Large Tunable Thermoelectric Effects in Superconducting Spin Valves with Commercially Available Materials

TL;DR

This paper demonstrates large, tunable thermoelectric effects in superconducting spin valves using commercially available materials, showing significant changes in thermoelectric output with magnetic configuration, and models these effects theoretically.

Contribution

It provides the first experimental and theoretical analysis of large thermoelectric effects in fully epitaxial F/S/F junctions with commercially available materials, enabling practical cryogenic thermoelectric devices.

Findings

Seebeck coefficients reach about 100 μV/K in parallel magnetic alignment.

Switching from parallel to antiparallel alignment reduces thermoelectric signal by over an order of magnitude.

Theoretical models qualitatively match experimental results, highlighting the role of spin-dependent electron-hole asymmetry.

Abstract

Recent studies have revealed magnetically controllable thermoelectric effects in superconductor/ferromagnet (S/F) structures. A tunable cryogenic thermoelectric generator needs not only a high conversion factor between electricity and heat, but also a large change in the thermoelectric output when switching the magnetic state of the device. However, the reported modifications in thermoelectric power are either minimal, involve superconductors with relatively low critical temperatures (below 1 K), or do not utilize commercially available spintronic materials. Here, we experimentally measure and numerically model thermoelectric effects in fully epitaxial F/S/F junctions based on commercially available, easily grown materials, as well as their dependence on the magnetic configuration of the F electrodes. We observe sizeable Seebeck coefficients for the parallel alignment of the ferromagnetic electrodes, reaching values of about $100$~$\mu$V/K. Importantly, we find a decrease of the thermoelectric signal of more than an order of magnitude when switching from a parallel to an antiparallel configuration, constituting a large thermoelectric spin-valve effect. Theoretical modeling based on a self-consistent non-equilibrium Keldysh-Usadel Green's function theory, combined with micromagnetic simulations, qualitatively reproduce the experimental findings. The thermoelectric effect is optimized when there is a large spin-dependent electron-hole asymmetry in the superconductor combined with spin-dependent transmission through the interfaces. These findings pave the way for the development of efficient and versatile cryogenic thermoelectric heat engines.