Magnetic Properties of FeCl2·2H2O

The magnetic properties of Fe${\mathrm{Cl}}_2$·2${\mathrm{H}}_2$O have been investigated by means of single-crystal magnetic susceptibility, high-field magnetization, and proton nuclear-magnetic-resonance (nmr) measurements. A transition to an antiferromagnetic state is observed at $T_N\approx 23°$K. The magnetic susceptibility tensor in the paramagnetic state has uniaxial symmetry about an axis ($\alpha$) lying in the ac plane ($\alpha ={\alpha }^{*}+58\mathrm{}°$). In the range 30-100°K the measured susceptibilities follow a Curie-Weiss relation with $g_{\alpha }=2.4\mathrm{}$, $g_{\beta }\approx g_{\gamma }=1.9\mathrm{}$, and effective Weiss constants ${\Delta }_{\alpha }=12\mathrm{}°$K, ${\Delta }_{\beta }\approx {\Delta }_{\gamma }=5\mathrm{}°$K. The zero-field proton nmr in the ordered state consists of a single spin-spin doublet with extrapolated center frequency at 0°K ${\nu }_0=9.231\mathrm{}\pm 0.001$ Mc/sec. Proton nmr measurements in weak external magnetic fields at 4.0°K give four symmetry-related proton local-field directions. The magnetic susceptibility and proton nmr experiments give evidence for a magnetic structure ($\frac{\mathrm{Pc}2\mathrm{}}{m}$) in which ferromagnetic chains, parallel to the $c$ axis, are coupled antiferromagnetically to adjacent chains. The direction of sublattice magnetization coincides with the $\alpha$ axis. The magnetization behavior at 4.0°K and with $\mathrm{H}\parallel \alpha$ exhibits two metamagnetic transitions, $H_{c1\mathrm{}}=39\mathrm{}\pm 1$ kOe and $H_{c2\mathrm{}}=46\mathrm{}\pm 1$ kOe. The corresponding magnetic moments are $M_1=1.4\mathrm{}\pm 0.1{\mu }_B$ and $M_2=4.25\mathrm{}\pm 0.05{\mu }_B$. The first magnetization discontinuity is shown to result from an antiferromagnetic intrasublattice interaction which makes the two-sublattice zero-field configuration unstable with respect to a six-sublattice modification in sufficiently strong external fields.