Particle-size dependent structural transformation of skyrmion lattice

R Takagi,V Ukleev,Y Yamasaki,T Arima,S Seki,Y Yokoyama,T Yokouchi,H Nakao,Y Tokura

doi:10.1038/s41467-020-19480-8

Abstract

Magnetic skyrmion is a topologically protected particle-like object in magnetic materials, appearing as a nanometric swirling spin texture. The size and shape of skyrmion particles can be flexibly controlled by external stimuli, which suggests unique features of their crystallization and lattice transformation process. Here, we investigated the detailed mechanism of structural transition of skyrmion lattice (SkL) in a prototype chiral cubic magnet Cu2OSeO3, by combining resonant soft X-ray scattering (RSXS) experiment and micromagnetic simulation. This compound is found to undergo a triangular-to-square lattice transformation of metastable skyrmions by sweeping magnetic field (B). Our simulation suggests that the symmetry change of metastable SkL is mainly triggered by the B-induced modification of skyrmion core diameter and associated energy cost at the skyrmion-skyrmion interface region. Such internal deformation of skyrmion particle has further been confirmed by probing the higher harmonics in the RSXS pattern. These results demonstrate that the size/shape degree of freedom of skyrmion particle is an important factor to determine their stable lattice form, revealing the exotic manner of phase transition process for topological soliton ensembles in the non-equilibrium condition.

Highlights

Magnetic skyrmion is a topologically protected particle-like object in magnetic materials, appearing as a nanometric swirling spin texture
Our results reveal the unique manner of phase transition process of skyrmion lattice (SkL) in the non-equilibrium condition and suggest that the size/shape degree of freedom of skyrmion particle plays an important role in the determination of their stable lattice form
The stability of square SkL phase has been discussed in several theoretical works[2,20,24,25,26], while they mostly focused on the case of the equilibrium ground state in the positive field region, i.e., not the non-equilibrium metastable state in the negative field region as studied here

Summary

Introduction

Magnetic skyrmion is a topologically protected particle-like object in magnetic materials, appearing as a nanometric swirling spin texture. Our simulation suggests that the symmetry change of metastable SkL is mainly triggered by the B-induced modification of skyrmion core diameter and associated energy cost at the skyrmion-skyrmion interface region Such internal deformation of skyrmion particle has further been confirmed by probing the higher harmonics in the RSXS pattern. Similar topological solitons are known to appear in various physical context, such as skyrmions, hopfions, and heliknotons in liquid crystals[7,8,9,10] or Abrikosov vortices in type-II superconductors[11] These particle-like objects generally prefer to form a periodic lattice in the similar manner as atomic or molecular crystals, implying that topological solitons can be a unique building block for a rich variety of tunable ordered structures. These compounds are characterized by the chiral cubic crystal structure, where Dzyaloshinskii–Moriya (DM)

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Results

Conclusion