Three-dimensional Topological Superstructure of Magnetic Hopfions Threaded by Meron Strings in Easy-plane Magnets

TL;DR

This paper demonstrates the stable formation of three-dimensional superstructures of magnetic hopfions in easy-plane magnets, revealing a new hierarchical level of topological spin textures with potential for novel quantum phenomena.

Contribution

It introduces a stable, three-dimensional hopfion superstructure with a periodic arrangement, expanding the hierarchy of topological magnetic textures.

Findings

Stable 3D hopfion superstructure constructed from staggered chains.

Superstructure remains metastable over a range of densities.

Higher Hopf number superstructures can also be stabilized.

Abstract

Topological spin textures exhibit a hierarchical nature. For instance, magnetic skyrmions, which possess a particle-like nature, can aggregate to form superstructures such as skyrmion strings and skyrmion lattices. Magnetic hopfions are also regarded as superstructures constructed from closed loops of twisted skyrmion strings, which behave as another independent particles. However, it remains elusive whether such magnetic hopfions can also aggregate to form higher-level superstructures. Here, we report a stable superstructure with three-dimensional periodic arrangement of magnetic hopfions in a frustrated spin model with easy-plane anisotropy. By comprehensively examining effective interactions between two hopfions, we construct the hopfion superstructure by a staggered arrangement of one-dimensional hopfion chains with Hopf number $H=+1$ and $H=-1$ running perpendicular to the easy plane. Each hopfion chain is threaded by a magnetic meron string, resulting in a nontrivial topological texture with skyrmion number $N_{\rm sk}=2$ per magnetic unit cell on any two-dimensional cut parallel to the easy plane. We show that the hopfion superstructure remains robust as a metastable state across a range of the hopfion density. Furthermore, we demonstrate that superstructures with higher Hopf number can also be stabilized. Our findings extend the existing hierarchy of topological magnets and pave the way for exploring new quantum phenomena and spin dynamics.