A new way to generate electricity from waste heat: Using an
antiferromagnet for solid devices

Date:
November 24, 2021
Source:
Max Planck Institute for Chemical Physics of Solids
Summary:
Researchers have discovered a giant thermoelectric effect
in an antiferromagnet. The study shows, surprisingly, that
antiferromagnets can have the same value of the anomalous Nernst
effect as conventional ferromagnets, but without any stray magnetic
fields that would otherwise affect surrounding devices. The newly
discovered recipe for generating large Nernst voltages opens a
new research direction for the development of highly efficient
thermoelectric devices.



FULL STORY ========================================================================== Forcing electrons to flow perpendicularly to a heat flow requires an
external magnetic field - this is known as the Nernst effect. In a
permanently magnetized material (a ferromagnet), an anomalous Nernst
effect (ANE) exists that can generate electricity from heat even without
a magnetic field. The anomalous Nernst effect scales with the magnetic
moment of the ferromagnet. An antiferromagnet, with two compensating
magnetic sublattices shows no external magnetic moment and no measurable external magnetic field and therefore should not exhibit any ANE. However,
we have recently understood that by the new concept of topology can be
applied to achieve large Nernst effects in magnets.

In particular, we have learned that the quantity known as the Berry
phase is related to the ANE and can greatly increase it. However, the
ANE in antiferromagnets is still largely unexplored, in part because the
ANE was not thought to exist. Remarkably, a joint research team from
the Max Planck Institute for Chemical Physics of Solids in Dresden,
Germany, together with collaborators at the Ohio State University
and the University of Cincinnati, has found a large anomalous Nernst
effect, larger than is known in almost all ferromagnets in YbMnBi2,
an antiferromagnet.


==========================================================================
The ANE that has been observed is likely a result of topology, the
high spin- orbit coupling, and the complex and not fully compensated
magnetic structure of YbMnBi2. The canted spin structure in YbMnBi2 breaks
time reversal symmetry and provides a non-zero Berry curvature. At the
same time, the large spin-orbit coupling of the heavy bismuth element
helps to produce a large extrinsic contribution. Based on this recipe,
a certain class of antiferromagnets with a non-collinear spin structure
and with large spin-orbit coupling can exhibit a large anomalous Nernst
effect. The researchers were surprised when they observed such a large ANE
in YbMnBi2, reaching 6 mV/K, which is a record value for antiferromagnets
and as high as those values previously observed for the best ferromagnets.

For practical applications, one could use this new phenomenon to make
simple energy converters: a transverse thermoelectric device where the
voltage is generated perpendicular to the heat flow. The device consists
of only one block of material. The commercially available thermoelectric generators based on the Seebeck effect are complex assemblies built
from small blocks of n- and p-type semiconductor materials. Unlike ferromagnets, which often suffer from low carrier mobility,
antiferromagnets can also exhibit higher mobilities and therefore show
better electrical conductivity. Together with low thermal conductivity,
an anomalous thermoelectric figure of merit (zT) is achieved in YbMnBi2,
which is an order of magnitude higher than that of all known ferromagnets.

"Although the ANE value is surprisingly large and the zT value is much
higher than that of ferromagnets, the overall thermoelectric performance
still needs to be improved for practical applications," says Yu Pan,
group leader in the department of Solid State Chemistry at the MPI
CPfS in Dresden. She continues, "Nevertheless, this study shows the
great potential of antiferromagnets for thermoelectric applications,
as they have much better performance than ferromagnets. We
believe our work is just the beginning of the discovery of
even more interesting thermoelectric materials in the future." ========================================================================== Story Source: Materials provided by Max_Planck_Institute_for_Chemical_Physics_of_Solids.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Yu Pan, Congcong Le, Bin He, Sarah J. Watzman, Mengyu Yao, Johannes
Gooth, Joseph P. Heremans, Yan Sun, Claudia Felser. Giant anomalous
Nernst signal in the antiferromagnet YbMnBi2. Nature Materials,
2021; DOI: 10.1038/s41563-021-01149-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211124154052.htm

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