  Permanent magnets are used in various products and essential for human
society. If omnipresent permanent magnets could directly convert heat into
electricity, they would lead to innovative energy harvesting and thermal
management technologies. However, achieving such "multifunctionality" has been
difficult because of the poor thermoelectric performance of conventional
magnets. Here, we develop a multifunctional composite magnet that enables giant
transverse thermoelectric conversion. The developed composite material,
comprising alternately and obliquely stacked
SmCo$_{5}$/Bi$_{0.2}$Sb$_{1.8}$Te$_{3}$ multilayers, exhibits large remanent
magnetization and coercivity as well as an excellent figure of merit of 0.22
for transverse thermoelectric conversion at room temperature owing to the
optimized anisotropic composite structure and extremely low interfacial
electrical and thermal resistances. While having versatile transverse geometry,
the thermopile module based on these composite materials generates 204 mW at a
temperature difference of 152 K. The corresponding power density normalized by
heat transfer area and temperature gradient is not only record-high among the
transverse thermoelectric modules but also comparable to that of the
longitudinal thermoelectric modules including commercial products based on the
Seebeck effect. The additive magnetic functionality enables efficient thermal
energy harvesting and integrates thermoelectric conversion capabilities
wherever permanent magnets are currently used.
