Stabilized biskyrmion states in annealed CoFeB bilayer with different interfaces

This study demonstrates that annealing CoFeB/Ta/CoFeB bilayers enhances interfacial Dzyaloshinskii-Moriya interaction and crystallinity to stabilize the spontaneous coexistence of skyrmions and biskyrmions at room temperature, revealing their distinct interaction behaviors and potential for advanced spintronic applications.

The Gist

Imagine you are looking at a microscopic dance floor made of layers of metal. On this floor, tiny magnetic particles (called spins) are dancing. Usually, they all line up in neat rows, pointing either up or down. But sometimes, under the right conditions, they decide to break formation and swirl into tiny, stable tornadoes. These magnetic tornadoes are called Skyrmions.

This paper is about discovering not just single tornadoes, but a special pair of tornadoes that stick together, called Biskyrmions, and figuring out how to make them appear and stay stable.

Here is the story of how they did it, explained simply:

1. The Stage: A Magnetic Sandwich

The scientists built a very thin "sandwich" of materials.

• The Bread: Layers of Platinum (Pt) and Cobalt (Co) at the bottom.
• The Filling: Two layers of a special metal alloy called CoFeB (Cobalt-Iron-Boron).
• The Separator: A tiny, invisible layer of Tantalum (Ta) sits right between the two CoFeB layers.
• The Topping: A layer of Magnesium Oxide (MgO) on top.

Think of the Tantalum layer as a traffic controller. Its job is to tell the magnetic dancers in the top layer and the bottom layer how to spin. Because the layers are different, the "rules" for spinning are different for the top dancers and the bottom dancers.

2. The Heat Treatment: Cooking the Sandwich

The scientists baked this sandwich in an oven at two different temperatures to see what happened:

• Baking at 230°C: The sandwich got crispier (the crystals aligned better), and it worked well electrically. However, the magnetic tornadoes (skyrmions) wouldn't form on their own. You had to push them with an external magnetic field (like a gentle wind) to get them to appear.
• Baking at 330°C: This was the "Goldilocks" temperature. The heat improved the structure so much that the magnetic tornadoes appeared spontaneously. They didn't need any external wind; they just formed naturally and stayed there.

3. The Discovery: Single Tornadoes vs. Twin Tornadoes

Using a super-powered microscope (Magnetic Force Microscopy), they looked at the dance floor and saw two things:

• Skyrmions: Single magnetic tornadoes spinning in one direction.
• Biskyrmions: A unique structure where two tornadoes are partially overlapping and stuck together.

The Secret Ingredient: The "Handedness" (Chirality)
Here is the magic trick. Because of the Tantalum separator, the top CoFeB layer wants its tornadoes to spin Left-Handed, while the bottom layer wants them to spin Right-Handed.

• The Lonely Dancers (Same Handedness): If you have two tornadoes spinning the same way (both Left), they are like two magnets with the same pole facing each other. They repel (push away) from each other. They want to stay far apart.
• The Twin Dancers (Opposite Handedness): If you have one Left-spinning tornado and one Right-spinning tornado, they are like puzzle pieces. Their edges fit perfectly together. They attract (pull toward) each other and merge into a stable pair. This merged pair is the Biskyrmion.

4. The Computer Simulation: The Virtual Lab

To prove this wasn't just a lucky accident, the scientists used a computer program to simulate the physics.

• They created a virtual version of their sandwich.
• They programmed the top layer to spin one way and the bottom layer the other.
• The Result: The computer showed that when the spins were opposite, they naturally grabbed onto each other and formed a stable twin structure (the biskyrmion). When they were the same, they pushed apart. This confirmed that the "traffic controller" (the Tantalum layer) was successfully creating the conditions for these twin tornadoes to form.

Why Does This Matter? (The Big Picture)

Why do we care about these tiny magnetic tornadoes?

• Future Hard Drives: Imagine a hard drive where data isn't stored as "0" or "1" on a flat surface, but as these tiny, stable tornadoes moving along a track (like cars on a racetrack).
• Efficiency: These tornadoes are incredibly small (nanoscale) and can be moved with very little electricity. This means future computers could be much faster and use much less battery power.
• Stability: The fact that these "twin tornadoes" (biskyrmions) can form spontaneously and stay stable without needing constant external help is a huge step forward for making this technology practical.

In a Nutshell:
The scientists baked a special magnetic sandwich. By adjusting the heat and the layers, they taught the magnetic particles to dance in pairs. They discovered that when two magnetic tornadoes spin in opposite directions, they naturally stick together to form a stable "twin tornado" (biskyrmion). This discovery brings us one step closer to building super-fast, super-efficient computers for the future.

Technical Summary

Here is a detailed technical summary of the research paper "Stabilized biskyrmion states in annealed CoFeB bilayer with different interfaces."

1. Problem Statement

Magnetic skyrmions are topologically non-trivial spin textures promising for next-generation spintronic applications (e.g., racetrack memory) due to their nanoscale size, stability, and low current manipulation requirements. However, biskyrmions (bound states of two skyrmions with a topological charge of $Q = \pm 2$) are less understood in perpendicular magnetic tunnel junctions (p-MTJs). While biskyrmions are typically stabilized by magnetic dipolar interactions in centrosymmetric materials, stabilizing them in engineered multilayer systems with interfacial Dzyaloshinskii-Moriya interaction (DMI) is challenging. The specific challenge addressed in this study is achieving the spontaneous coexistence of skyrmions and biskyrmions in a CoFeB/Ta/CoFeB bilayer free layer without requiring external magnetic fields, and understanding how interface engineering (specifically Ta layer thickness and annealing) influences DMI chirality and topological stability.

2. Methodology

The study employed a combination of experimental fabrication, characterization, and numerical simulation:

• Sample Fabrication:

  • Structure: A magnetic tunnel junction (MTJ) stack was deposited on oxidized silicon wafers. The free layer consisted of a CoFeB(1.2 nm)/Ta(0.3 nm)/CoFeB(1.2 nm) trilayer.
  • Underlayer: An antiferromagnetically coupled synthetic antiferromagnet (SAF) structure: $[Co(0.5)/Pt(0.6)]_6/Co(0.3)/Ru(0.4)/[Co(0.5)/Pt(0.6)]_2/Co(0.3)/Ta(0.25)/CoFeB(0.85)$.
  • Barrier/Capping: A 1.5 nm MgO tunnel barrier and a Ta capping layer were used to enhance Tunnel Magnetoresistance (TMR) and interfacial anisotropy.
  • Processing: Samples were annealed at 230°C and 330°C for 30 minutes in a high-vacuum environment. Nanopillars (1–2 µm diameter) were fabricated via photolithography and ion milling.

• Characterization Techniques:

  • Magneto-Optical Kerr Effect (MOKE): Used to measure hysteresis loops and verify perpendicular magnetic anisotropy (PMA).
  • Transport Measurements: TMR was measured using a four-point contact geometry on single nanopillars.
  • Magnetic Force Microscopy (MFM): Performed in both z-mode (height shift) and $\Delta$f-mode (frequency shift) to image magnetic textures (skyrmions and biskyrmions) at room temperature. $\Delta$f-mode was prioritized for its higher sensitivity to weak magnetic fields and reduced tip-sample interaction.

• Micromagnetic Simulations:

  • Software: MuMax3.
  • Model: Solved the Landau-Lifshitz-Gilbert (LLG) equation.
  • Parameters: $M_s = 900$ kA/m, Exchange stiffness $A_{ex} = 1.3 \times 10^{-11}$ J/m, Damping $\alpha = 0.1$, PMA constant $K_u = 0.6 \times 10^6$ J/m³.
  • DMI Modeling: Interfacial DMI was implemented as an effective field. Simulations tested DMI constants ($D$) of $+1.0$ and $-1.0$ mJ/m² to model opposite chiralities.

3. Key Contributions

• Demonstration of Spontaneous Biskyrmions: The study successfully demonstrated the spontaneous formation of biskyrmions in a CoFeB bilayer system at room temperature without an external magnetic field, achieved through high-temperature annealing (330°C).
• Interface-Driven Chirality Control: The research elucidates how varying the Ta spacer thickness and annealing conditions creates competing interfacial DMI contributions. This leads to the coexistence of skyrmions with opposite chiralities (helicities) within the same bilayer, a prerequisite for biskyrmion formation.
• Interaction Mechanism Validation: Through micromagnetic simulations, the authors confirmed that skyrmions with identical chirality repel each other, while those with opposite chirality attract, merging to form stable biskyrmions. This provides a theoretical basis for the experimental observations.
• Optimization of MTJ Performance: The work correlates structural annealing with electrical performance, achieving a high TMR of ~41% while simultaneously stabilizing complex topological states.

4. Key Results

• Electrical Properties:

  • Annealing at 230°C yielded a TMR of ~41%.
  • Annealing at 330°C resulted in a more tilted hysteresis loop with lower remanence, suggesting a reduction in perpendicular anisotropy (PMA) due to potential intermixing of Ta into CoFeB, yet it facilitated topological stability.

• Magnetic Texture Observations (MFM):

  • As-deposited & 230°C Annealed: Skyrmions were observed only under an external magnetic field ($H = 15$ mT). Biskyrmions were not spontaneously stable.
  • 330°C Annealed: Spontaneous coexistence of skyrmions and biskyrmions was observed at room temperature without any external magnetic field.
  • Imaging: Both z-mode and $\Delta$f-mode MFM confirmed the presence of these textures. The $\Delta$f-mode provided clearer topological characterization.

• Simulation Insights:

  • Repulsion: Two skyrmions with the same chirality (e.g., $D = +1.0$ mJ/m²) exhibited repulsive forces, maintaining separation.
  • Attraction & Merging: When simulating a bilayer scenario with opposite DMI signs (mimicking the two CoFeB interfaces), skyrmions with opposite helicities attracted each other, merged, and formed a bound state (biskyrmion).
  • Mechanism: The attraction arises because the domain walls of opposite chirality are complementary, allowing them to share a partial wall and lower the total system energy.

5. Significance and Impact

• Fundamental Physics: The study advances the understanding of how interfacial engineering (specifically Ta thickness and annealing) can tune DMI strength and sign to control spin texture chirality. It confirms that biskyrmions can be stabilized in non-centrosymmetric MTJ stacks via interfacial DMI competition, not just dipolar interactions.
• Spintronic Applications:
  • Data Storage: The ability to generate and stabilize biskyrmions ($Q=\pm 2$) and skyrmions ($Q=\pm 1$) spontaneously at room temperature is a critical step toward high-density racetrack memory. Biskyrmions offer higher information density per unit area compared to single skyrmions.
  • Device Design: The findings suggest that optimizing the free layer structure (e.g., using bilayers with distinct interfaces) is a viable strategy for creating complex spin textures without the need for continuous external magnetic fields, which is essential for practical device integration.
• Material Optimization: The work highlights the delicate balance required between PMA and DMI. While excessive annealing (330°C) reduced PMA, it enhanced crystallinity and DMI interactions sufficiently to stabilize topological states, providing a roadmap for future material processing.