Alternating spin chains coupled by frustrated interactions in the multiferroic FeTe2O5Br

Density functional theory calculations performed for the multiferroic FeTe2O5Br reveal surprisingly strong super-superexchange pathways through Fe-O-Te-O-Fe bridges, implying that the magnetic lattice must be treated as a system of alternating Fe chains coupled by frustrated interactions. We find firm support for this model in the magnetization and the antiferromagnetic resonance measurements as well as in the matching of the incommensurate magnetic vector. Its peculiar temperature dependence is explained by interchain exchangestriction being responsible for the emergent net electric polarization.