Proximity effect in ferromagnet/superconductor hybrid systems.

摘要

The bulk of this thesis deals with proximity effects resulting from placing a ferromagnet (F) in contact with a superconductor (S). The hybrid structure combines conventional superconductivity, which brings about the coupling of electrons with opposite spins, with ferromagnetism, which favors the parallel alignment of electron spins. The superconducting correlations penetrate inside the ferromagnet over a length scale that depends upon its strength. In addition to its decay, the propagation of the order parameter is spatially modulated by the energy difference between the minority and majority spin bands.;There are several experimental consequences arising from the competing symmetries, including the reduction in the superconducting transition temperature, TC. The degree of suppression is directly related to the material thicknesses and the quality of the interface that they form. We have performed TC measurements on several F/S/F trilayer structures with strong ferromagnets. The results yielded slight differences in TC based on whether the magnetization of the ferromagnet are oriented parallel (P) or anti-parallel (AP), with the latter exhibiting a slightly higher TC.;The sensitivity of a superconductor to the magnetic orientation of its contacting ferromagnet lies at the heart of a theoretical prediction for the existence of a triplet pairing mechanism in a conventional superconductor. Surrounding a superconductor with ferromagnets having non-collinear magnetization is theoretically shown to induce triplet state correlations that extend out to distances much longer than those reached by the conventional singlet state. Using strong ferromagnets, our F/S/F measurements have laid the foundation for systems that may exhibit triplet pairing.