Field-induced transitions from incommensurate to commensurate phases in helical antiferromagnets

TL;DR

This paper analyzes how magnetic fields induce transitions between incommensurate and commensurate phases in helical antiferromagnets, providing analytical expressions and applying the theory to a specific compound.

Contribution

It offers a theoretical framework for field-induced phase transitions in helical antiferromagnets, including explicit critical field formulas and application to real materials.

Findings

Field can induce transitions between incommensurate and commensurate phases.

Analytical expressions for critical fields for n=2, 3, 4.

Application to RbFe(MoO_4)_2 explains experimental data.

Abstract

Heisenberg antiferromagnet with an easy-plane anisotropy is discussed in which a magnetic spiral is induced by Dzyaloshinskii-Moriya interaction and/or frustration of the exchange coupling. The distortion of the spiral by small in-plane magnetic field is described analytically. It is found that the field can gradually change the vector of the magnetic structure ${\bf k}_0$ and can produce transitions between phases with incommensurate and commensurate magnetic orderings when ${\bf k}_0$ is close to ${\bf g}/n$, where ${\bf g}$ is a reciprocal lattice vector and $n$ is integer. Analytical expressions for critical fields are derived for $n=2$, 3, and 4. Application of the theory to the triangular-lattice compound $\rm RbFe(MoO_4)_2$ is discussed alongside its potential applicability to other materials. As a by-product of the main consideration, model parameters are found which describe more accurately the full set of available experimental data suggested before for $\rm RbFe(MoO_4)_2$.