Anomalous Nernst effect beyond the magnetization scaling relation in the ferromagnetic Heusler compound Co2MnGa

Satya N Guin,Claudia Felser,Jonathan Noky,Sarah J Watzman,Walter Schnelle,Kaustuv Manna,Johannes Gooth,Yan Sun,Chenguang Fu,Nitesh Kumar,Chandra Shekhar

doi:10.1038/s41427-019-0116-z

Satya N Guin, Claudia Felser + Show 9 more

Open Access

https://doi.org/10.1038/s41427-019-0116-z

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Abstract

Applying a temperature gradient in a magnetic material generates a voltage that is perpendicular to both the heat flow and the magnetization. This phenomenon is the anomalous Nernst effect (ANE), which was long thought to be proportional to the value of the magnetization. However, more generally, the ANE has been predicted to originate from a net Berry curvature of all bands near the Fermi level (EF). Subsequently, a large anomalous Nernst thermopower ({boldsymbol{S}}_{{boldsymbol{yx}}}^{boldsymbol{A}}) has recently been observed in topological materials with no net magnetization but a large net Berry curvature [Ωn(k)] around EF. These experiments clearly fall outside the scope of the conventional magnetization model of the ANE, but a significant question remains. Can the value of the ANE in topological ferromagnets exceed the highest values observed in conventional ferromagnets? Here, we report a remarkably high {boldsymbol{S}}_{{boldsymbol{yx}}}^{boldsymbol{A}}-value of ~6.0 µV K−1 in the ferromagnetic topological Heusler compound Co2MnGa at room temperature, which is approximately seven times larger than any anomalous Nernst thermopower value ever reported for a conventional ferromagnet. Combined electrical, thermoelectric, and first-principles calculations reveal that this high-value of the ANE arises from a large net Berry curvature near the Fermi level associated with nodal lines and Weyl points.

Highlights

1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Introduction Conventional thermoelectric devices are based on the Seebeck effect in a two-terminal geometry, where two electronic reservoirs at different temperatures result in an electric voltage that arises in the direction of the imposed temperature gradient[1,2]
Prior to the transport experiments, the as-grown crystals were characterized by Laue X-ray diffraction (XRD), where sharp spots in the pattern indicate high crystal quality
In the ferromagnetic topological Heusler compound Co2MnGa, which is ~7 times larger than any value reported for conventional ferromagnets to date in the literature

Summary

Introduction

Conventional thermoelectric devices are based on the Seebeck effect in a two-terminal geometry, where two electronic reservoirs at different temperatures result in an electric voltage that arises in the direction of the imposed temperature gradient[1,2]. The problem is achieving good thermal insulation while maintaining good electrical conduction To circumvent this issue, multiterminal thermoelectric devices have recently attracted increasing attention[3,4,5,6,7,8,9,10,11,12,13] because they allow for the spatial separation of the heat reservoir from the electric circuitry. In principle, simpler to integrate technologically than conventional thermoelectrics because there is no need for both p- and n-type materials

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