Thermoelectric Effects in Superconductor-Ferromagnetic Heterostructures

Superconductor-ferromagnetic (S-F) heterostructures have been predicted to show large thermoelectric effects at cryogenic temperatures with a figure of merit of ~1.8. Confirmation of these theoretical predictions requires S-F heterostructures, controlled temperature gradients, cryogenic thermometry, thermal conductivity measurements, and electrical transport measurements. To this end, we have developed an experimental platform for the measurements of thermal and electrical transport in nanoscale devices at cryogenic temperatures. S-F heterostructures have been fabricated using lithography, evaporation, and sputtering, providing precise control over interfaces, and a wide choice of materials and geometries. Heaters and resistive thermometers, patterned via focused ion beam assisted platinum deposition, have been calibrated and optimized for thermal transport measurements at cryogenic temperatures. Finally, the Seebeck coefficient of S-F heterostructures has been measured in the presence of a magnetic field. This measurement sets the stage for systematic examination of the effects of dimensionality, ferromagnet polarization, and interface quality.