Thermoelectric Effects in Superconductor-Ferromagnetic Heterostructures

Superconductor-ferromagnetic (SF) structures have been shown to host a variety of interesting physics. In particular, thermoelectric effects have recently excited a wealth of research. While conventional superconductors are known to be poor thermoelectric materials, the combined effects of spin splitting and spin filtering creates an asymmetry in the density of states and generates large thermoelectric effects. These SF systems have been predicted to have a thermoelectric figure of merit (zT) of 1.8, far exceeding any other thermoelectric materials at cryogenic temperatures. Here, we directly measure the Seebeck coefficient (△V/△T) of an Al-Ni junction below 1 K. To accurately measure and generate a temperature gradient, we utilize focused-ion beam deposited platinum (FIB Pt) as local nanoscale resistive thermometers and an FIB Pt on-chip joule heater. We measure a local maximum in the Seebeck coefficient with respect to both temperature and magnetic field. These features are attributed to the competing effects of: 1) increased quasiparticle density and peak broadening with increasing temperatures and 2) increased Zeeman splitting and spin-scattering effects due to an increasing magnetic field.