Magnétisme et pompage de spin dans des systèmes hybrides à base de film supraconducteur

Superconducting thin films exhibit electromagnetic properties that significantly differ from those of bulk materials, mainly due to geometrical effects, disorder, and confinement. Understanding the mechanisms of flux penetration, vortex dynamics, and interfacial interactions is therefore a key issue, both from a fundamental perspective and for applications in superconducting electronics and spintronics. 

In this context, several questions remain open in the literature. On the one hand, the magnetic response of thin films, particularly in strongly demagnetizing geometries, exhibits unconventional behaviors that are not fully described by classical models. On the other hand, spin transport prop- erties at superconductor/ferromagnet interfaces still lead to conflicting results, especially regarding the evolution of the damping parameter below the superconducting transition temperature. 

This thesis aims to provide an experimental contribution to these issues through three main re- search directions. 

The first part focuses on the growth and control of superconducting niobium nitride (NbN) thin films. Particular attention is given to deposition on MgO substrates, which are well documented in the literature, as well as to the development of an original growth route using a Pt buffer layer. This approach leads to twinned epitaxial growth despite a large lattice mismatch, and enables the fabrication of heterostructures suitable for interfacial studies. 

The second part investigates the magnetic response of superconducting thin films. A systematic study of magnetization loops was carried out on various materials, including both type I and type II superconductors. The aim is to characterize regimes where geometrical effects and vortex dy- namics become dominant. In particular, an inversion of the magnetization loop was observed under specific conditions of temperature, magnetic field, and thickness. This constitutes a central result of this thesis. 

Finally, the third part addresses spin injection in NbN-based heterostructures using ferromagnetic resonance measurements. Systems of the type NbN/X/Py (X = Cu, Pt) were investigated in order to analyze the evolution of the damping parameter across the superconducting transition. The re- sults show a decrease in damping below Å, consistent with the opening of the superconducting gap in NbN and the associated reduction of dissipative spin absorption channels, with no clear signature of triplet correlations under the investigated conditions. 

Overall, this work provides new insights into the magnetic response of superconducting thin films and interfacial phenomena involving spin currents, in direct connection with recent developments in superconductivity and spintronics.