  Transverse thermoelectric devices produce electric fields perpendicular to an
incident heat flux. Classically, this process is driven by the Nernst effect in
bulk solids, wherein a magnetic field generates a Lorentz force on thermally
excited electrons. The spin Seebeck effect (SSE) also produces
magnetization-dependent transverse electric fields. SSE is traditionally
observed in thin metallic films deposited on electrically insulating
ferromagnets, but the films' high resistance limits thermoelectric conversion
efficiency. Combining Nernst and SSE in bulk materials would enable devices
with simultaneously large transverse thermopower and low electrical resistance.
Here we demonstrate experimentally this is possible in composites of conducting
ferromagnets (Ni or MnBi) containing metallic nanoparticles with strong
spin-orbit interactions (Pt or Au). These materials display positive shifts in
transverse thermopower attributable to inverse spin Hall electric fields in the
nanoparticles. This more than doubles the power output of the Ni-Pt materials,
establishing proof-of-principle that SSE persists in bulk nanocomposites.
