Moriya Interaction and the Problem of the Spin Arrangements inβMnS

A study is made of the nature of the anisotropic superexchange interaction of the form ${\mathrm{D}}_{\mathrm{ij}}·{\mathrm{S}}_i\times {\mathrm{S}}_j$ recently proposed by Moriya. This interaction is permitted in $\beta \mathrm{MnS}$, with symmetry requiring that ${\mathrm{D}}_{\mathrm{ij}}$ be normal to the plane defined by $i$, $j$, and the single intervening anion. It is conjectured that this interaction leads to a screw spin arrangement, with a 90° screw angle; in crystals considered heretofore the Moriya energy produces only a slight canting. The observed powder neutron diffraction pattern has been interpreted as indicating ordering of the third kind with spins normal to the ordering axis; this arrangement, however, does not have minimum dipolar energy. The present proposed arrangement leads to the same diffraction pattern, and the Moriya energy probably overbalances the dipolar.