Proximity-Coupled Normal Metals and Ferromagnets

Northwestern University, Evanston IL

Investigators

Abstract

This individual investigator award supports a project that will investigate the electrical and magnetic properties of metallic heterostructures fabricated from superconducting (S), normal metal (N), and ferromagnetic (F) elements. The goal is to explore the nature of the interaction between ferromagnetism and superconductivity in FS structures, and to investigate the influence of long-range phase coherence on the thermal transport in NS heterostructures. Electrical transport properties of FS heterostructures, will be used to attempt to verify the existence of long-range superconducting correlations in a diffusive ferromagnet placed in contact with a superconductor. In addition, effects of finite spin polarization in the ferromagnet on the transport properties of FS structures will be studied. Using a novel local thermometry technique, effects in the thermopower of NS devices, as well the thermal conductance of NS structures, will be explored. Samples will be fabricated by electron-beam lithography and measured at millikelvin temperatures using various low temperature cryostats, including a dilution refrigerator and a 3He refrigerator. The students and post-docs trained on this project will gain experience with both microfabrication and low temperature techniques. Thus becoming well-prepared for careers in industry, academia or government laboratories. Superconductors are materials that show a number of unusual electrical and magnetic properties at low temperatures, the best known property being their ability to carry an electrical current without resistance. These properties form the basis for a number of useful applications, such as very high field magnets. When a superconductor is placed in good contact with a conventional normal metal such as copper or gold, or a ferromagnet like iron or nickel, the interaction between the two different elements results in new effects with potential device applications. The goal of this project is to investigate the electrical and thermal transport properties of normal metals and ferromagnets placed in close proximity to a superconductor. Electrical properties of structures incorporating ferromagnets and superconductors will be studied to investigate the interplay between magnetism and superconductivity in very small devices. In addition, heat transport in a normal metal placed in contact with a superconductor will be explored. These measurements will help us gain further insight into the interaction between superconductors, ferromagnets and normal metals on very small size scales. The samples for these experiments will be fabricated using advanced electron-beam lithography techniques, and measured using sophisticated techniques at temperatures near absolute zero. Post-doctoral associates and graduate students trained on this project will be exposed to a number of experimental techniques which will be useful in future careers in either industry or academia.