Critical temperature oscillations in superconductor-ferromagnet trilayers.

摘要

The interplay between superconductivity (S) and ferromagnetism (F) results in many interesting physical effects. Strong suppression of superconductivity by magnetic impurities was discovered many years ago. Investigation of this phenomena evolved into the understanding of gapless superconductive state and subsequent discovery of the mid-gap localized electronic states. On the other hand, uniform material with both superconducting and magnetic interactions was predicted to develop a periodic Fulde-Ferrel-Larkin-Ovchinnikov state.; Advances in metal film growth lead to the emergence of hybrid superconductor-ferromagnet multilayer structures - the simplest intermediates between a single impurity in a superconducting matrix and uniform magnetic superconductors. The interplay between magnetism and superconductivity happens near the F/S boundaries but becomes important to their finite concentration. Even the simplest trilayer hybrid thin film structures exhibit novel properties. An example is the S/F/S Josephson junction, which can be put into a state where the phases of the adjacent superconductors differ by pi. Following the experimental observation of this "pi-state", the field of F/S multilayers experienced a surge of interest.; In the present thesis the F/S/F trilayers are investigated. Superconducting critical temperature of such structures was predicted to depend on the mutual orientation of the F-layer magnetizations. This result was recently confirmed experimentally in trilayers composed of sufficiently disordered films that permit analysis in the framework of Usadel equations. Experiments were in qualitative agreement with the theory, however the approximations employed in the literature to solve Usadel equations were clearly invalid in the parameter range of experimental interest. To get a quantitative description of the effect, a numerical method was developed to analyze the F/S/F structures. The method was generalized for the case non-symmetric structures with different F-layers and F/S boundary properties. Our numeric results show that experiments cannot be described by the theory of symmetric structures, but can be accounted for by introducing asymmetry in the F/S boundary transparencies. We then suggest how such asymmetry of boundary parameters can be independently tested. The dependence of critical temperature on the magnetic configuration is discussed in terms of superconductor spin-triplet order parameter arising at the F/S boundaries.