Magnetic Structure of RbMnCl3

The compound RbMn${\mathrm{Cl}}_3$ is paramagnetic at room temperature and belongs to the space group $\frac{D_{6h}^4-P{6}_3}{\mathrm{mmc}}$ with six molecules per unit cell. Neutron diffraction of RbMn${\mathrm{Cl}}_3$-powder sample was studied as a function of the sample temperature. A transition to antiferromagnetism was found at $T_N\sim 94$°K. The magnetic structure determined from the diffraction pattern at 4.2°K consists of ferromagnetic planes stacked in the sequence $A(+)\mathrm{}$, $B(-)\mathrm{}$, $B(+)\mathrm{}$, $A(-)\mathrm{}$, $C(+)\mathrm{}$, $C(-)\mathrm{}$, along the $c$ axis. Spin direction is perpendicular to the $c$ axis, defining an infinite number of structures which belong to a two-dimensional irreducible representation ${\Gamma }_6^{+}$ of $D_{6h}^4$. Two distinctly different structures of orthorhombic symmetry, $C{m}^{\prime }{c}^{\prime }m$ and $\mathrm{Cmcm}$, span the subspace of this representation and are consistent with the diffraction data. Dipolar energy calculations show that the direction perpendicular to the $\hat{z}$ direction is of lower dipolar energy than that parallel to $\hat{z}$. The existence of a spin component perpendicular to $\hat{z}$ is also confirmed with the aid of an external magnetic field. A magnetic moment of 4.1 ${\mu }_B$ is calculated for the ${\mathrm{Mn}}^{2+}$ ion from the diffraction pattern at 4.2°K.