Nonequilibrium and quantum transport phenomena in mesoscopic ferromagnet /superconductor heterostructures

Although the past twenty years have seen much progress in the understanding of mesoscopic electronic transport, many novel devices remain unexplored. In contrast to nonmagnetic, normal mesoscopic structures, our understanding of similarly scaled ferromagnetic devices is minimal. This is in part due to the experimental difficulties of analyzing transport in the presence of nonhomogeneous magnetic fields since ferromagnetic structures may support magnetic structure on length scales comparable to the submicron dimension of many proposed devices. Such difficulties must be eliminated when assembling devices in which such ferromagnetic elements may be important elements in more complicated heterostructures (e.g., ferromagnet/normal, ferromagnet/superconductor, etc.). In this sense it is important to understand the electronic transport properties of the single submicron ferromagnetic elements that may comprise these more complicated heterostructures.;In this dissertation we examine the electronic transport properties of single submicron ferromagnets and demonstrate how macroscopic ferromagnetic transport issues translate to submicron dimension devices. Our measurements indicate that many of the considerations taken in understanding mesoscopic nonmagnetic normal metals (e.g., quantum interference and probe switching symmetries) are also applicable to submicron ferromagnetic metals with additional complications due to the inherent magnetic structure. These complications manifest themselves through anisotropic contributions to the resistivity tensor which are a function of magnetization direction within a ferromagnetic structure and are much more pronounced when the dimensions of the element are reduced to the micrometer scale.;Finally, we apply this knowledge of single submicron ferromagnetic transport to investigate the possibility of a superconducting proximity effect in ferromagnet/superconductor heterostructures. It is found that such an effect is minimal in our systems yet the resistance of the interface between the ferromagnet and superconductor can display strong dependences on the temperature, magnetic field and energy bias. These results are discussed within the framework of the Blonder-Tinkham-Klapwijk model of transport in a normal/superconductor interface with modifications for the spin-polarized current of the ferromagnetic metal.