  The effect of thermal fluctuations on the phase structure of monoaxial
helimagnets with external magnetic field parallel to the chiral axis is
analyzed by means of a saddle point expansion of the free energy. The phase
transition that separates the conical and forced ferromagnetic phases is
changed to first order by the thermal fluctuations. In a purely monoaxial
system the pitch of the conical state remains independent of temperature and
magnetic field, as in mean field theory, even when fluctuations are taken into
account. However, in presence of weak Dzyaloshinskii-Moriya interactions in the
plane perpendicular to the chiral axis, thermal fluctuations induce a
dependence of the pitch on temperature and magnetic field. This may serve to
determine the nature of magnetic interactions in such systems.
