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Influence of chemical ordering on magnetocrystalline anisotropy and magnetoelastic properties in Weyl magnetic semimetal Co2MnGa thin films

This study demonstrates that increasing chemical ordering in Co2MnGa thin films, particularly the formation of the L21 phase, significantly enhances both magnetocrystalline anisotropy and magnetoelastic properties, with the L21-ordered structure exhibiting giant magnetoelastic anisotropy distinct from non-Weyl semimetal counterparts.

Original authors: O. Chumak, A. Nabiałek, L. T. Baczewski, T. Seki, J. Wang, K. Takanashi, H. Szymczak

Published 2026-06-24

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CC BY 4.0

Original authors: O. Chumak, A. Nabiałek, L. T. Baczewski, T. Seki, J. Wang, K. Takanashi, H. Szymczak

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 a thin film of a special metal alloy called Co2MnGa as a crowded dance floor. The dancers are atoms, and the way they arrange themselves determines how the whole group behaves. This specific alloy is a "Weyl semimetal," which is a fancy way of saying it has some unique, topological properties that make it interesting for future electronics, but for this story, let's just focus on how the atoms dance.

The researchers wanted to see how changing the order of these dancers affects two main things:

Magnetocrystalline Anisotropy: Which direction the group "wants" to face (like a compass needle).
Magnetoelastic Properties: How the group stretches or squishes when you pull on the floor they are standing on (strain).

The Experiment: Rearranging the Dance Floor

The scientists took thin films of this alloy and heated them (annealed) at different temperatures: 300°C, 400°C, and 500°C. Think of heating as giving the dancers more energy to move around and find their perfect spots.

At 300°C: The dancers are messy and scattered. This is called the A2 phase. It's like a chaotic crowd where everyone is bumping into each other randomly.
At 400°C: The crowd starts to organize. About 87% of the dancers find a specific, semi-ordered pattern called the B2 phase.
At 500°C: The dance floor becomes highly organized. A new, perfectly structured pattern called the L21 phase appears (making up about 23% of the floor), mixed with the B2 phase. This L21 phase is the "VIP section" where the unique topological properties live.

What Happened to the "Compass" (Magnetism)?

The researchers measured which way the magnetic "compass" of the film pointed.

In the messy (300°C) and semi-ordered (400°C) films, the compass liked to point in one specific direction (the [110] direction).
However, once the L21 phase appeared at 500°C, the compass suddenly flipped! It decided it preferred a completely different direction (the [100] direction).
The Takeaway: The appearance of the highly ordered L21 phase acted like a switch, completely changing the magnetic "personality" of the material.

What Happened to the "Stretch" (Magnetoelasticity)?

This is where things got really surprising. The scientists used a special technique (SMFMR) to see how the material reacted when they physically stretched it in different directions.

The Messy and Semi-Ordered Films: When they stretched these films, the magnetic properties changed a little bit, but it was pretty much the same whether they pulled them left-right or up-down. It was a gentle, uniform reaction.
The Highly Ordered Film (500°C): When they stretched the film with the L21 phase, the reaction was wildly different depending on the direction.
- If they pulled in one direction, the magnetic properties changed a huge amount.
- If they pulled in the perpendicular direction, the change was almost zero.
- The Analogy: Imagine a rubber band. If you pull a normal rubber band, it stretches evenly. But imagine a rubber band that stretches easily if you pull it horizontally, but is as hard as a steel rod if you pull it vertically. That is what the researchers found in the 500°C film. They call this a "giant anisotropy" of magnetoelastic properties.

The Big Comparison: Is it the "Topological" Magic?

The researchers compared their Co2MnGa film to a "cousin" alloy called Co2MnSi.

Co2MnSi is chemically very similar (just swapping one atom, Ga for Si), but it is not a Weyl semimetal. It doesn't have those special topological properties.
When they looked at the ordered Co2MnSi, it behaved normally. Even when it had the same ordered L21 structure, pulling it in different directions gave similar results. It didn't have that "giant" difference.
The Conclusion: The fact that Co2MnGa (the Weyl semimetal) showed this massive, direction-dependent stretching effect, while Co2MnSi (the non-topological one) did not, suggests that the unique electronic structure of the Weyl semimetal is likely responsible for this giant sensitivity.

Summary

In simple terms, the paper shows that:

Order matters: Heating the film to create a highly ordered atomic structure (L21 phase) drastically changes how the material behaves.
Direction is key: In this highly ordered state, the material becomes incredibly sensitive to which way you stretch it.
Topological connection: This extreme sensitivity seems to be a special trait of the Weyl semimetal nature of Co2MnGa, because a chemically similar but non-topological material didn't show the same behavior.

The study didn't test specific devices or future gadgets; it simply mapped out how the internal atomic order changes the fundamental magnetic and stretching rules of this specific material.

Technical Summary: Influence of Chemical Ordering on Magnetocrystalline Anisotropy and Magnetoelastic Properties in Weyl Magnetic Semimetal Co2MnGa Thin Films

Problem Statement
The magnetic Weyl semimetal Co2MnGa (CMG) exhibits topological properties and large anomalous Hall and Nernst effects, making it a candidate for spintronic and spin-caloritronic applications. However, the realization of these properties in thin films is heavily dependent on the degree of chemical ordering (ranging from disordered A2 to partially ordered B2 and fully ordered L21 phases). While previous studies have linked chemical ordering to the emergence of Weyl semimetallic features and magnetic properties, the specific correlation between chemical ordering, magnetocrystalline anisotropy (MCA), and magnetoelastic properties in CMG thin films remains unexplored. Understanding these relationships is critical for optimizing film performance, particularly given that practical applications often involve strain-induced modifications and that thin-film properties can differ significantly from bulk materials.

Methodology
The study utilized 75 nm thick epitaxial Co2MnGa thin films grown on MgO (001) substrates and capped with a 2 nm Al layer (oxidized to prevent parasitic phase formation). To induce varying degrees of chemical ordering, samples were annealed at three distinct temperatures: 300 °C, 400 °C, and 500 °C. Structural characterization confirmed that these temperatures yielded samples dominated by the A2 phase (300 °C), the B2 phase (400 °C), and a mixture containing approximately 23% of the ordered L21 phase (500 °C).

The investigation employed two primary magnetic characterization techniques:

Ferromagnetic Resonance (FMR): Used to determine magnetocrystalline anisotropy constants ( $K_1$ and $K_p$ ) by measuring angular dependencies of the resonance field in both in-plane and out-of-plane directions.
Strain-Modulated Ferromagnetic Resonance (SMFMR): Used to measure anisotropic magnetoelastic properties. This technique involved applying periodic strain to the films via a quartz rod and measuring the resulting shifts in the FMR resonance field. This allowed for the calculation of magnetoelastic constants ( $b_1, b_2$ ) and magnetostriction constants ( $\lambda_{100}, \lambda_{111}$ ).

The results were compared against previously studied non-topological Heusler alloys, specifically the Co2FexMn1-xSi system, to isolate the influence of topological properties.

Key Results

Chemical Ordering and Phase Evolution: Annealing at 500 °C successfully introduced the highly ordered L21 phase (approx. 23% volume fraction) alongside the B2 phase, whereas lower temperatures resulted in A2 or predominantly B2 structures.
Magnetocrystalline Anisotropy (MCA):
- The cubic MCA constant ( $K_1$ ) exhibited a sign reversal and magnitude increase correlated with the appearance of the L21 phase. For samples annealed at 300 °C and 400 °C (A2/B2 phases), $K_1$ was negative ( $\approx -0.4 \times 10^4$ erg/cm³), indicating an easy axis along [110]. For the 500 °C sample (containing L21), $K_1$ became positive ( $2.2 \times 10^4$ erg/cm³), shifting the easy axis to [100].
- The perpendicular anisotropy constant ( $K_p$ ) remained negative (order of $10^5$ erg/cm³) for all samples, favoring in-plane magnetization, with the most significant change occurring upon the formation of the B2 phase.
Magnetoelastic Properties:
- A "giant anisotropy" in magnetoelastic properties was observed specifically in the sample containing the L21 phase (500 °C). The magnetoelastic constants $b_1$ and $b_2$ differed by nearly two orders of magnitude ( $b_1 \approx -1.43 \times 10^7$ erg/cm³ vs. $b_2 \approx -2 \times 10^5$ erg/cm³).
- This resulted in a massive disparity between the calculated magnetostriction constants: $\lambda_{100} = 7.7 \times 10^{-6}$ and $\lambda_{111} = 0.05 \times 10^{-6}$ .
- In contrast, samples with only A2 or B2 phases showed relatively weak anisotropy between $b_1$ and $b_2$ .
Comparison with Non-Topological Systems: When compared to Co2MnSi (which has a similar crystallographic structure and degree of ordering but lacks Weyl semimetal properties), the L21-ordered Co2MnGa exhibited a unique giant anisotropy in magnetoelastic constants. In Co2MnSi, $b_1$ and $b_2$ were nearly equal. This suggests that the topological nature of the L21 CMG phase may play a role in this anisotropy, although the authors note that compositional changes in non-topological systems (e.g., Fe substitution in Co2FexMn1-xSi) can also induce anisotropy.

Significance and Claims
The paper claims that the degree of chemical ordering is a crucial factor in determining the magnetic and magnetoelastic behavior of Co2MnGa thin films. Specifically, the emergence of the L21 ordered phase is directly correlated with:

A significant increase in the magnitude of the cubic magnetocrystalline anisotropy constant and a change in the easy magnetization axis.
The appearance of a "giant anisotropy" in magnetoelastic properties, characterized by a drastic difference between the $b_1$ and $b_2$ constants.

The authors suggest that the band structure of the L21-ordered Co2MnGa alloy is highly sensitive to both the crystallographic direction of magnetization and the orientation of the strain tensor. While they propose that the specific giant anisotropy observed in CMG compared to Co2MnSi may be linked to the topological properties of the Weyl semimetal, they explicitly state that this hypothesis requires further theoretical verification (e.g., band structure calculations under strain) and is not definitively proven by this study alone. The work underscores the importance of controlling chemical ordering for tuning the performance of Heusler alloy thin films in potential device applications.