Spin-lattice coupling in ferroelectric spiral magnets. Comparison between the cases of (Tb, Dy)MnO3 and CoCr2O4

Lattice modulation expected for various types of spiral magnetic ordering with a propagation vector Qm is discussed in terms of the symmetric (Si·Sj type) and antisymmetric (Si x Sj type) exchange interactions between neighboring spins (Si and Sj). We show that the conical ordering causes the superlattice modulation with Qm via the antisymmetric interaction, while the elliptical deformation of spiral induces a 2Qm modulation in lattice, due to the symmetric and antisymmetric interactions. This theoretical prediction was experimentally confirmed for a transverse elliptical antiferromagnet (Tb, Dy)MnO3 and a transverse conical ferrimagnet CoCr2O4, both accompanied by ferroelecricity (lattice deformation with Q=0) as well. (author)