  The phase diagram of the monoaxial chiral helimagnet as a function of
temperature (T ) and magnetic field with components perpendicular (H x ) and
parallel (H z ) to the chiral axis is theoretically studied via the variational
mean field approach in the continuum limit. A phase transition surface in the
three dimensional thermodynamic space separates a chiral spatially modulated
phase from a homogeneous forced ferromagnetic phase. The phase boundary is
divided into three parts: two surfaces of second order transitions of
instability and nucleation type, in De Gennes terminology, are separated by a
surface of first order transitions. Two lines of tricritical points separate
the first order surface from the second order surfaces. The divergence of the
period of the modulated state on the nucleation transition surface has the
logarithmic behavior typical of a chiral soliton lattice. The specific heat
diverges on the nucleation surface as a power law with logarithmic corrections,
while it shows a finite discontinuity on the other two surfaces. The soliton
density curves are described by a universal function of H x if the values of T
and H z determine a transition point lying on the nucleation surface;
otherwise, they are not universal.
