  We describe a Peierls dimerization which occurs in ferromagnetic spin chains
at finite temperature, within the modified spin-wave theory. Usual spin-wave
theory is modified by introducing a Lagrange multiplier which enforces a
nonmagnetic state at finite temperature. It is shown that this method gives
results in excellent agreement with the density--matrix renormalization group
applied to transfer matrices for dimerized ferromagnetic chains. We study bond
correlation functions and explore the characteristic features of dimerization
in the specific heat.
