Coherent coupling and nonlinear dynamics in planar magnet-superconductor hybrid systems.

Magnonics is an emerging research field of magnetism, considered as a subfield of spintronics, dealing with the investigation of spin waves in ferromagnetic materials, i.e. the propagation of a magnetization variable in space inside the material. In this context, ferro- or ferri-magnetic materials such as the Yttrium Iron Garnet (YIG) have been widely investigated because of the high spin density and low ferromagnetic resonance damping rates, which makes it perfect for the study of spin wave excitations. Coplanar waveguide (CPW) transmission lines are planar circuits having lateral dimensions much smaller than the wavelength of the transmitted signal. Usually, they are made up of two conductive ground lines and a central conductive line through which the signal passes. While CPW lines allow for broadband transmission of microwaves in a wide range, CPW resonators present two additional capacitive gaps at the extremities of the transmission line which allows one to confine stationary waves, giving rise to resonant modes at a precise frequency. Superconducting films allow for the realization of planar circuits with very confined electromagnetic fields, which can integrate magnetic materials to realize a hybrid device. High critical temperature (HCT) superconductors have been widely studied in the last 4 decades; in particular, Yttrium Barium Copper Oxide (YBCO) is characterized by unique properties including high critical temperature above the liquid-nitrogen boiling point and strong resilience to applied magnetic fields. This thesis project focuses on two main topics. The first one regards the investigation of coherent coupling between YBCO superconducting resonators and spin waves in YIG films; the second one concerns the exploration of non-linear effects in the hybrid superconduct-magnet system. Because of the presence of the superconductor, all of the measurements were performed in a cryogenic environment. For the first topic, a 100-nm-thick YIG film was positioned on top of the YBCO resonator. Once the frequency of the resonator has been chosen, which depends on its length, the magnetic field corresponding to the ferromagnetic resonance (FMR) line is set to obtain coupling between electromagnetic and spin wave modes. The evolution of frequency and coupling strength of hybrid modes as a function of temperature was characterized, showing effects that can be interpreted as a signature of the interplay between superconductor and ferrimagnet. The second part of this work explores nonlinear effects in hybrid YIG/YBCO systems. Their origin lies in the nonlinearity of YBCO which is added to nonlinear processes in YIG. Specifically, I studied intermodulation distortion products on the YBCO coplanar transmission line by using two microwave sources to generate two microwave signals with different frequencies. This leads to the generation of additive harmonic peaks with a characteristic frequency shift. When the YIG film is positioned above the superconductor and a resonant field is applied, an enormous enhancement of the peaks occurs, causing the formation of a frequency comb. Furthermore, some complex physical effects are hiding behind an unusual behavior of the peaks' intensity. This intriguing phenomenon, related to proximity effects between the superconductor and the ferrimagnet, evidences the complexity behind this kind of physical systems.