Spin-polarized transport in superconducting and ferromagnetic nanostructures

In this thesis we investigate how superconductivity affects the spin-polarized transport of phase-coherent, magnetic nanostructures such as magnetic multilayers and ferromagnet/superconductor interfaces. Within the multiple scattering approach we calculate the conductance of such structures using an efficient recursive Green's function technique based on the tight-binding model. The first main result is the finding that a superconducting contact strongly suppresses the Giant Magnetoresistance (GMR) of magnetic multilayers. The extent of this suppression is found to depend on details of band structures and on the amount of disorder present within the layers. This apparent contradiction with the well-established experimental evidence of 'giant' values of GMR ratio can be removed once the possibility of spin-mixing processes is taken into account. We prove this by allowing spin-flip scattering to take place either in the multilayer itself or in the superconducting lead. We demonstrate that, in both cases spin-mixing greatly enhances the GMR ratio, which reaches 'giant' values exceeding 100 %. The case where spin-orbit coupling is present in the superconductor seems to be the one more likely to represent experiments involving [Co/Co] or [Co/Ag] multilayers, in which the spin-diffusion length is believed to be very large. GMR in multilayers with superconducting contacts is therefore possible only if spin-mixing processes are present, in contrast with the case of normal-metallic leads. The second main result of this thesis concerns a method recently proposed for determining the degree of polarization of the current in a ferromagnet, which makes use of ferromagnet/superconductor nanojunctions. By comparing the differential conductance curves of an 'exact' spd-model with the corresponding curves of a simple single-band model, we find an extremely large error in the determination of such polarization and conclude that such a method is unreliable. (author)