First-principles study of collective spin excitations in noncollinear magnets

The pace of the current data revolution depends on the world's technological capability to store and process information. A great share of that is done by manipulating magnetic materials with astonishing speed and precision, which involves several dynamical processes. Among the latter are the collective spin excitations known as spin waves. Just like the strings of a guitar, spin waves are the natural "tunes" of a material's magnetization, and knowing their properties allows to predict, design and control technological devices. In this thesis, we study the properties of spin waves in complex magnets focusing on systems of low-dimensionality. The manifestation of spin waves in collinear magnets, such as ferromagnets, has been extensively investigated. However, spin waves in noncollinear magnets are not fully understood yet. For instance, no experimental data is available concerning large-wavevector spin waves in thin films and surfaces. Nevertheless, novel noncollinear spin textures, such as the topologically nontrivial skyrmions, are at the heart of many recent proposals of information nanotechnologies for the future. Therefore, we develop in this thesis an atomistic description of the spin waves in noncollinear magnets applicable to real materials. We achieve that by combining the density functional theory, as implemented within the Korringa-Kohn-Rostoker method, with the spin-wave adiabatic approximation. Effectively, we parametrize from first-principles a generalized quantum Heisenberg Hamiltonian accounting for relativistic effects of the spin-orbit coupling. Thus, besides calculating the magnetic exchange interaction, we also have access to the Dzyaloshinskii-Moriya interaction(DMI) and the magneto crystalline anisotropy. To further relate our results with experimental works, we calculate the inelastic-electron-scattering spectrum using timedependent perturbation theory. This led us to propose spin-resolved electron-energy-loss spectroscopy (SREELS) as an experimental tool to probe large-wavevector spin waves in noncollinear magnets. [...