The magnetoelastic behaviour of antiferromagnets having helical spin configurations

The magnetoelastic behaviour of antiferromagnetic substances possessing a helical spin system has been investigated. It is shown that all magnetoelastic phenomena arise from two distinct effects, termed the anisotropy and exchange magnetostrictions. Equations governing the magnetostriction of single and polycrystals as a function of external field strength have been obtained. The behaviour of a helical spin system in the presence of an axial tension leads to a stress dependence of the ferromagnetic-antiferromagnetic transition temperature. It also leads to the Δ E effect. Recent measurements of the crystal distortion and magnetostriction in dysprosium crystals are analysed and it is shown that at low temperatures both measurements yield the same information. An estimate of the twofold and sixfold magnetostriction constants in the basal plane of dysprosium has been evaluated at 20 °K. The experimental magnetostriction-field data for dysprosium at 144 °K are in good agreement with the predicted behaviour, and the characteristic feature of the magnetostriction of a helical spin system, namely the appearance of a strain along the c axis when a field is applied wholly within the basal plane, is correctly anticipated. The behaviour of holmium is similar, but is complicated by the large hexagonal anisotropy.