondence with that of the sublattice moment, this result is in poor agreement with the model just cited and with all others presently available.