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 $H_x$ if the values of $T$ and $H_z$ determine a transition point lying on the nucleation surface; otherwise, they are not universal.

• Received 12 October 2016
• Revised 11 May 2017

DOI:https://doi.org/10.1103/PhysRevB.95.224410