Magnetic Properties of FeCl2·2H2O

Abstract

The magnetic properties of Fe${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}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 ${T}_{N}\ensuremath{\approx}23\ifmmode^\circ\else\textdegree\fi{}$K. The magnetic susceptibility tensor in the paramagnetic state has uniaxial symmetry about an axis ($\ensuremath{\alpha}$) lying in the ac plane ($\ensuremath{\alpha}={\ensuremath{\alpha}}^{*}+58\ifmmode^\circ\else\textdegree\fi{}$). In the range 30-100\ifmmode^\circ\else\textdegree\fi{}K the measured susceptibilities follow a Curie-Weiss relation with ${g}_{\ensuremath{\alpha}}=2.4$, ${g}_{\ensuremath{\beta}}\ensuremath{\approx}{g}_{\ensuremath{\gamma}}=1.9$, and effective Weiss constants ${\ensuremath{\Delta}}_{\ensuremath{\alpha}}=12\ifmmode^\circ\else\textdegree\fi{}$K, ${\ensuremath{\Delta}}_{\ensuremath{\beta}}\ensuremath{\approx}{\ensuremath{\Delta}}_{\ensuremath{\gamma}}=5\ifmmode^\circ\else\textdegree\fi{}$K. The zero-field proton nmr in the ordered state consists of a single spin-spin doublet with extrapolated center frequency at 0\ifmmode^\circ\else\textdegree\fi{}K ${\ensuremath{\nu}}_{0}=9.231\ifmmode\pm\else\textpm\fi{}0.001$ Mc/sec. Proton nmr measurements in weak external magnetic fields at 4.0\ifmmode^\circ\else\textdegree\fi{}K give four symmetry-related proton local-field directions. The magnetic susceptibility and proton nmr experiments give evidence for a magnetic structure ($\frac{\mathrm{Pc}2}{m}$) in which ferromagnetic chains, parallel to the $c$ axis, are coupled antiferromagnetically to adjacent chains. The direction of sublattice magnetization coincides with the $\ensuremath{\alpha}$ axis. The magnetization behavior at 4.0\ifmmode^\circ\else\textdegree\fi{}K and with $\mathrm{H}\ensuremath{\parallel}\ensuremath{\alpha}$ exhibits two metamagnetic transitions, ${H}_{c1}=39\ifmmode\pm\else\textpm\fi{}1$ kOe and ${H}_{c2}=46\ifmmode\pm\else\textpm\fi{}1$ kOe. The corresponding magnetic moments are ${M}_{1}=1.4\ifmmode\pm\else\textpm\fi{}0.1{\ensuremath{\mu}}_{B}$ and ${M}_{2}=4.25\ifmmode\pm\else\textpm\fi{}0.05{\ensuremath{\mu}}_{B}$. 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.