Nucleation, instability, and discontinuous phase transitions in monoaxial helimagnets with oblique fields.

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 DeGennes 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 a 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 Hx if the values of T and Hz determine a transition point lying on the nucleation surface; otherwise, they are not universal.