磁気エネルギーの紐は、曲げたり、くねらせたり、再接続したりすることを示す。(Strings of magnetic energy shown to flex, wiggle and reconnect)

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ナノ磁性体のアレイは複雑なトポロジー過程を持つストリング構造を持つ Arrays of nanomagnets have string structures with complex topological processes

2023-05-05 ロスアラモス国立研究所(LANL)

2023-05-04
A microscopy snapshot of a lattice of frustrated nanomagnets. The red lines connect dynamic points of high energy at the vertices of the lattice, which are indicated by the yellow dots.

◆集団振る舞いの物理を探究する多機関チームが、一連の揺れる紐のような振る舞いをするモデルのナノ磁気アレイを開発し、測定した。紐は高エネルギーの連結点から構成され、伸縮し、再接続することができる。紐の特別な点は、特定のエンドポイントに限定され、特定の方法で接続する必要がある。
◆物理学者がトポロジカル振る舞いと呼ぶこの制約は、ドーナツの形から最先端の半導体を通る電子の移動まで、広範なトピックに関連している。この研究は、トポロジカルな機能が純粋に古典的な材料系で現れる例であるため、研究が容易で特性が確認しやすいという点で重要である。

<関連情報>

ナノ磁性体配列のトポロジカル運動論的相互作用 Topological kinetic crossover in a nanomagnet array

Xiaoyu Zhang,Grant Fitez,Shayaan Subzwari,Nicholas S. Bingham,Ioan-Augustin Chioar,Hilal Saglam,Justin Ramberger,Chris Leighton,Cristiano Nisoli and Peter Schiffer
Science  Published:4 May 2023
DOI:https://doi.org/10.1126/science.add6575

Editor’s summary

Frustrated magnetic systems, such as spin ices, do not have a ground state in which each spin is at an energy minimum. In an artificial spin ice geometry called Santa Fe ice, this frustration can be viewed through the magnetic moment configurations at the vertices of the magnetic array. X. Zhang et al. monitored the kinetics of the strings formed by “unhappy” vertices—those not in their lowest energy state. Using x-ray magnetic circular dichroism photoemission electron microscopy, the researchers took images of the system once a second and analyzed the observed changes in string configurations. The system underwent a crossover between a low-temperature regime with limited string motion to a high-temperature one characterized by changes in string topology. —Jelena Stajic

Abstract

Ergodic kinetics, which are critical to equilibrium thermodynamics, can be constrained by a system’s topology. We studied a model nanomagnetic array in which such constraints visibly affect the behavior of the magnetic moments. In this system, magnetic excitations connect into thermally active one-dimensional strings whose motion can be imaged in real time. At high temperatures, our data showed the merging, breaking, and reconnecting of strings, resulting in the system transitioning between topologically distinct configurations. Below a crossover temperature, the string motion is dominated by simple changes in length and shape. In this low-temperature regime, the system is energetically stable because of its inability to explore all possible topological configurations. This kinetic crossover suggests a generalizable conception of topologically broken ergodicity and limited equilibration.

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