# Formation of spiral ordering by magnetic field in frustrated anisotropic antiferromagnets

## TL;DR

This paper theoretically investigates phase transitions in frustrated anisotropic antiferromagnets under magnetic fields, revealing new first-order transitions to spiral phases at moderate anisotropy, with findings applicable to multiferroic materials.

## Contribution

It introduces a theoretical framework for first-order transitions to spiral phases in frustrated antiferromagnets with moderate anisotropy, expanding understanding beyond well-known spin-flop scenarios.

## Key findings

- Derived critical fields for phase transitions in mean-field approximation.
- Identified conditions for first-order transitions to spiral phases.
- Reproduced experimental and numerical results for MnWO₄.

## Abstract

We discuss theoretically phase transitions in frustrated antiferromagnets with biaxial anisotropy or dipolar forces in magnetic field applied along the easy axis at $T=0$. There are well-known sequences of phase transitions upon the field increasing: the conventional spin-flop transition and the flop of the spiral plane at strong and weak easy-axis anisotropy, respectively. We argue that much less studied scenarios can appear at moderate anisotropy in which the magnetic field induces transitions of the first order from the collinear state to phases with spiral orderings. Critical fields of these transitions are derived in the mean-field approximation and the necessary conditions are found for the realization of these scenarios. We show that one of the considered sequences of phase transitions was found in multiferroic MnWO$_4$ both experimentally and numerically (in a relevant model) and our theory reproduces quantitatively the numerical findings.