Spin-flop transition driven by competing magnetoelastic anisotropy terms in a spin-spiral antiferromagnet

Holmium, the archetypical system for spin-spiral antiferromagnetism, undergoes an in-plane spin-flop transition earlier attributed to competing symmetry-breaking and fully symmetric magnetoelas-tic (MEL) anisotropy terms [Phys. Rev. Lett. 94, 227204 (2005)], which underlines the emergence of six-fold MEL constants in heavy rare earth (RE) metals, as otherwise later studies suggested [Phys. Rev. Lett. 98, 267201 (2007) and Phys. Rev. B 89, 134421 (2014)]. A model that encompasses MEL contributions to the in-plane sixfold magnetic anisotropy is laid out to elucidate the mechanism behind the spin-flop transition. The model, which is tested in a Ho-based superlat-tice, shows that the interplay between competing fully symmetric α-MEL and symmetry-breaking γ-MEL anisotropy terms triggers the spin-reorientation. This also unveils the dominant role played by the six-fold exchange magnetostriction constant, where D 66 α2 0.32 GPa against its crystal-field counterpart M 66 α2 −0.2 GPa, in contrast to the crystal-field origin of the symmetry-breaking mag-netostriction in RE metals.