Spin Seebeck effect in bulk composite materials

The spin-Seebeck effect (SSE) is a totally new thermal-to-electrical energy conversion technology akin to Nernst-Ettingshausen thermoelectrics. As a thin-film effect, SSE cannot generate much power. Here we demonstrate experimentally a practical bulk geometry for SSE capable of generating higher power in composites comprised of ferromagnetic (FM) conductors (Fe${_3}$O${_4}$ or MnBi) containing normal metal (NM) nanoparticles (Au or Pt) with strong spin-orbit interactions. Pure Fe${_3}$O${_4}$ and MnBi display transverse (Nernst) thermopowers with comparable magnitude but opposite polarity, enabling us to isolate the SSE contribution due to the NMs. Several FM/NM combinations give consistent results: All display positive shifts in their transverse thermopower attributable to an inverse spin Hall electric field generated within the NM particles. This results in the largest transverse thermopowers observed in any ferromagnetic material, along with power factors an order of magnitude larger than those of multilayer SSE devices, establishing proof-of-principle for this new design of transverse thermoelectrics.