A computer simulation (digital twin) of a three‑dimensional magnetic hopfion inside a thin film of iron germanium (FeGe).
Image Credit: Philipp Rybakov.
Scientific Frontline: Extended "At a Glance" Summary: Magnetic Hopfions
The Core Concept: A magnetic hopfion is a highly stable, three-dimensional magnetic structure in which electron spins exhibit all possible directions within a limited volume, forming closed and linked loops.
Key Distinction/Mechanism: Unlike traditional magnetism where electron spins typically align in uniform directions, hopfions are complex, knot-like 3D arrangements. They are formed by striking chiral magnetic crystals with femtosecond laser pulses, which push the material out of equilibrium and allow the spins to overcome energy barriers to reorganize into these stable shapes.
Major Frameworks/Components:
- Chiral Magnetic Crystals (FeGe): Asymmetrical structural materials (like left and right hands) that intrinsically force magnetic spins into complex arrangements.
- Femtosecond Laser Excitation: Ultra-short flashes of light, lasting a millionth of a billionth of a second, utilized as a remote control to rapidly alter magnetic states at the nanoscale.
- Topological Mathematics: The mathematical study of shapes and knots used to formally identify hopfions as distinct objects that remain stable under continuous deformation.
- Digital Twins and Excalibur Software: Advanced computational simulations used to recreate the behavior of millions of interacting spins to verify experimental findings against theoretical models.
Branch of Science: Condensed Matter Physics, Spintronics, and Topology.
Future Application: Because hopfions are dense and highly stable, they are exceptionally promising for the field of spintronics—a technology that uses electron spin rather than electrical charge to drastically improve data storage density and information processing efficiency.
Why It Matters: This discovery bridges a major gap between theoretical physics and experimental reality, proving that light can be used as a universal tool to unlock and manipulate entirely new, complex magnetic states for next-generation nanotechnology.
Flashes of femtosecond laser light, lasting just a few trillionths of a second, have made it possible to observe new magnetic structures for the first time. By using light as a remote control, researchers were able to switch magnetism into previously unseen three-dimensional states at the nanoscale.
Magnetism is often imagined as something simple, pointing in one direction or another. At very small scales, however, magnetism can behave in far more complex ways. Magnetism originates from a quantum property of electrons known as spin, which can be thought of as a tiny internal compass carried by each electron. When many spins interact inside a solid material, they can organize into stable patterns.
Observed for the First Time
In this study, a Swedish–German–Luxembourg–Chinese collaboration of researchers has observed magnetic hopfions. A hopfion is a three-dimensional magnetic structure in which electron spins exhibit all possible directions in a limited volume of the material.
Counterparts of magnetic hopfions have previously been observed in non-magnetic systems. In magnetic materials, however, their independent existence had so far only been predicted by theory, and direct experimental observation had remained a major challenge.
“Hopfions are fascinating because of their structure. They are three-dimensional objects made of spins that form closed and linked loops. Once they appear, they keep their form and are largely unaffected by their surroundings,” says Philipp Rybakov, a researcher at the Department of Physics and Astronomy at Uppsala University and one of the researchers behind the study.
The experiments were carried out on chiral magnetic crystals. In a chiral magnetic crystal, the structure comes in two variants that are mirror images of each other, like a left hand and a right hand. Although they are made of the same atoms, the two forms cannot be perfectly aligned by rotation. This built-in asymmetry strongly influences how magnetic spins arrange themselves inside the material.
Created Using Femtosecond Laser Pulses
The researchers studied thin films of iron germanium (FeGe) with a thickness of about 110–200 nanometers. Although magnetic hopfions had been predicted by theory for several years, observing them experimentally proved extremely difficult. Under normal conditions, the magnetic system does not easily reach these states because it must overcome energy barriers.
What made the breakthrough possible was the use of femtosecond laser pulses. A femtosecond is an extremely short moment in time, one-millionth of a billionth of a second. Laser pulses can briefly disturb the spin system and push it out of equilibrium, allowing new magnetic states to form.
In the experiment, a relatively large surface was covered with FeGe and illuminated with femtosecond laser light once per second. After the laser exposure, the researchers examined the magnetic state of the material using advanced electron-based microscopy techniques. The experiment could then be repeated under the same conditions, making it possible to carefully test and verify the results.
Experiments, Theoretical Calculations, and Simulations
At the same time as the experiments were performed, the same magnetic structures were recreated in detailed computer simulations using Excalibur, a simulation program previously developed by Rybakov and adopted by the research team. The software models how millions of interacting spins evolve and organize into complex three-dimensional patterns. These simulations act as digital twins of the experiments.
When the measurements were compared with simulations, the observed structures were consistent with theoretical models of magnetic hopfions.
A key part of the work was the topological analysis of the magnetic states. Topology is a branch of mathematics that describes the properties of shapes and more complex geometric objects that remain unchanged under continuous deformations, such as knots or linked loops. Philipp Rybakov led the theoretical and topological work that made it possible to identify hopfions as distinct and stable three-dimensional magnetic structures.
The study is the result of a close collaboration between theory and experiment, with experimental work and theoretical modeling developed in parallel.
“Theory helped point us in the right direction, experiments made the structures visible, and simulations and topology helped us interpret what we were seeing,” says Philipp Rybakov.
Parallel Experiment
The results are not limited to hopfions in a single material. In parallel work carried out at the Synergetic Extreme Condition User Facility (SECUF), the same light-based approach was used to control magnetism in a different chiral material. In that study, researchers demonstrated so-called bimerons, two-dimensional magnetic structures that can be seen as counterparts of three-dimensional hopfions. Taken together, these studies show that laser light can serve as a general tool for accessing new magnetic states in different materials and in both two and three dimensions.
New Opportunities Using Spintronics
The discovery opens up new opportunities for future research. Because hopfions are stable three-dimensional magnetic structures, they are of interest for spintronics, where electron spin is used instead of electric charge to store and process information.
“Using femtosecond laser light, we now have a way to switch magnetism into these complex states. That allows us to explore magnetic phenomena in ways that were not possible before,” says Philipp Rybakov.
Published in journal:
- Nature Communications
- Nature Physics
Title:
Authors:
- Kaixin Zhu, Filipp N. Rybakov, Zhan Wang, Wenli Gao, Shuaishuai Sun, Wentao Wang, Jun Li, Huanfang Tian, Olle Eriksson, Huaixin Yang, Ying Zhang, Nikolai S. Kiselev, Zian Li, and Jianqi Li
- Xiaowen Chen, Donghai Yang, Zefang Li, Jiangteng Guo, Haixue Wang, Yue Hu, Vladyslav M. Kuchkin, Andrii S. Savchenko, Huai Zhang, Bei Ding, Zhipeng Hou, Wen Shi, Filipp N. Rybakov, Olle Eriksson, Stefan Blügel, Yu Han, Rafal E. Dunin-Borkowski, Nikolai S. Kiselev, Xuewen Fu, and Fengshan Zheng
Source/Credit: Uppsala University | Camilla Thulin
Edited by: Scientific Frontline
Reference Number: phy052226_01
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