Magnetic Raman scattering of the ordered tetrahedral spin-? clusters in Cu2Te2O5(Br1 ? xClx)2 compounds

Raman light-scattering experiments in the antiferromagnetic phase of the Cu2Te2O5(Br1 − xClx)2 compounds are analyzed in terms of a dimerized spin model for the tetrahedral Cu-clusters. It is shown that the longitudinal magnetic excitation in the pure Br system hybridizes with a localized singlet excitation due to the presence of a Dzyaloshinskii-Moriya anisotropy term. The drastic change of the magnetic scattering intensities observed when a proportion of Br is replaced by Cl ions, is proposed to be caused by a change of the magnetic order parameter. Instead of being parallel/antiparallel with each other, the spins in the two pairs of spin-½ order perpendicular to each other, when the composition x is larger than about 0.25.