Current driven instabilities and non-lineairities in magnetic systems

In this Thesis we developed new ideas to address two central questions in the field of spintronics and magnonics. Namely, how does one extend the lifetime of spin waves and how does one coherently inject spin waves using direct currents (DC)? Most of the proposals we developed use analogies between high-energy and condensed matter physics. For instance, antiparticlelike negative energy states can exist in driven magnetic systems. We showed that these negative energy modes can be used to enhance spin waves of an interface and coherently inject spin waves using DC currents. Besides this, this thesis also contains a chapter on non-linear magnetization dynamics of coupled macrospins and two chapters on the interplay between exchange- and magnetostatic interactions.