Micromagnetic description of twisted spin spirals in the B20 chiral magnet FeGe from first principles

TL;DR

This paper combines first-principles calculations and micromagnetic modeling to analyze twisted spin spirals in B20 FeGe, revealing their structure, energy differences, and proposing new experimental directions.

Contribution

It introduces an advanced micromagnetic model for twisted spin spirals in B20 FeGe based on first-principles data, highlighting the structure and energetics of these states.

Findings

Twisted spin-density waves propagate with different phases and axes.

The spin-spiral twist magnitude matches global spiraling.

Energy difference aligns with experimental data, unlike untwisted spirals.

Abstract

Using the model of classical Heisenberg exchange and Dzyaloshinskii-Moriya (DM) interaction we show that the ground state of B20 FeGe chiral magnet is a superposition of twisted helical spin-density waves formed by different sublattices of the crystal. Such twisted spin-density waves propagate in the same direction but with different phases and different directions of the rotation axes. We derive an advanced micromagnetic expression describing the exchange and DM interaction for such magnetic structures. By employing first-principles calculations based on density functional theory and using our micromagnetic model we show that the magnitude of the spin-spiral twist in B20 FeGe is of the same order as global spiraling. While the energy difference between the ground state of twisted spirals and the FM state is in good agreement with the experimental results, for the spin spirals without a twist it is smaller by factor three. In addition, we verify our results by employing spin dynamics simulations. This calls for new experiments exploring the ground state properties of B20 chiral magnets.