  The low-temperature free energy of the spin S quantum Heisenberg
ferromagnetic chain in a strong magnetic field is obtained in a two-particle
approximation by using exact solution of two-spin-wave problem. The result is
beyond the perturbation theory because it incorporates the both bound and
scattering state contributions, and the scattering effect is essential as well
as the bound state one. In particular the main temperature renormalization of
an exchange constant is found to be linear in temperature instead $T^{3/2}$
corresponded to the perturbation theory result.
