Internal structure of hexagonal skyrmion lattices in cubic helimagnets

TL;DR

This study uses high-resolution electron microscopy to analyze the internal magnetic structure of hexagonal skyrmion lattices in FeGe, revealing how their size and symmetry change with magnetic field and confirming theoretical models.

Contribution

It provides detailed experimental measurements of skyrmion internal structure and validates phenomenological models predicting their behavior under magnetic fields.

Findings

Skyrmion core size decreases with increasing magnetic field.

Hexagonal symmetry and internal structure are confirmed by measurements.

Variations in individual skyrmion structures are observed and analyzed.

Abstract

We have utilised a high spatial resolution imaging method, Differential Phase Contrast (DPC) performed in a scanning transmission electron microscope (STEM), for precise measurement of the magnetic induction distribution in skyrmion states in noncentrosymmetric magnetically ordered materials. Applied to investigate the internal structure of hexagonal skyrmion lattice cells, stabilised by an out-plane applied magnetic field in an FeGe nanowedge specimen, mapping of the in-plane component of magnetic induction has yielded "average" skyrmion profiles and observation of internal six-fold symmetry. With increasing field strength, the diameter of "average" skyrmion cores was observed to decrease accompanied by a non-linear variation of the lattice periodicity. Variations in structure for individual skyrmions were studied utilising an advanced DPC detection scheme with a variety of symmetry lowering distortions being observed. Our observations are consistent with a theoretical phenomenological model, which has predicted the structure of hexagonal skyrmion lattice cells and also that twisting states near to the material surfaces provide a basis for energetic stabilisation of the skyrmion lattice over the conical phase. There was good agreement with experiment for predictions of bulk skyrmion structure and their response (core-size & lattice periodicity variation) to an applied field.