- The paper shows that the anomalous Nernst thermopower in Co₂MnGa reaches ~6.0 µV/K at 1 T, far exceeding conventional magnetization-based predictions.
- It employs detailed electrical measurements and DFT calculations to reveal that a large Berry curvature, arising from nodal lines and Weyl points, drives the high thermopower.
- The findings suggest new thermoelectric design strategies using topologically nontrivial Heusler compounds, decoupling thermal and electrical transport.
Anomalous Nernst Effect Beyond Magnetization Scaling in Ferromagnetic Heusler Compound Co2MnGa
The study presented examines the anomalous Nernst effect (ANE) in the ferromagnetic topological Heusler compound Co2MnGa, illustrating a significant departure from conventional ANE theories traditionally linked to magnetization. This paper provides compelling evidence that the anomalous Nernst thermopower, which generates a voltage perpendicular to both the temperature gradient and magnetization, can be extraordinarily high in certain topological materials, independent of net magnetization.
Overview of Findings
The researchers observed an anomalous Nernst thermopower value of approximately 6.0 µV K-1 at 1 T and room temperature in Co2MnGa. Notably, this value surpasses any previously reported for conventional ferromagnetic systems by a factor of seven, challenging the conventional model that scales ANE with magnetization. The discovery is underpinned by comprehensive electrical, thermoelectric, and first-principles calculations, pointing to a large net Berry curvature near the Fermi level arising from nodal lines and Weyl points as a critical factor in this phenomenon.
Theoretical and Experimental Insights
The paper elaborates on the intrinsic connection between ANE and the Berry phase effects within the band structure. Traditionally, ANE was considered a thermoelectric counterpart to the anomalous Hall effect (AHE), with both effects presumed to originate from the band magnetization. However, recent theoretical advancements suggest that the Berry curvature, a consequence of the nontrivial topology of bands near the Fermi level, plays a vital role. This study exemplifies this concept through detailed ab-initio calculations using density-functional theory (DFT) with a focus on Berry curvature distribution.
Key Experimental Results
- Electric Transport: The study notes a decrease in electrical resistivity (ρxx) with temperature, affirming metallic behavior, and measures a Seebeck coefficient indicative of electron-dominated transport.
- Magnetization and Hall Conductivity: Co2MnGa exhibits ferromagnetic characteristics with a high Curie temperature (Tc = 687 K) and saturation magnetization that decreases with temperature. Importantly, the anomalous Hall conductivity (AHC) was shown to depend strongly on the Fermi level, implicating Berry curvature.
- Nernst Thermopower Measurements: The Nernst effect displayed significant deviations from conventional scaling, with the researchers attributing the large ANE values to mechanisms beyond the scope of traditional magnetization-based theories.
Practical and Theoretical Implications
From an application perspective, the findings suggest potential for Co2MnGa in the development of more efficient multi-terminal thermoelectric devices, which would benefit from the non-reliance on p- and n-type materials. The ability to decouple thermal and electrical paths might simplify the integration and increase the operability of thermoelectric devices.
Theoretically, these results advocate for a refined understanding of Nernst and Hall effects, especially in topologically non-trivial materials. They suggest further exploration into Heusler compounds, given their flexible electronic structures, which can lead to novel implementations harnessing Berry curvature to optimize the anomalous Nernst effect.
Future Directions
This research opens avenues for exploring other topological materials with potential high ANE values, thereby extending the utility of thermoelectric effects in magnetic applications. Future studies could focus on tuning the crystal and magnetic symmetries and manipulating band structures to exploit Berry curvature effects fully. There is also potential for discovering new classes of materials that exhibit similar high thermopower due to their topology rather than their magnetization alone.
In summary, this paper provides significant evidence that challenges conventional understanding of the anomalous Nernst effect in ferromagnetic systems, presenting Co2MnGa as a potential candidate for innovative thermoelectric applications through its high ANE, derived predominantly from its electronic topology and Berry curvature.