Magnetic Structure and Two-Dimensional Behavior of ${\mathrm{Rb}}_2$Mn${\mathrm{Cl}}_4$ and ${\mathrm{Cs}}_2$Mn${\mathrm{Cl}}_4$

A neutron-diffraction study of powder samples of ${\mathrm{Rb}}_2$Mn${\mathrm{Cl}}_4$ and ${\mathrm{Cs}}_2$Mn${\mathrm{Cl}}_4$ was performed. These compounds are paramagnetic at room temperature with ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$-type structure ($D_{4h}^{17}-\frac{I4\mathrm{}}{\mathrm{mmm}}$). It was found that both compounds order antiferromagnetically at $T_N\sim 55$°K. In both compounds the magnetic structure below $T_N$ is collinear with antiferromagnetic axis parallel to $z$, isomorphic with that of low-temperature ${\mathrm{K}}_2\mathrm{Ni}{\mathrm{F}}_4\left(F_A{m}^{\prime }{m}^{\prime }m\right)$. Calculations of the dipolar energy show that $z$ is indeed a direction of lower dipolar energy than $x$. In addition, the diffraction patterns exhibit a "ridge" - a broad reflection below about 180°K which indicates the existence of two-dimensional spin correlations. Such correlations in the planes parallel to (001) are expected and have already been found in ${\mathrm{K}}_2$Ni${\mathrm{F}}_4$ and isomorphic compounds.