Resumen de Out of equilibrium dynamics in magnetic and superconducting nanostructures
Historically, high frequency magnetic response (microwave magnetic permeability) has been first studied in macroscopic systems close to equilibrium. The more recent efforts towards miniaturization of devices have resulted in systems which are often found in conditions out of equilibrium (metastable) states. Examples of this are superconducting systems with pinned vortices, the formation of a pinned vortex Bean’s critical state or magnetic structures in some complex magnetic states involving domain walls.
This thesis studies the high frequency response of magnetic and superconducting systems in out of equilibrium conditions. As a continuation of the work by Dr. A. A. Awad, this thesis starts by exploring ways of stabilizing metastable states (double vortex states) in magnetic micron sized dots with domain walls present. In such systems it was observed how spin waves can propagate only through the domain walls (the so called “Winter’s magnons”, and not anywhere else in the dot. This has interesting properties, since it allows to direct spin waves along specific and narrow paths, without them “spilling” everywhere, as usually tends to happen. The domain walls present in the double vortex state are metastable, and even if they can be stabilized for some hours in real samples, thanks to pinning and defects, they eventually move, and the metastable double vortex system decays into the ground state single vortex state.
Chapter 3 explores a way to pin vortices and domain walls, to stabilize such states, by drilling holes in the magnetic dots, capable to trap them.
Next, in chapter 4, we explore the possiblity of propagaing spin waves through domain walls in dots again, only that this time in non decaying systems. The system of choice is triangular magnetic dots, where domain walls have a more stable structure. First, there are domain walls in the ground state (vortex state). Second, under an applied field, new regions near the dot edges accumulate an excess of exchange energy (similar to a domain wall, only that it lays next to the dot border), and again, waves propagate especially well in those regions, compared to the inside of the dot. This case is another example of perturbations out of equilibrium, since these regions can only be maintained under the external action of a bias magnetic field.
Superconducting systems can also exhibit non trivial behaviors out of equilibrium, in this case close to the superconducting transtion, where stimulation of superconductivity by microwaves is a noticeable phenomenon. In chapter 6 we explore the effect of moving vortices close to the critical temperature, in this stimulated regime, whose effects we have used as a tool to study the dissipation of vortices. Also, to understand the experimental results, we have developed a simulation software to solve the Time Dependent Ginzburg Landau equation. This program is explained in detail in chapter 5.
Finally, the last out of equilibrium phenomenon studied in this work is magnetic flux avalanches, again close to the superconducting transition. We have studied how the effect of stimulation of superconductivity takes place to strenghthen the potential barriers that must be overcome for avalanches to enter the sample.
