🔬 mesoscale physics

Thickness-Driven Control of Room Temperature Ferrimagnetic Skyrmions and their Topological Hall signature in GdFe Single Layers

This study demonstrates that precise control of film thickness in GdFe single layers enables the engineering of room-temperature ferrimagnetic skyrmions with tunable size and density, evidenced by correlated magnetic force microscopy and topological Hall resistivity measurements, thereby establishing a viable pathway for high-density skyrmionic devices.

Original authors: Saroj Kumar Mishra, Y. K. Takahashi, C. Malavika, Karthik V. Raman, Jyoti Ranjan Mohanty

Published 2026-02-26

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CC BY 4.0

Original authors: Saroj Kumar Mishra, Y. K. Takahashi, C. Malavika, Karthik V. Raman, Jyoti Ranjan Mohanty

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 you are trying to build a super-fast, super-dense hard drive for your computer. Instead of using tiny magnetic dots to store data (like a traditional hard drive), scientists are trying to use magnetic skyrmions.

Think of a skyrmion not as a dot, but as a tiny, swirling tornado of magnetism. It's a stable, knot-like structure made of electron spins. Because these "tornadoes" are topologically protected (meaning they are very hard to untie or destroy), they are perfect candidates for storing data. They are small, stable, and can be moved with very little energy.

However, there's a catch: making these skyrmions appear, stay stable, and pack tightly together at room temperature is incredibly difficult. Usually, scientists have to build complex "sandwiches" of different materials (multilayers) to force them to exist.

This paper is about a simpler, smarter way to do it using just one layer of material.

Here is the story of what the researchers discovered, explained simply:

1. The Material: A Magnetic "Smoothie"

The researchers used a material called GdFe (Gadolinium-Iron). Imagine this as a magnetic smoothie where two different ingredients (Gadolinium and Iron) are mixed together so thoroughly that they look like a single, uniform substance. Unlike the complex "sandwiches" used before, this is just a single, flat film.

2. The Secret Ingredient: Thickness

The big discovery here is that the thickness of this film is the "remote control" for the skyrmions.

The Experiment: They made three films: one thin (60 nanometers), one medium (70 nanometers), and one thick (80 nanometers).
The Result: By simply changing how thick the film was, they could change the size and number of the magnetic tornadoes.
- Thinner film: The skyrmions were larger and fewer in number.
- Thicker film: The skyrmions shrank down to be very small (about the size of a virus!) and packed much more tightly together.

The Analogy: Think of it like making popcorn. If you have a small pot (thin film), you get a few big kernels. If you have a huge pot (thick film), the kernels get smaller and you can fit way more of them in the same space. The researchers found the perfect "pot size" to get the maximum number of tiny, stable skyrmions.

3. Why Does This Happen? (The "Gradient" Mystery)

You might wonder: If the film is just one smooth layer of mixed metal, why does the thickness change anything?

The researchers looked at the film under a super-powerful microscope (like a high-tech X-ray vision) and found a secret: The smoothie isn't perfectly mixed from top to bottom.

The Gradient: As the film was being built, the Iron atoms settled faster than the Gadolinium atoms. This created a gradient (a gradual change in composition) from the bottom of the film to the top.
The Effect: This slight imbalance breaks the "symmetry" of the material. In physics, breaking symmetry is like tilting a table; it forces things to roll in a specific direction. This tilt creates a hidden force (called Dzyaloshinskii–Moriya Interaction) that acts like a glue, holding the magnetic tornadoes together and giving them their twisty shape.

4. Proving It Works: The "Hall Effect" Detective Work

How did they know the skyrmions were actually there and not just random magnetic noise?

They used two methods:

Magnetic Force Microscopy (MFM): This is like taking a high-resolution photo of the magnetic surface. They literally saw the circular skyrmions.
Topological Hall Effect (THE): This is a clever electrical trick. When electricity flows through a magnetic tornado, it gets "deflected" slightly, creating a tiny extra voltage.
- The researchers measured this voltage and found a clear signal that matched the number of skyrmions they saw in the photos.
- The Analogy: Imagine driving a car through a windy valley. If the wind (the skyrmion) is strong, your car gets pushed sideways. By measuring how much the car was pushed, they could calculate exactly how strong the wind was, even without seeing it.

5. Why Is This a Big Deal?

This research is a game-changer for future technology for three reasons:

Simplicity: You don't need to build complex, expensive multi-layer sandwiches. A single layer of material is easier and cheaper to make.
Density: They managed to pack the skyrmions very tightly (high density). This means future hard drives could store massive amounts of data in a tiny space.
Efficiency: Because these are made of a special type of magnet (ferrimagnet), they are naturally better at moving without wobbling off course (a problem called the "Skyrmion Hall Effect" that plagues other materials). This makes them perfect for "racetrack memory," where data zips along a wire like cars on a highway.

The Bottom Line

The researchers found a simple "knob" (film thickness) to tune a complex magnetic system. By adjusting the thickness of a single layer of GdFe, they created a stable, high-density forest of magnetic tornadoes at room temperature. This paves the way for the next generation of super-fast, ultra-dense, and energy-efficient computer memory.

1. Problem Statement

Magnetic skyrmions are promising candidates for next-generation high-density data storage and energy-efficient computing due to their nanoscale size, topological stability, and low current-driven motion requirements. However, realizing practical devices faces several challenges:

Material Constraints: Most skyrmion-hosting systems rely on multilayer stacks (e.g., Ir/Fe/Co/Pt) where skyrmion stability depends on interfacial Dzyaloshinskii–Moriya interaction (DMI). This limits tunability, as interfacial DMI typically decreases as the magnetic layer thickness increases, which is often required for thermal stability.
Ferromagnetic Limitations: In ferromagnetic materials, the Skyrmion Hall Effect (SkHE) causes skyrmions to deflect transversely during motion, hindering controlled unidirectional movement in racetrack memory.
Single-Layer Gap: While amorphous rare-earth–transition metal (RE-TM) ferrimagnets (like GdFe) offer intrinsic advantages (low net magnetization, suppressed SkHE), the direct visualization and control of skyrmions in single-layer films without complex interface engineering remain unexplored. Specifically, the mechanism for stabilizing bulk DMI in amorphous single layers and the ability to tune skyrmion size and density via film thickness were unknown.

2. Methodology

The study employed a multi-modal approach combining experimental synthesis, advanced microscopy, electrical transport measurements, and numerical simulations:

Sample Fabrication: Amorphous GdFe single-layer films with thicknesses of 60 nm, 70 nm, and 80 nm were deposited on Si substrates via electron-beam evaporation under ultra-high vacuum. A 3 nm Cr capping layer was added in situ to prevent oxidation.
Structural Analysis: Scanning Transmission Electron Microscopy (STEM) combined with Energy-Dispersive X-ray Spectroscopy (EDS) was used to perform depth-resolved microstructural analysis. This included high-angle annular dark-field (HAADF) imaging and elemental mapping to detect compositional gradients and structural asymmetry.
Magnetic Characterization:
- SQUID Magnetometry: Used to measure out-of-plane (OOP) and in-plane (IP) hysteresis loops to determine saturation magnetization ( $M_s$ ), anisotropy fields ( $H_k$ ), and magnetic anisotropy energy ( $K_u$ ).
- Magnetic Force Microscopy (MFM): Performed at room temperature (RT) under varying OOP magnetic fields to directly image spin textures (domain walls, bubbles, and skyrmions).
Electrical Transport: Magnetotransport measurements (Hall effect) were conducted using a van der Pauw geometry. The Topological Hall Effect (THE) was isolated by subtracting the Ordinary Hall Effect (OHE) and Anomalous Hall Effect (AHE) contributions from the total Hall resistivity.
Micromagnetic Simulations: Mumax3 software was used to simulate domain evolution and skyrmion dynamics. Parameters ( $M_s$ , $K_u$ , exchange stiffness $A$ ) were derived from experimental data, while the bulk DMI strength ( $D$ ) was systematically varied to match experimental observations.

3. Key Contributions

First Demonstration of RT Skyrmions in Single-Layer GdFe: The study provides the first microscopic (MFM) and electrical (THE) evidence of room-temperature ferrimagnetic skyrmions in a single-layer amorphous GdFe film, independent of multilayer interfaces.
Thickness-Driven Tunability: It establishes film thickness (60–80 nm) as a primary control knob for engineering skyrmion properties. Increasing thickness leads to a decrease in skyrmion size and a significant increase in skyrmion density.
Identification of Bulk DMI Origin: The research identifies a compositional gradient (Fe/Gd ratio variation) across the film thickness as the source of structural inversion symmetry breaking. This gradient generates a bulk DMI capable of stabilizing skyrmions, overcoming the limitations of interface-dependent DMI.
Correlation of THE and Skyrmion Density: The study quantitatively links the magnitude of the Topological Hall resistivity ( $\Delta\rho_{xy}$ ) to the skyrmion density ( $n_{sk}$ ) and topological charge ( $\langle Q_{sk} \rangle$ ), validating THE as a robust probe for skyrmion detection in ferrimagnets.

4. Key Results

Structural and Magnetic Properties

Compositional Gradient: STEM-EDS revealed a vertical gradient in Gd and Fe concentrations. The gradient slope ( $\beta$ ) decreased as film thickness increased, indicating that thicker films become more uniform, yet sufficient asymmetry remains to induce bulk DMI.
Magnetic Parameters: As thickness increased from 60 to 80 nm:
- Saturation magnetization ( $M_s$ ) increased from 240 to 370 kA/m.
- Anisotropy field ( $H_k$ ) increased from 0.06 T to 0.24 T.
- Magnetic anisotropy energy ( $K_u$ ) increased from 50 to 160 kJ/m³.
- The films exhibited weak perpendicular magnetic anisotropy (PMA), a regime favorable for skyrmion formation.

Skyrmion Imaging and Dynamics (MFM)

Evolution: MFM images showed a transition from labyrinthine domains at low fields to isolated circular textures (skyrmions) at intermediate fields, and finally to saturation.
Size and Density:
- 60 nm film: Few skyrmions observed; diameter $\sim$ 60 nm.
- 80 nm film: High density of skyrmions; diameter decreased to $\sim$ 60 nm (experiment) / 55 nm (simulation).
- Density: Skyrmion density increased from a few $\mu m^{-2}$ at 60 nm to $\sim$ 26 $\mu m^{-2}$ at 80 nm.
Stability: Skyrmions were stable over a wide magnetic field range and persisted at room temperature.

Topological Hall Effect (Transport)

THE Signature: A characteristic double-peak feature (peak and dip) in the topological Hall resistivity ( $\Delta\rho_{xy}$ ) was observed, corresponding to the reversal of skyrmion core polarity during the magnetic field sweep.
Thickness Dependence: The magnitude of $\Delta\rho_{xy}$ $Δ ρ_{x y}$ increased systematically with thickness:
- 60 nm: $\sim$ 0.36 $\mu\Omega\cdot$ cm
- 70 nm: $\sim$ 0.73 $\mu\Omega\cdot$ cm
- 80 nm: $\sim$ 1.18 $\mu\Omega\cdot$ cm
Correlation: The increase in $\Delta\rho_{xy}$ directly correlated with the increased skyrmion density observed in MFM, confirming the topological origin of the signal.

Simulation Validation

Micromagnetic simulations using Mumax3 successfully reproduced the experimental domain evolution and hysteresis loops.
To match experimental results, the bulk DMI strength ( $D$ ) had to be increased with thickness (0.4 mJ/m² for 60 nm $\to$ 0.6 mJ/m² for 80 nm), consistent with the interplay between anisotropy, exchange, and dipolar interactions.
Simulations confirmed the Bloch-type helicity of the skyrmions and the sequential stabilization of two distinct skyrmion phases (opposite polarity) during field sweeps.

5. Significance and Impact

Simplified Device Architecture: This work demonstrates that complex multilayer stacks are not strictly necessary for stabilizing room-temperature skyrmions. Single-layer ferrimagnetic films offer a simpler, more scalable platform for spintronic devices.
Overcoming the Skyrmion Hall Effect: By utilizing ferrimagnetic GdFe, the study leverages the intrinsic low net magnetization of these materials, which significantly suppresses the Skyrmion Hall Effect, enabling more reliable unidirectional motion for racetrack memory applications.
Tunability: The ability to tune skyrmion size and density simply by adjusting film thickness provides a powerful engineering route for optimizing storage density and device performance.
Fundamental Insight: The discovery that compositional gradients in amorphous alloys can break inversion symmetry and generate bulk DMI expands the understanding of topological spin textures in disordered systems.

In conclusion, this research establishes GdFe single-layer films as a viable, high-performance platform for high-density, energy-efficient spintronic devices, offering a robust method to engineer room-temperature skyrmions through thickness control.