Incommensurate--commensurate transitions in the mono-axial chiral helimagnet driven by the magnetic field

TL;DR

This paper thoroughly analyzes the phase transitions in mono-axial chiral helimagnets under magnetic fields, revealing the nature of incommensurate-commensurate transitions, tricritical points, and the influence of anisotropy, with implications for materials like CrNb3S6.

Contribution

It provides a detailed phase diagram analysis, identifying the conditions for continuous and discontinuous transitions and the role of anisotropy in mono-axial chiral helimagnets.

Findings

Transition types depend on field direction relative to the helical axis.

Identification of tricritical points with specific singular behavior.

Universal transition line form for large anisotropy, applicable to CrNb3S6.

Abstract

The zero temperature phase diagram of the mono-axial chiral helimagnet in the magnetic field plane formed by the components parallel and perpendicular to the helical axis is thoroughly analyzed. The nature of the transition to the commensurate state depends on the angle between the field and the helical axis. For field directions close to the directions parallel or perpendicular to the helical axis the transition is continuous, while for intermediate angles the transition is discontinuous and the incommensurate and commensurate states coexist on the transition line. The continuous and discontinuous transition lines are separated by two tricritical points with specific singular behaviour. The location of the continuous and discontinuous lines and of the tricritical points depend strongly on the easy-plane anisotropy, the effect of which is analyzed. For large anisotropy the conical approximation locates the transition line very accurately, although it does not predict the continuous transitions nor the tricitical behaviour. It is shown that for large anisotropy, as in CrNb3S6, the form of the transition line is universal, that is, independent of the sample, and obeys a simple equation. The position of the tricritical points, which is not universal, is theoretically estimated for a sample of CrNb3S6