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

TL;DR

This paper theoretically investigates the phase diagram of monoaxial chiral helimagnets under oblique magnetic fields, revealing complex transition surfaces, tricritical points, and soliton lattice behavior using a mean field approach.

Contribution

It introduces a detailed theoretical phase diagram including first and second order transitions, tricritical points, and soliton lattice divergence in monoaxial helimagnets with oblique fields.

Findings

Identification of a phase transition surface separating modulated and ferromagnetic phases.

Discovery of tricritical points dividing first and second order transition surfaces.

Logarithmic divergence of the soliton lattice period on the nucleation transition surface.

Abstract

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.