  A combination of density functional theory calculations, many-body model
considerations, magnetization and electron spin resonance measurements shows
that the multiferroic FeTe$_2$O$_5$Br should be described as a system of
alternating antiferromagnetic $S=5/2$ chains with strong Fe-O-Te-O-Fe bridges
weakly coupled by two-dimensional frustrated interactions, rather than the
previously reported tetramer models. The peculiar temperature dependence of the
incommensurate magnetic vector can be explained in terms of interchain exchange
striction being responsible for the emergent net electric polarization.
