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John O | January 2018

New research uses magnet to generate electricity from heat


Researchers from the University of Tokyo (Japan) and from RIKEN, a research institute located in Wako, Japan, have discovered the anomalous Nernst effect in an antiferromagnet for the first time, which demonstrates potential future exploration of thermoelectric materials.

 


In the anomalous Nernst effect, the electromotive force is generated perpendicular to both the magnetization and thermal gradient. (Takahiro Tomita)

 

The anomalous Nernst effect, according to a report on the University of Tokyo website, is spontaneously generated voltage that is created when a magnetic conductor is exposed to a temperature gradient, regardless of the absence of a magnetic field.

 

“Since its discovery about a century ago, this effect was long assumed as being proportional to the magnetization of the material, thus appearing only in ferromagnets, which have strong magnetization, and was not expected to occur in materials with zero magnetization like antiferromagnets,” the article explained. “Strikingly, the joint research team found a large anomalous Nernst effect in manganese-tin alloy Mn3Sn, an antiferromagnetic metal.”

 

The anomalous Nernst effect in the Mn­­3Sn was more than 100 times larger than researchers had estimated from the material’s magnetization.

 

“Experiments on two crystals with different compositions were quantitatively consistent with calculations performed by the team at RIKEN, suggesting the presence of particles called Weyl fermions as the source of the large fictitious magnetic field,” the article continued. “If the presence of Weyl fermions in Mn3Sn is verified through other experiments in the future, it would make the alloy the long-sought magnetic Weyl metal.”

 

The research was recently published in Nature Physics. The abstract read:

 

“A temperature gradient in a ferromagnetic conductor can generate a transverse voltage drop perpendicular to both the magnetization and heat current. This anomalous Nernst effect has been considered to be proportional to the magnetization, and thus observed only in ferromagnets. Theoretically, however, the anomalous Nernst effect provides a measure of the Berry curvature at the Fermi energy, and so may be seen in magnets with no net magnetization.

 

“Here, we report the observation of a large anomalous Nernst effect in the chiral antiferromagnet Mn 3Sn. Despite a very small magnetization ∼0.002 μB per Mn, the transverse Seebeck coefficient at zero magnetic field is ∼0.35 μV K−1 at room temperature and reaches ∼0.6 μV K−1 at 200 K, which is comparable to the maximum value known for a ferromagnetic metal.

 

“Our first-principles calculations reveal that this arises from a significantly enhanced Berry curvature associated with Weyl points near the Fermi energy.

 

“As this effect is geometrically convenient for thermoelectric power generation—it enables a lateral configuration of modules to cover a heat source—these observations suggest that a new class of thermoelectric materials could be developed that exploit topological magnets to fabricate efficient, densely integrated thermopiles.”

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