h1

%0 Thesis
%A Stellhorn, Annika
%T Interplay of proximity effects in superconductor/ferromagnet heterostructures
%I RWTH Aachen University
%V Dissertation
%C Aachen
%M RWTH-2021-03280
%P 1 Online-Ressource : Illustrationen, Diagramme
%D 2021
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, RWTH Aachen University, 2021
%X Proximity effects in superconductor(S)/ferromagnet(F) thin film heterostructures are gaining tremendous interest in fundamental studies scrutinizing exotic new quantum states as well as in the application of superconducting spin valves or fluxonic devices for quantum computing. In particular, an inhomogeneous magnetization of the neighboring ferromagnet enables modifications of the superconducting state, which emerges as stray-field generated domain-superconductivity or direct exchange related proximity effects such as spin-triplet superconducting correlations.In this thesis, a combination of both phenomena in one heterostructure system, and their interplay and controllability by an external magnetic field, is studied. A systematic study on thin film heterostructures of Nb(S)/FePd(F) with Perpendicular Magnetic Anisotropy (PMA) and a lateral magnetic domain pattern is performed addressing three main objectives: (i) growing the Nb/FePd bilayers reproducibly with varying strength of PMA and a controllable domain configuration, (ii) characterizing macroscopic magnetotransport measurements of stray-field generated and direct exchange proximity effects which reveal their interplay and tunability by an external magnetic field, and (iii) characterizing the inverse phenomenon where the superconducting state affects the lateral magnetic depth-profile of FePd. Grazing-Incidence Small-Angle Neutron Scattering (GISANS) is the method of choice for such inverse proximity effects that appear in S/F heterostructures with a lateral domain pattern on the mesoscopic length scale. Room-temperature characterizations reveal a successfully reproducible growth of thin film FePd with an adjustable strength of PMA and a controllable domain configuration. GISANS measurements confirm triangular shaped closure domains at the layer surfaces of stripe-patterned FePd. Results at low temperatures demonstrate isolated, stray-field generated domain-wall- and reverse-domain-superconductivity in samples of high PMA. This is consistent with theoretical models based on the Ginzburg-Landau approach. These confinement effects are accompanied by a change of magnetic fluctuations inside the FePd domain pattern and an increase of the domain wall width probed by neutron scattering in an out-of-plane applied guiding field. Polarized GISANS studies on S/F heterostructure systems exhibiting PMA are an entirely new research field and are demonstrated to reveal small changes in the magnetic orientation by the onset of superconductivity. Results on Nb/FePd with low PMA and a non-collinear magnetization promote spin-triplet Cooper pair generation with spin alignment along the F-layer magnetization in an in-plane applied field. The variation of the S critical temperature due to spin-triplet Cooper pairs (dTc) of 100mK is a promising large value for a bilayer system with capability for higher dTc in corresponding trilayer systems. In-plane measurements on samples with higher magnetocrystalline anisotropy and an angle-dependent study on the low-PMA Nb/FePd suggest a spin-triplet Cooper pair generation also in high-PMA Nb/FePd bilayers.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2021-03280
%U https://publications.rwth-aachen.de/record/816569