🔬 materials science

Room-Temperature Polarity Control of the Anomalous Nernst Effect in a High-Magnetic-Anisotropy Topological Nodal-Line MnAlGe

This study demonstrates room-temperature polarity control of the anomalous Nernst effect in high-magnetic-anisotropy MnAlGe thin films through Al/Ge compositional tuning, enabling enhanced thermoelectric output in practical devices by modulating intrinsic Berry curvature via sublattice-selective carrier doping.

Original authors: Nanhe Kumar Gupta, Keisuke Masudaa, Masaaki Kakoki, Benugopal Bairagya, Satoki Tazawaa, Weinan Zhoua, Hirofumi Sutoa, Ryo Toyama, Akio Kimura, Yuya Sakuraba

Published 2026-08-20

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Nanhe Kumar Gupta, Keisuke Masudaa, Masaaki Kakoki, Benugopal Bairagya, Satoki Tazawaa, Weinan Zhoua, Hirofumi Sutoa, Ryo Toyama, Akio Kimura, Yuya Sakuraba

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 world where the waste heat from your laptop, your car engine, or even your body could be captured and turned directly into electricity. This is the promise of thermoelectric technology, a field dedicated to converting temperature differences into electrical power. For decades, scientists have focused on a method called the Seebeck effect, where heat flows through a material to create a voltage. However, a newer, more versatile approach has emerged: the anomalous Nernst effect. Unlike its predecessor, which requires a long, straight path for heat to travel, this effect works in a flat, two-dimensional sheet. When heat moves across a magnetic material, it generates an electric voltage sideways, perpendicular to the heat flow. This geometry is far easier to build into compact devices, making it ideal for sensors and small power generators. Yet, a major hurdle has stood in the way of practical use: finding a material that is both magnetically stable at room temperature and capable of producing this voltage with a specific direction. Ideally, engineers would want to combine materials that produce positive voltage with those that produce negative voltage to stack them together, much like batteries in a flashlight, to boost the total power. But finding a single material that can be tuned to switch between these two directions while remaining stable has been a elusive goal.

A team of researchers has now solved this puzzle by working with a specific crystal called MnAlGe, a layered magnetic material that acts as a topological nodal-line semimetal. In simple terms, this means the material's electrons move in a way that is protected by the symmetry of its atomic structure, creating unique pathways for electricity. The researchers discovered that by carefully adjusting the ratio of aluminum to germanium atoms within this crystal, they could shift the energy level of the electrons without breaking the material's magnetic stability. This adjustment acts like a precise dial, moving the electronic structure just enough to flip the direction of the anomalous Nernst voltage from positive to negative, all while keeping the material's strong magnetic properties intact at room temperature.

To achieve this, the team created thin films of the material where the composition varied continuously from aluminum-rich to germanium-rich. They found that in the middle range of this mix, the material maintained a single, stable crystal structure known as the C38 phase. By using advanced spectroscopy, they confirmed that adding more germanium effectively added electrons to the system, shifting the energy landscape of the electrons. This shift was not random; it moved the electrons across a specific feature in the material's electronic structure, a point where the energy bands cross in a way that generates a strong magnetic response. As the researchers tuned the composition, they observed the voltage signal flip its sign. At one end of the spectrum, the material generated a positive voltage; at the other, a negative one. Crucially, this reversal happened without destroying the material's high magnetic anisotropy, a property that ensures the magnet stays aligned in a specific direction even without an external magnetic field, which is essential for reliable device operation.

The study went beyond just observing the effect in a lab setting; it demonstrated how this discovery could be used in a real device. The researchers built a small, meander-shaped sensor by alternating strips of the material tuned to produce positive voltage with strips tuned to produce negative voltage. When they applied a temperature difference across this device, the voltages from the different strips added up, creating a much stronger total signal than a single strip could produce on its own. This proved that the polarity control was robust enough to work even when the material was not a perfect single crystal, but rather a polycrystalline film, which is easier to manufacture for mass production. The results suggest that by engineering the atomic composition of such topological magnets, scientists can now design thermoelectric devices that are more efficient and easier to integrate into everyday technology.

While the magnitude of the voltage generated was not as large as some theoretical models predicted, the researchers identified the likely cause: the thermal agitation of the magnetic spins within the material, which tends to scatter the electrons and reduce the signal. This insight points the way forward, suggesting that future improvements will come from finding ways to stabilize these magnetic spins at higher temperatures. The work establishes a clear path for creating a new generation of transverse thermoelectric devices, where a single material platform can be tuned to provide the exact electrical response needed, eliminating the need to combine different, incompatible materials. By mastering the control of electron energy levels through simple chemical substitution, the team has opened a door to more powerful and versatile ways of harvesting waste heat.

Technical Summary: Room-Temperature Polarity Control of the Anomalous Nernst Effect in High-Magnetic-Anisotropy Topological Nodal-Line MnAlGe

Problem Statement
The efficient conversion of waste heat into electricity via the anomalous Nernst effect (ANE) offers a promising pathway for thermoelectric applications, particularly due to the simple planar geometry of ANE-based devices compared to conventional Seebeck-effect modules. However, a critical bottleneck remains: the lack of materials that simultaneously exhibit large anomalous Nernst coefficients ( $S_{ANE}$ ), high magnetic anisotropy ( $K_u$ ), and controllable polarity (bipolarity) at room temperature (RT). While some materials show large $S_{ANE}$ and others show high $K_u$ , achieving both in a single system is challenging. Furthermore, realizing controllable polarity reversal in topological ferromagnets at RT is difficult because chemical substitution often distorts the underlying topological band structure, and many topological magnets (e.g., Co $_3$ Sn $_2$ S $_2$ ) only exhibit finite ANE at low temperatures due to low Curie temperatures. There is a specific need for a single-phase material platform where the Fermi level ( $E_F$ ) can be tuned to reverse the ANE sign without compromising the robust magnetic anisotropy required for stable device operation.

Methodology
The authors investigated the quasi-two-dimensional topological ferromagnet MnAlGe, specifically the C38-type Mn(Al $_{1-x}$ Ge $_x$ ) $_2$ system. The study employed a combinatorial approach to fabricate composition-spread epitaxial thin films on MgO(001) substrates, covering the range $0 \le x \le 1$ .

Structural and Electronic Tuning: The strategy relied on sublattice-selective carrier doping. By varying the Al/Ge ratio, the authors aimed to shift the Fermi level while preserving the Mn-derived magnetic layers and the topological nodal-line features.
Characterization: Structural integrity was verified via X-ray diffraction (XRD). Electronic structure changes were probed using Hard X-ray Photoemission Spectroscopy (HAXPES) to track core-level binding energy shifts. Transport properties, including Anomalous Hall Effect (AHE) and ANE, were measured across the composition range at room temperature.
Theoretical Support: First-principles calculations (DFT and KKR methods) were performed to model the spin-resolved density of states (DOS), band structures, and intrinsic anomalous transport coefficients ( $\sigma_{xy}^{int}$ and $\alpha_{xy}^{int}$ ) as a function of chemical potential.
Device Fabrication: To demonstrate practical feasibility, a meander-structured thermopile was fabricated on SiO $_2$ substrates using polycrystalline MnAlGe films with positive ( $x=0.39$ ) and negative ( $x=0.54$ ) $S_{ANE}$ polarities connected in series.

Key Results

Structural and Magnetic Stability: The C38-type single phase was stabilized in the composition range $0.35 \le x \le 0.70$ . Crucially, the material maintained robust perpendicular magnetic anisotropy ( $K_u \approx 0.40 - 0.55$ MJm $^{-3}$ ) and high effective in-plane saturation fields across this entire range, indicating that the Mn-derived electronic structure responsible for magnetism remained largely unperturbed by Al/Ge substitution.
Fermi Level Tuning: HAXPES measurements confirmed an upward shift in the chemical potential with increasing Ge concentration, consistent with electron doping. Theoretical calculations indicated a total Fermi level shift of approximately 267 meV between $x=0.3$ and $x=0.7$ .
ANE Polarity Reversal: The most significant finding was the room-temperature reversal of the ANE sign. $S_{ANE}$ changed from positive ( $+0.09$ $\mu$ V K $^{-1}$ at $x \approx 0.41$ ) to negative ($-0.63$ $\mu$ V K $^{-1}$ at $x \approx 0.46$ ). This reversal occurred without a sign change in the Seebeck coefficient ( $S_{SE}$ ), which remained positive (hole-dominated) throughout, confirming that the polarity switch was driven by the anomalous Hall conductivity term ( $\alpha_{xy}^A$ ).
Berry Curvature Origin: The sign reversal was attributed to the shifting of the Fermi level across Berry curvature hotspots associated with the topological nodal lines in the minority-spin channel. While the experimental magnitudes of $\sigma_{xy}^A$ and $\alpha_{xy}^A$ were suppressed by nearly an order of magnitude compared to theoretical predictions (likely due to finite-temperature spin fluctuations and electron-magnon scattering), the qualitative trends and the position of the sign reversal aligned well with the calculated intrinsic Berry curvature dependence on chemical potential.
Device Performance: A thermopile device combining alternating positive and negative polarity segments demonstrated a systematic increase in output voltage with the number of pairs, validating the concept of monolithic ANE thermopiles using a single material system.

Significance and Claims
The paper claims to demonstrate a viable strategy for engineering room-temperature transverse thermoelectric devices by decoupling the control of transport polarity from magnetic anisotropy. By utilizing sublattice-selective carrier doping in a layered topological magnet, the authors achieved controllable ANE polarity reversal within a single material system (MnAlGe) while preserving high $K_u$ . This approach eliminates the need to integrate disparate positive and negative ANE materials, simplifying device fabrication. The work establishes that tuning the Fermi level relative to the nodal-line electronic structure is an effective route to modulate Berry curvature-driven transport. The authors conclude that while spin fluctuations currently limit the magnitude of the effect, this platform provides a foundation for next-generation thermoelectric and sensing applications, with future improvements expected from suppressing these fluctuations to better realize the theoretically predicted performance.

More like this