Competition between helimagnetism and commensurate quantum spin correlations in LiCu₂O₂

The nature of the low temperature ordered state in LiCu₂O ₂, a S=1÷2 spin chain compound with competing interactions was determined using neutron diffraction and bulk measurements. It was found that the spin structures were helimagnetic with a propagation vector (0.5, ζ, 0), ζ=0.174. It was also found that the nearest-neighbor exchange constant and the frustration ratio were J₁=5.8 meV and J ₂÷J₁=0.29 respectively. The temperature dependence of the magnetic propagation vector in LiCu₂O₂ was attributed to a competition between incommensurate helimagnetism in the classical spin model and commensurability in the quantum case.