Large magnon-induced anomalous Nernst conductivity in single-crystal MnBi

Bin He,Cüneyt Şahin,Michael E Flatté,Claudia Felser,Joseph P Heremans,Brian C Sales,Stephen R Boona,Yu Pan

doi:10.1016/j.joule.2021.08.007

Abstract

SummaryThermoelectric modules are a promising approach to energy harvesting and efficient cooling. In addition to the longitudinal Seebeck effect, transverse devices utilizing the anomalous Nernst effect (ANE) have recently attracted interest. For high conversion efficiency, it is required that the material have a large ANE thermoelectric power and low electrical resistance, which lead to the conductivity of the ANE. ANE is usually explained in terms of intrinsic contributions from Berry curvature. Our observations suggest that extrinsic contributions also matter. Studying single-crystal manganese-bismuth (MnBi), we find a high ANE thermopower (∼10 μV/K) under 0.6 T at 80 K, and a transverse thermoelectric conductivity of over 40 A/Km. With insight from theoretical calculations, we attribute this large ANE predominantly to a new advective magnon contribution arising from magnon-electron spin-angular momentum transfer. We propose that introducing a large spin-orbit coupling into ferromagnetic materials may enhance the ANE through the extrinsic contribution of magnons.

Highlights

With its ability to convert heat directly into electricity and vice versa, thermoelectricity plays a significant role in energy harvesting as well as static cooling applications.[1,2,3,4] Since over 90% of the energy humanity uses today comes from thermal processes, even a very slight improvement in efficiency translates into a large amount of energy saved, for example, by recovering the heat wasted in the exhaust of internal-combustion engines
Most thermoelectric research has focused on the longitudinal Seebeck effect, in which the temperature gradient is parallel to the voltage generated
Of a magnetic field, magnon spin flux can transfer spin-angular momenta ( S ) to the itinerant electrons during magnon-electron scattering in the bulk of the FM itself, thereby dynamically spin-polarizing the itinerant electrons beyond what is expected from the thermodynamic equilibrium band structure

Summary

Introduction

With its ability to convert heat directly into electricity and vice versa, thermoelectricity plays a significant role in energy harvesting as well as static cooling applications.[1,2,3,4] Since over 90% of the energy humanity uses today comes from thermal processes, even a very slight improvement in efficiency translates into a large amount of energy saved, for example, by recovering the heat wasted in the exhaust of internal-combustion engines. Most thermoelectric research has focused on the longitudinal Seebeck effect, in which the temperature gradient is parallel to the voltage generated In this case, the thermopiles have to be connected in series to generate a high voltage. Transverse thermoelectric devices, in which the voltage generated is perpendicular to the applied temperature gradient, can avoid these disadvantages This configuration vastly simplifies the fabrication procedure by making it possible to apply the electrical contact only to a colder side of the thermoelectric material. It reduces the thermal resistance, where a designated voltage can be generated by making the device longer or thicker. The low contact resistance losses of transverse devices compared with Peltier coolers

Results

Discussion

Conclusion