S. Grytsiuk | Searcharxiv

arXiv2021

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

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.

cond-mat.mtrl-sci↗

arXiv2019

Ab initio analysis of magnetic properties of prototype B20 chiral magnet FeGe

FeGe in the B20 phase is an experimentally well-studied prototypical chiral magnet exhibiting helical spirals, skyrmion lattices and individual skyrmions with a robust length of 70~nm. While the helical spiral ground state can be verified by first-principles calculations based on density functional theory, this feature size could not be reproduced even approximately. To develop a coherent picture of the discrepancy between experiment and theory, we investigate in this work the magnetic properties of FeGe from first-principles using different electronic-structure methods. We study atomistic as well as micromagnetic parameters describing exchange and Dzyaloshinskii-Moriya interactions, and discuss their subtle dependence on computational, structural, and correlation parameters. In particular, we quantify how these magnetic properties are affected by changes of the lattice parameter, different atomic arrangements, exchange and correlation effects, finite Fermi-function broadening, and momentum-space sampling. In addition, we use the obtained atomistic parameters to determine the corresponding Curie temperature, which agrees well with experiments. Our results indicate that the well-known and well-accepted relation between the micromagnetic parameters and the period of the helical structure, is not valid for FeGe. This calls for new experiments exploring the relation by measuring independently the spin stiffness, the spiralization and the period of the helical spin spiral.

cond-mat.mtrl-sci↗