🔬 mesoscale physics

Current-induced creation and dynamics of embedded magnetic skyrmion bags

This study demonstrates the direct current-induced creation and manipulation of embedded magnetic skyrmion bags in FeGe nanoplates under zero magnetic field, revealing a spin-transfer-torque-driven mechanism that enables electrical control over complex topological structures like magnetic monopoles and bobbers for future spintronic applications.

Original authors: Yaodong Wu, Jialiang Jiang, Lingyao Kong, Meng Shi, Shouguo Wang, Mingliang Tian, Haifeng Du, Jin Tang

Published 2026-07-27

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CC BY 4.0

Original authors: Yaodong Wu, Jialiang Jiang, Lingyao Kong, Meng Shi, Shouguo Wang, Mingliang Tian, Haifeng Du, Jin Tang

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the world of tiny magnets not as solid blocks, but as a swirling, invisible ocean of invisible arrows called "spins." In certain special materials, these arrows don't just point randomly; they twist and turn into beautiful, stable patterns, like whirlpools in a stream. Scientists call these whirlpools "skyrmions." Think of them as magnetic bubbles or knots that are incredibly tough to untie. What makes them super exciting for the future of computers is that they can carry information, much like the 1s and 0s in your phone, but they are smaller, faster, and use less energy.

Usually, to create these magnetic knots, scientists have to use strong external magnets, which is like trying to sculpt a snowflake while holding a giant magnet in your hand—it works, but it's clunky and hard to control. The big dream in this field is to create and move these knots using only electricity, like sending a signal down a wire to tell a magnet where to go. This would be the key to building the next generation of super-fast, tiny computers. But there's a catch: creating the most complex and useful versions of these knots, which look like a bag full of smaller bubbles inside a bigger one, has been a massive puzzle. Until now, no one had figured out how to make these "magnetic bags" just by pushing electricity through them.

This paper tells the story of how a team of researchers finally cracked that code. They worked with a thin slice of a material called FeGe, which is a bit like a microscopic stage for these magnetic actors. Instead of using giant external magnets, they used tiny, super-fast bursts of electricity—pulses lasting just 70 nanoseconds (that's 70 billionths of a second)—to shake the magnetic spins into new shapes.

Here is what they discovered: By sending these electrical pulses through the material, they could turn a messy, twisted starting state into a perfectly organized "skyrmion bag." Imagine a large, hollow magnetic bubble that traps several smaller magnetic bubbles inside it. The researchers found they could create these bags, and even more complex "nested" bags (a bag inside a bag inside a bag), all while sitting in zero magnetic field. They watched this happen in real-time using a powerful electron microscope, seeing the magnetic patterns shift and settle into these new, stable shapes.

The team also figured out how it happens. It turns out that the electricity doesn't just push the magnets; it twists them in a specific way that causes the edges of the magnetic patterns to snap and re-form, trapping the inner bubbles. They also discovered that by changing the direction of the current, they could flip the magnetic "charge" of these bags, turning them inside out.

But the story doesn't end with just making them. The researchers also watched what happens when they keep pushing the current. Sometimes, the magnetic tubes that make up the bag would collapse. When they did, they saw evidence of strange, 3D magnetic defects appearing and disappearing, like magnetic "monopoles" (single north or south poles that usually don't exist alone) and "bobbers" (knots that float in the middle of the material). It's like watching a magician pull a rabbit out of a hat, but the rabbit is a fundamental law of physics breaking and reforming right before your eyes.

The paper is very careful to say that while they proved this works, the process isn't perfectly predictable yet; sometimes the bags form, sometimes they don't, depending on tiny details. They also ruled out that heat from the electricity was the main cause, showing it was actually the twisting force of the current itself doing the heavy lifting. This work doesn't just show a cool trick; it proves that we can build these complex, high-tech magnetic structures using only electricity, paving the way for future devices that could store and process information in ways we've only dreamed of.

Technical Summary: Current-Induced Creation and Dynamics of Embedded Magnetic Skyrmion Bags

Problem Statement
Magnetic skyrmion bags—vortex-like structures hosting multiple skyrmions with tunable topological charge ( $Q$ )—are promising candidates for next-generation spintronic computing due to their ability to encode information in customizable topological states. While the creation of these structures using magnetic fields has been demonstrated, their direct electrical generation remains a significant challenge. Previous methods often rely on complex two-step magnetic field protocols (e.g., applying a negative field to create internal clusters, then reversing to form boundary spirals) to stabilize multiple- $Q$ solitons. Experimental confirmation of electrical creation for these complex topological textures has remained elusive.

Methodology
The authors investigated a 100-nm-thick FeGe nanoplate (lamella) equipped with two electrodes, fabricated for in-situ Lorentz transmission electron microscopy (LTEM). The experiments were conducted at zero external magnetic field and low temperatures (95 K).

Stimulus: The team applied nanosecond current pulses (70 ns duration) with varying densities ( $3.5 \times 10^{10}$ to $5.5 \times 10^{10}$ A/m²) and polarities.
Imaging: Magnetic textures were captured using LTEM in Fresnel mode with the objective lens switched off to maintain field-free conditions.
Simulation: Micromagnetic simulations were performed using MuMax3 to model the total free energy (exchange, Dzyaloshinskii-Moriya interaction, and demagnetization) and to reproduce the observed spin configurations and collapse dynamics.
Control: A standardized procedure was employed where a high-density pulse ( $5.6 \times 10^{10}$ A/m²) reset the system to a helical ground state, followed by approximately 150 lower-density pulses to induce skyrmion formation.

Key Contributions and Results

Direct Electrical Creation of Skyrmion Bags: The study reports the first direct current-induced formation of embedded skyrmion bags in FeGe under zero magnetic field. The process relies on the two-fold degeneracy of skyrmions ( $Q=1$ and $Q=-1$ ) at zero field.
Mechanism of Formation: The transformation is driven by spin-transfer torque (STT). Current pulses induce a horizontal helix state with a $q$ $q$ -vector perpendicular to the current. The instability of the helix ends, combined with the skyrmion Hall effect, causes the fragmentation of the helix. Individual skyrmions with opposite topological charges (seeds from the initial disordered state) become embedded within larger closed helix domains, forming nested structures denoted as $S(n)$ $S (n)$ or $S(n_1, S(n_2))$ $S (n_{1}, S (n_{2}))$ .
- $S(n)$ bags consist of $n$ interior skyrmions ( $Q=1$ ) surrounded by a boundary with $Q=-1$ , yielding a total charge of $Q = n-1$ .
- Nested bags $S(n_1, S(n_2))$ involve a complex hierarchy of charges.
Role of Current Parameters: The topological sign of the induced structures is determined solely by the current polarity (STT), while Joule heating plays an auxiliary role in assisting the splitting of skyrmions from helix ends. The Oersted field was ruled out as a primary nucleation mechanism due to its in-plane orientation and low magnitude relative to the required out-of-plane fields.
Topological Transitions and Defects: The authors observed current-induced transitions between skyrmion bags of different $Q$ $Q$ values (e.g., $S(n) \to S(n-1)$ $S (n) \to S (n - 1)$ ). These transitions are linked to the collapse of skyrmion tubes within the bags.
- Monopoles: Merging of two skyrmion tubes facilitated by Y-shaped configurations.
- Bobbers: Breakage of a skyrmion tube from the interior, evidenced by intermediate states with weak, dotted-like Fresnel contrast.
- These observations provide experimental signatures of magnetic monopoles and bobbers, which are typically difficult to observe electrically.
Stability and Dynamics: Skyrmion bags were found to be metastable high-energy phases. Their formation probability decreases with increasing temperature and pulse width due to thermal fluctuations and continuous STT driving them toward isolated skyrmions. However, once formed, they exhibit long-term stability (over 13 hours) under constant zero-field conditions. The study also noted that skyrmion bags undergo counter-clockwise rotation within the lattice, reflecting the rotation of the entire skyrmion lattice.

Significance
This work establishes a foundation for the all-electrical control of high- $Q$ topological spin textures and topological defects without the need for external magnetic fields. By demonstrating that STT can drive the fracture of helical states to create complex nested structures, the study opens new avenues for exploring the dynamics of topological defects like monopoles and bobbers. The ability to electrically write, manipulate, and potentially read (via topological Hall effect or tunnel magnetoresistance) these high- $Q$ structures suggests a pathway toward functional spintronic devices capable of utilizing customizable topological charges for information encoding. The authors note that while the current stochastic nature of formation limits deterministic operation, the underlying physics rooted in topology and dynamics suggests these findings could be generalized to other chiral magnet systems.

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