  The Seebeck and Peltier effects have been widely studied and used in various
thermoelectric technologies, including thermal energy harvesting and
solid-state heat pumps. However, basic and applied studies on the Thomson
effect, another fundamental thermoelectric effect in conductors, are limited
despite the fact that the Thomson effect allows electronic cooling through the
application of a temperature gradient bias rather than the construction of
junction structures. In this article, we report the observation of a giant
Thomson effect that appears owing to magnetic phase transitions. The Thomson
coefficient of FeRh-based alloys reaches large values approaching $-$1,000
$\mu$VK$^{-1}$ around room temperature because of the steep temperature
dependence of the Seebeck coefficient associated with the
antiferromagnetic-ferromagnetic phase transition. The Thomson coefficient is
several orders of magnitude larger than the Seebeck coefficient of the alloys.
Using the active thermography technique, we demonstrate that the Thomson
cooling can be much larger than Joule heating in the same material even in a
nearly steady state. The operation temperature of the giant Thomson effect in
the FeRh-based alloys can be tuned over a wide range by applying an external
magnetic field or by slightly changing the composition. Our findings provide a
new direction in the materials science of thermoelectrics and pave the way for
thermal management applications using the Thomson effect.
