Estudo do efeito de proximidade em interfaces supercondutor/ferromagnética por espectroscopia de absorção de raios X / Vanessa Carreño Diaz.

Abstract

The study of thin films systems with superconducting materials has been a great interest in the research of new materials, since in addition to having wide technological applicability, they offer fascinating possibilities to observe new phenomena in superconductivity. This last feature is the main motivation for this dissertation work, which deals with the study of the proximity effect that occurs when a superconducting material is in contact with a ferromagnetic material in the form films. As a superconducting material, MoRe films were grown using magnetron sputtering by codeposition, and different techniques were used for the magnetic, electrical, chemical and crystalline characterization of these films, in order to obtain the appropriate growth parameters for the fabrication of SC/FM bilayers. The growth of MoRe films was one of the challenges since, in order to study the proximity effect, they should have a high critical temperature (close to 10 K) and low roughness. For the growth of the bilayers, as ferromagnetic material, Co and Py (NiFe) films were grown with different thicknesses varying between 1 and 50 nm and the decrease of the superconducting critical temperature was observed as a function of the thickness of the ferromagnetic layer. This critical temperature behavior in SC/FM bilayers has been studied in the literature, but very little is reported on the proximity effect from a microscopic point of view. In this work, we propose to study the charge transfer that occurs at the interfaces of the SC/FM bilayers due to the proximity effect. For this, the X-ray absorption spectroscopy technique was used in grazing incidence due to its chemical and orbital selectivity, and its sensitivity to the depth of the material. These XANES spectra were performed at the L2,3 edges of Re and the K edge of Co for different temperatures. Differences in the XANES spectra were observed at the L2,3 edges of Re at low temperatures close to the superconducting transition and at higher temperatures quite far from this transition, indicating changes in the electronics of Re that may be associated with a transfer of charge by the proximity effect or a spin-orbit coupling at the edges L2,3 of Re.