Understanding the H-T phase diagram of the mono-axial helimagnet

TL;DR

This paper theoretically investigates the phase diagram of monoaxial helimagnets under perpendicular magnetic fields, revealing complex phase transitions, a tricritical point, and universal soliton lattice features consistent with experiments.

Contribution

It introduces a simplified continuum model capturing the phase diagram and soliton behavior, including the tricritical point and temperature-dependent features, advancing understanding of monoaxial helimagnets.

Findings

Identification of a tricritical point separating different phase transition orders.

Universal logarithmic scaling of the chiral soliton lattice below the tricritical temperature.

Temperature independence of normalized soliton density curves below the tricritical point.

Abstract

Some unexpected features of the phase diagram of the monoaxial helimagnet in presence of an applied magnetic field perpendicular to the chiral axis are theoretically predicted. A rather general hamiltonian with long range Heisenberg exchange and Dzyaloshinskii--Moriya interactions is considered. The continuum limit simplifies the free energy, which contains only a few parameters which in principle are determined by the many parameters of the hamiltonian, although in practice they may be tuned to fit the experiments. The phase diagram contains a Chiral Soliton Lattice phase and a forced ferromagnetic phase separated by a line of phase transitions, which are of second order at low T and of first order in the vicinity of the zero-field ordering temperature, and are separated by a tricritical point. A highly non linear Chiral Soliton Lattice, in which many harmonics contribute appreciably to the spatial modulation of the local magnetic moment, develops only below the tricritical temperature, and in this case the scaling shows a logarithmic behaviour similar to that at T=0, which is a universal feature of the Chiral Soliton Lattice. Below the tricritical temperature, the normalized soliton density curves are found to be independent of T, in agreement with the experimental results of magnetorresistance curves, while above the tricritical temperature they show a noticeable temperature dependence. The implications in the interpretation of experimental results of CrNb3S6 are discussed.