🔬 materials science

Facilitating electrical and laser-induced skyrmion nucleation with a dipolar-field enhanced effective DMI

This study demonstrates that engineering a layer-dependent Dzyaloshinskii-Moriya interaction (DMI) sign in Ir/Co/Pt multilayers aligns the in-plane dipolar field with the effective DMI, thereby enhancing the effective DMI to significantly improve the nucleation density and magnetic field stability of skyrmions under both electrical and laser excitation.

Original authors: Mark C. H. de Jong, Dinar Khusyainov, Julian Hintermayr, Bart Sanders, Dmitry Kozodaev, Aleksei V. Kimel, Bert Koopmans, Theo H. M. Rasing, Reinoud Lavrijsen

Published 2026-02-03

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Mark C. H. de Jong, Dinar Khusyainov, Julian Hintermayr, Bart Sanders, Dmitry Kozodaev, Aleksei V. Kimel, Bert Koopmans, Theo H. M. Rasing, Reinoud Lavrijsen

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

The Big Idea: Building Better Magnetic "Vortexes"

Imagine you are trying to create tiny, stable whirlpools (called skyrmions) in a magnetic material. These whirlpools are special because they can carry information, acting like bits of data in future computers. However, making them is tricky. They need a specific "twist" to stay stable, and if the conditions aren't just right, they fall apart or are very hard to create.

This paper is about a team of scientists who figured out how to make these magnetic whirlpools much easier to create and much more stable by rearranging the layers of their magnetic "sandwich."

The Ingredients: The Magnetic Sandwich

The scientists built their devices using a stack of very thin layers, like a multi-layered sandwich. The main ingredients were:

Platinum (Pt) and Iridium (Ir): Heavy metals that act like the "spices" that give the magnetic layers their special twisting properties.
Cobalt (Co): The magnetic "meat" where the whirlpools live.

In a normal sandwich, you might stack the layers in the same order all the way through (e.g., Bread-Meat-Bread-Meat). In this experiment, the scientists created two different types of sandwiches:

The Uniform Sandwich: The layers are stacked in the exact same order from top to bottom.
The "Enhanced" Sandwich: The bottom half of the stack is one way, and the top half is flipped upside down (reversed order).

The Problem: The "Wind" Inside the Stack

Inside these magnetic layers, there is a natural force called the Dipolar Field. You can think of this like a gentle wind blowing through the layers.

In a normal stack, this "wind" sometimes blows against the natural twist the layers want to have. It's like trying to spin a top while someone is blowing air against it. This makes it harder to create the whirlpools (skyrmions) and makes them less stable.
The scientists realized that by flipping the order of the layers in the top half of their "Enhanced" sandwich, they could make the "wind" blow in the same direction as the natural twist.

The Analogy: The Rowing Team

Imagine the magnetic layers are a team of rowers in a boat.

The DMI (The Twist): This is the natural rhythm the rowers want to use to turn the boat.
The Dipolar Field (The Wind): This is a gust of wind.

In the Uniform Sandwich, the wind blows against half the rowers. Some rowers are rowing with the wind, but the others are fighting it. The team struggles to turn the boat quickly or keep it stable.

In the Enhanced Sandwich, the scientists flipped the order of the top half of the team. Now, the wind is blowing in the same direction as everyone's rowing stroke. The wind actually helps them turn the boat. The "effective twist" of the whole team is much stronger because the wind is helping, not hindering.

What They Found

The scientists tested these two sandwiches using two different methods to create the skyrmions:

Electric Current: Sending a quick pulse of electricity through the wire.
Laser Pulse: Hitting the material with a super-fast flash of light.

Here is what happened:

The "Enhanced" Sandwich (Wind Helping): When they used the sandwich where the wind helped the twist, they created 20 times more skyrmions than with the "Reduced" sandwich (where the wind fought the twist).
Stability: The skyrmions in the "Enhanced" sandwich could survive in stronger magnetic fields. They were tougher and didn't disappear as easily.
The Threshold: Interestingly, the amount of electricity or laser power needed to start creating the skyrmions was about the same for both sandwiches. The difference wasn't in starting the process, but in how many successful whirlpools survived and how many were created once the process started.

The Conclusion

The paper shows that by simply flipping the order of the layers in half of the stack, you can align the internal "wind" (dipolar field) to help the magnetic twist (DMI).

This doesn't just make the whirlpools easier to find; it makes them much more abundant and stable. The scientists proved that this works whether you use electricity or lasers to create them. It's a simple trick of rearrangement that turns a struggle against the wind into a helpful breeze, allowing for much better control over these tiny magnetic data carriers.

Technical Summary: Facilitating Electrical and Laser-Induced Skyrmion Nucleation with a Dipolar-Field Enhanced Effective DMI

Problem Statement
The stability and controllability of chiral magnetic textures, such as skyrmions, rely heavily on the Dzyaloshinskii-Moriya interaction (DMI). In magnetic multilayers, interfacial DMI stabilizes Néel-type domain walls and skyrmions, ensuring uniform chirality and enabling current-driven motion. However, in multilayers with significant thickness and magnetic moment, long-range dipolar interactions can compete with the DMI. Specifically, the dipolar field generated by out-of-plane magnetization in domains can create an in-plane field within domain walls that opposes the DMI in certain layers, potentially leading to heterochiral walls and reduced stability. While this dipolar effect is often viewed as detrimental to current-driven motion, it is also essential for the small size and high stability of skyrmions. The challenge lies in isolating the effect of the effective DMI on skyrmion nucleation without simultaneously altering other magnetic parameters (such as saturation magnetization or anisotropy), which typically occurs when modifying layer thickness or composition.

Methodology
The authors fabricated four distinct Ir/Co/Pt-based magnetic multilayer stacks to systematically vary the effective DMI while maintaining identical average saturation magnetization ( $M_s$ ) and uniaxial anisotropy ( $K_u$ ). The building blocks were two trilayers: $[Pt(1) | Co(1) | Ir(1)]$ (positive intrinsic DMI) and $[Ir(1) | Co(1) | Pt(1)]$ (negative intrinsic DMI).

Uniform+ and Uniform- Stacks: Contained six repeats of a single trilayer type, resulting in uniform intrinsic DMI signs throughout the stack.
Enhanced Stack: Composed of three repeats of the positive DMI trilayer followed by three repeats of the negative DMI trilayer. This configuration aligns the DMI effective field parallel to the dipolar field in all layers, theoretically enhancing the effective DMI.
Reduced Stack: Composed of three repeats of the negative DMI trilayer followed by three repeats of the positive DMI trilayer. This configuration aligns the DMI effective field antiparallel to the dipolar field, theoretically reducing the effective DMI.

The samples were characterized using SQUID-VSM for magnetic parameters and Magnetic Force Microscopy (MFM) to measure equilibrium domain widths and determine the effective DMI strength ( $|D_{eff}|$ ) using a model by Lemesh et al. Skyrmion nucleation was induced via two methods:

Current-driven: Using 50 ns current pulses in narrow current lines.
Laser-induced: Using single 70 fs laser pulses.
In both cases, nucleation was performed under an applied out-of-plane bias field, and the resulting skyrmion density and stability were analyzed.

Key Results

Effective DMI Modulation: The characterization confirmed that the Enhanced and Reduced stacks possessed nearly identical $M_s$ and $K_u$ values (averages of the two uniform stacks). However, the effective DMI differed significantly. The Enhanced stack exhibited an effective DMI approximately 2.5 times larger than the Reduced stack. Specifically, the Enhanced stack showed an increase of $(0.34 \pm 0.08) \text{ mJ m}^{-2}$ over the average, while the Reduced stack showed a decrease of $(0.6 \pm 0.1) \text{ mJ m}^{-2}$ . These experimental values aligned well with MuMax3 simulations and analytical models predicting dipolar contributions.
Skyrmion Nucleation Density: The most significant finding was the impact on nucleation density. In the Enhanced stack, the density of nucleated skyrmions was up to a factor of $\sim 20$ higher than in the Reduced stack for current-driven nucleation, and 4 times higher for laser-induced nucleation at saturation.
Thresholds: Surprisingly, the threshold current density ( $J_c$ ) and threshold fluence ( $F_c$ ) required to initiate nucleation were nearly identical for both stack types. This suggests that the effective DMI primarily influences the stability and survival of the nucleated skyrmions rather than the initial energy barrier for nucleation.
Stability: Skyrmions in the Enhanced stack remained stable in stronger magnetic fields compared to the Reduced stack, further indicating improved stability due to the enhanced effective DMI.
Dynamics: Preliminary measurements indicated that spin-orbit torque (SOT) could still drive domain wall and skyrmion motion in the Enhanced stack, despite the inverted stacking order in half the device, as the sign change in SOT and chirality compensated for each other.

Significance and Claims
The paper claims to demonstrate experimentally that skyrmion nucleation is strongly dependent on the magnitude of the effective DMI in a magnetic multilayer. By utilizing the internal dipolar field in combination with a specific layer-dependent stacking order, the authors successfully controlled the effective DMI without altering other material parameters.

The authors state that this approach provides an effective route to facilitate the nucleation and stability of chiral magnetic textures. They highlight that their results offer the first insight into the dependence of laser-induced skyrmion nucleation on internal dipolar interactions in magnetic multilayers. The work suggests that engineering the stacking order to align dipolar fields with the DMI is a viable strategy for optimizing skyrmion-based devices, particularly for improving the density and stability of nucleated skyrmions, even though the nucleation thresholds themselves remain largely unaffected by this specific modification.