Abstract
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 also produces magnetization-dependent transverse electric fields. It is traditionally observed in thin metallic films deposited on electrically insulating ferromagnets, but the films' high resistance limits thermoelectric conversion efficiency. Combining Nernst and spin Seebeck effect in bulk materials would enable devices with simultaneously large transverse thermopower and low electrical resistance. Here we demonstrate experimentally that 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 the spin Seebeck effect persists in bulk nanocomposites.
Highlights
Transverse thermoelectric devices produce electric fields perpendicular to an incident heat flux
A temperature gradient rxT is applied to a ferromagnetic insulator (FMI) possessing magnetization Mz set by applied field Hz. rxT generates a magnon flux that carries heat[8] and spin along x
At the surface these magnons impinge on an adjacent normal metal (NM) film whose thickness is comparable to its spin diffusion length ls, typically 1–10 nm
Summary
Transverse thermoelectric devices produce electric fields perpendicular to an incident heat flux. Together with low electrical resistivity r, the transverse power factor PFxyz 1⁄4 Sx2yz/r in the composites is increased 2–5 times compared with the reference sample, and the transverse thermoelectric figure of merit zTxyz 1⁄4 T Á PFxyz/k (k being the thermal conductivity and T the absolute temperature) increases by an order of magnitude This result establishes that can SSE be observed in bulk nanocomposites, but it can be utilized to produce significantly more electrical power from the same temperature gradient relative to single-phase magnetic materials
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