Moriya Interaction and the Problem of the Spin Arrangements in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>β</mml:mi><mml:mi mathvariant="normal">MnS</mml:mi></mml:math>

Abstract

A study is made of the nature of the anisotropic superexchange interaction of the form ${\mathrm{D}}_{\mathrm{ij}}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{S}}_{i}\ifmmode\times\else\texttimes\fi{}{\mathrm{S}}_{j}$ recently proposed by Moriya. This interaction is permitted in $\ensuremath{\beta}\mathrm{MnS}$, 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\ifmmode^\circ\else\textdegree\fi{} 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.