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Large magnon-induced anomalous Nernst conductivity in single-crystal MnBi
Journal article   Open access   Peer reviewed

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

Bin He, Cüneyt Şahin, Stephen R. Boona, Brian C. Sales, Yu Pan, Claudia Felser, Michael E. Flatté and Joseph P. Heremans
Joule, Vol.5(11), pp.3057-3067
11/17/2021
DOI: 10.1016/j.joule.2021.08.007
PMCID: PMC8604385
PMID: 34841198
url
https://doi.org/10.1016/j.joule.2021.08.007View
Published (Version of record) Open Access

Abstract

Thermoelectric 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. [Display omitted] •MnBi shows a remarkable anomalous Nernst thermopower of 10 μV/K at 80 K•Anomalous Nernst conductivity reaches 40 A/Km, the highest value reported•Magnon-electron drag is likely the source of the large anomalous Nernst effect Thermoelectricity offers the prospect of generating electric power from heat and controlling temperatures in a directed manner. The oldest thermoelectric devices, dating to the 1800s, were based on Seebeck’s observation of electric fields forming in the direction of a temperature gradient. More recent devices have been based on the transverse thermoelectric effect, in which the voltage and thermal gradients are perpendicular. In the anomalous Nernst effect, a magnetic field normal to a thermal gradient produces a voltage drop transverse to both. This Hall-like effect is poorly understood but has great potential for applications in thermoelectric energy harvesting and cooling. We demonstrate a startlingly large transverse thermoelectric response in MnBi crystals, which traditional approaches cannot fully explain. We believe that it is due to interactions between magnons and electrons. This mechanism may operate in other materials, allowing major advances in thermoelectrics. Transverse thermoelectrics utilizing the anomalous Nernst effect (ANE) can be a novel approach to energy sustainability. We investigate the thermoelectric transport properties in ferromagnetic MnBi and observe one of the largest ANEs ever reported. We attribute this giant ANE to the coexistence of ferromagnetism and the heavy Bi atom. Our discovery proposes an alternative recipe to generate large ANE, which introduce a large spin-orbit coupling to ferromagnetic systems.
 anomalous Nernst effect ferromagnetic bismuthide magnons thermoelectricity

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