  Ferromagnetic, in contrast to antiferromagnetic, Heisenberg chains can
undergo a Spin-Peierls dimerization only at finite temperatures. They show
reentrant behavior as a function of temperature, which might play a role for
systems with small effective elastic constants as, for example, monatomic
chains on surfaces. We investigate the physical properties of the dimerized
ferromagnetic Heisenberg chain using a modified spin-wave theory. We calculate
the exponentially decaying spin and dimer correlation functions, analyze the
temperature dependence of the corresponding coherence lengths, the
susceptibility, as well as the static and dynamic spin structure factor. By
comparing with numerical data obtained by the density-matrix renormalization
group applied to transfer matrices, we find that the modified spin wave theory
yields excellent results for all these quantities for a wide range of
dimerizations and temperatures.
