Thermodynamically stable skyrmion lattice in tetragonal frustrated antiferromagnet with dipolar interaction

TL;DR

This paper presents an analytical mean-field model explaining the formation of a stable square skyrmion lattice in tetragonal frustrated antiferromagnets with dipolar interactions, aligning with recent experimental observations.

Contribution

It introduces a simple analytical framework for the high-temperature phase diagram of centrosymmetric tetragonal frustrated antiferromagnets with dipolar forces, predicting stable skyrmion lattices.

Findings

Square skyrmion lattice with topological charge ±1 is stabilized by dipolar anisotropy.

The model explains the nanometer size of skyrmions in these materials.

Additional single-ion anisotropy can alter the phase diagram, relevant to GdRu₂Si₂.

Abstract

Motivated by recent experimental results on GdRu$_2$Si$_2$ [Khanh, N.D., Nakajima, T., Yu, X. et al., \emph{Nat. Nanotechnol.} \textbf{15}, 444-449 (2020)], where nanometric square skyrmion lattice was observed, we propose simple analytical mean-field description of the high-temperature part of the phase diagram of centrosymmetric tetragonal frustrated antiferromagnets with dipolar interaction in the external magnetic field. In the reciprocal space dipolar forces provide momentum dependent biaxial anisotropy. It is shown that in tetragonal lattice in the large part of the Brillouin zone for mutually perpendicular modulation vectors in the $ab$ plane this anisotropy has mutually perpendicular easy axes and collinear middle axes, what leads to double-Q modulated spin structure stabilization. The latter turns out to be a square skyrmion lattice in the large part of its stability region with the topological charge $\pm 1$ per magnetic unit cell, which is determined by the frustrated exchange coupling, and, thus, nanometer-sized. In the presence of additional single-ion easy-axis anisotropy, easy and middle axes can be swapped, which leads to different phase diagram. It is argued that the latter case is relevant to GdRu$_2$Si$_2$.