  We demonstrate that a Peierls dimerization can occur in ferromagnetic spin
chains activated by thermal fluctuations. The dimer order parameter and
entanglement measures are studied as functions of the modulation of the
magnetic exchange interaction and temperature, using a spin-wave theory and the
density-matrix renormalization group. We discuss the case where a periodic
modulation is caused by spin-phonon coupling and the case where electronic
states effectively induce such a modulation. The importance of the latter for a
number of transition metal oxides is highlighted.
