Thermal and thermal electric measurements in superconductor-ferromagnetic heterostructures

Abstract

Electrical and spin transport measurements at cryogenic temperatures have powered new arenas of research and applications. However, exploration into thermal effects at cryogenic temperatures has only just begun. Recent thermal transport measurements at low temperatures have led to interesting novel physics in the fields of quantized heat flows, quantum thermodynamics, thermal Josephson effects, quantum heat engines, and thermoelectric materials. In particular, superconductor-ferromagnetic (S-F) systems have recently been proposed as effective thermoelectric devices. While conventional superconductors are known to be poor thermoelectric materials, the combined effects of spin splitting and spin filtering provided by an external magnetic field and magnetic material are predicted to create an asymmetry in the superconducting density of states and generate thermoelectric effects. These S-F systems have been predicted to have a thermoelectric figure of merit (zT) of 1.8, far exceeding any other thermoelectric materials at cryogenic temperatures. If these predictions hold true, S-F thermoelectrics could have applications in nanoscale cooling and as radiation or single photon detectors.

In this thesis, we have directly measured the Seebeck voltage in S-F structures down to 8 mK and developed the hardware and techniques necessary to make such a measurement at cryogenic temperatures. First, we review the background and theoretical underpinnings which inform and motivate the study of S-F systems as thermoelectric materials. Second, we report our development of methods for the electrodeposition of superconductor and ferromagnetic nanowires, including the first fabrication of niobium nanowires via electrodeposition. Third, as methods of thermal measurements at cryogenic temperatures are not well established, we have developed an experimental platform for low dimensional temperature measurements. Our experimental advances in this area include the development of an on-chip cryogenic thermometer that is sensitive down to 8 mK and can be placed on the chip with 100s of nm precision in a lithography free process. We also have quantified local, on-chip heating using AC and DC power as a function of distance, power, frequency, and sample configurations. Finally, we directly measure the Seebeck coefficient of Al, Ni, and an Al-Ni junction below 1 K.