β-Tantalum, a better candidate for spin-to-charge conversion

Development of thermoelectric energy harvesting devices has hit a stumbling block due to the intrinsically linked electrical and thermal conductivities of materials. However, this field can still be improved by employing devices that take advantage of spin-based effects. A temperature gradient can be converted to a spin-polarized current in a ferrimagnetic insulator by the spin Seebeck effect (SSE), and that spin current can be converted to an electrical voltage in a heavy metal by the Inverse spin Hall effect (ISHE). Thus, the thermal energy capture and charge production steps can be separated into two distinct regions of the thermoelectric device, allowing separate tuning of electrical and thermal conductivities. The second step of this process, spin current to electrical voltage conversion, is controlled by the strength and sign of ISHE in the metal, and platinum has become the standard for this purpose. However, here we report a better candidate, β-Tantalum, which shows a spin Seebeck voltage approximately 4 times higher than that of Pt at room temperature. The temperature dependence of spin Seebeck in YIG/β-Ta also closely follows that of YIG/Pt, consistent with magnon spin current theory. The sign of the spin Seebeck voltage in Ta found to be opposite to that of Pt, making the two materials highly complementary for fabricating spintronics-based thermoelectric modules for practical applications.