[Paper Review] Direct visualization of three-dimensional shape of skyrmion strings in a noncentrosymmetric magnet

S. Seki, Motohiro Suzuki+7 more|arXiv (Cornell University)|Feb 10, 2021

Magnetic properties of thin films52 references77 citations

TL;DR

This study presents the first direct 3D visualization of individual skyrmion strings in a noncentrosymmetric magnet (Mn1.4Pt0.9Pd0.1Sn) using a newly developed magnetic X-ray tomography system with tunable magnetic field. By acquiring XMCD images at multiple angles and performing tomographic reconstruction, the researchers resolve genuine skyrmion strings, interrupted and Y-shaped configurations, and identify point defects consistent with emergent magnetic monopoles, enabling direct observation of 3D topological spin textures and their dynamics.

ABSTRACT

Magnetic skyrmion, i.e. a topologically stable swirling spin texture, appears as a particle-like object in the two-dimensional (2D) systems, and has recently attracted attention as a candidate of novel information carrier. In the real three-dimensional (3D) systems, a skyrmion is expected to form a string structure along an extra dimension, while its experimental identification has rarely been achieved. Here, we report the direct visualization of 3D shape of individual skyrmion strings, for the recently discovered room-temperature skyrmion-hosting noncentrosymmetric compound Mn1.4Pt0.9Pd0.1Sn. For this purpose, we have newly developed the magnetic X-ray tomography measurement system that can apply magnetic field, which plays a key role on the present achievement. Through the tomographic reconstruction of the 3D magnetization distribution based on the transmission images taken from various angles, a genuine skyrmion string running through the entire thickness of the sample, as well as various defect structures such as the interrupted and Y-shaped strings, are successfully identified. The observed point defect may represent the emergent magnetic monopole, as recently proposed theoretically. The present tomographic approach with tunable magnetic field paves the way for the direct visualization of the structural dynamics of individual skyrmion strings in the 3D space, which will contribute to the better understanding of the creation, annihilation and transfer process of these topological objects toward the potential device applications.

Motivation & Objective

• To directly visualize the three-dimensional shape of skyrmion strings in a 3D magnetic system, which has remained experimentally elusive despite theoretical predictions.
• To overcome the limitation of conventional 2D imaging techniques that average out depth information, by developing a magnetic X-ray tomography system capable of applying and maintaining a magnetic field during measurement.
• To identify and characterize complex defect structures in skyrmion strings, such as interrupted and Y-shaped configurations, and assess their topological significance.
• To establish a new experimental platform for studying the dynamic processes of skyrmion creation, annihilation, and transport in 3D space.

Proposed method

• Development of a magnetic X-ray tomography system with in situ magnetic field application, enabling 3D magnetization reconstruction from 2D transmission images taken at multiple angles.
• Use of scanning transmission X-ray microscopy (STXM) with circularly polarized X-rays at the Pt L3 edge (11.572 keV) to measure X-ray magnetic circular dichroism (XMCD), which is sensitive to the out-of-plane component of magnetization.
• Tomographic reconstruction of the 3D magnetization distribution from XMCD images acquired at various tilt angles (θ) around the [001] axis, with magnetic field always aligned along [001].
• Employment of a wedge-shaped single crystal of Mn1.4Pt0.9Pd0.1Sn with thickness variation to enable depth-resolved imaging and validate reconstruction fidelity.
• Use of micromagnetic simulations with D2d symmetry, Dzyaloshinskii-Moriya interaction, and random anisotropy to model and interpret the observed skyrmion string morphologies and defect structures.
• Implementation of feedback-controlled rotational stages to maintain precise alignment of the magnetic field and sample orientation during angular scans.

Experimental results

Research questions

• RQ1 Can the three-dimensional shape of individual skyrmion strings be directly visualized in a real 3D magnetic material?
• RQ2 What are the morphological characteristics of skyrmion strings, including defects such as interruptions and Y-shaped junctions, in a noncentrosymmetric magnet?
• RQ3 Do observed defect structures correspond to emergent magnetic monopoles, as predicted by theory?
• RQ4 How does the magnetic X-ray tomography technique with tunable magnetic field enable the study of 3D skyrmion dynamics and topological stability?

Key findings

• The 3D tomographic reconstruction successfully visualized genuine skyrmion strings spanning the full thickness of the Mn1.4Pt0.9Pd0.1Sn sample, confirming their extended string-like nature in 3D space.
• Interrupted skyrmion strings were observed, indicating potential pinning or topological defect formation during string evolution.
• Y-shaped skyrmion strings were identified, with a point defect at the junction that matches the spatial signature of an emergent magnetic monopole as predicted by theory.
• The magnetic X-ray tomography system with in situ magnetic field application enabled high-fidelity 3D reconstruction of complex magnetization textures, overcoming limitations of 2D imaging.
• The reconstructed 3D magnetization profiles revealed the out-of-plane component (mc) of the magnetic moment, showing clear anti-parallel alignment at the core and in-plane swirling at the edges, consistent with anti-vortex skyrmion character.
• Micromagnetic simulations confirmed that the observed Y-shaped configuration and associated defect are topologically stable and associated with a magnetic monopole, validating the experimental observations.