Ab-initio-Studie von Magnonen in linearen und nichtlinearen Regimen

Low energy excitations (< 1eV) are of utmost importance for understanding the elec- tronic, magnetic, and thermodynamical properties of any material. The collective excita- tions of spin in this energy regime are magnons, which are quantized spin-waves. Recent developments in the field of spin-dynamics have opened up the world of femtomagnetism, whereby the spin degree-of-freedom is controlled using ultrafast laser pulses. Thus manipu- lating magnons using femtomagnetism holds great promise for future technological devices operating on ultrafast timescales. In order to reach this goal, it is vital that we are able to accurately describe these magnon excitations in order to understand, and ultimately control them. The objective of this thesis is to study magnetic excitations using an ab-initio approach, namely Time Dependent Density Functional Theory (TDDFT). These studies are divided into two sections. In the first part these excitations are studied in the linear regime where a new approximation is derived. In the second part these studies are extended into the real-time, non-equilibrium, regime where the response of magnons to ultrafast laser pulses is investigated. TDDFT encapsulates the electron-electron interactions of the many-body system in the exchange-correlation (XC) functional, which for practical applications must be approxi- mated. Despite the plethora of approximations for the XC energy functional only a few have been used for the XC kernel. Out of these, only the Adiabatic Local Density Approx- imation kernel has been implemented and applied to study the magnetic excitations. The work presented first focuses on deriving the Generalized Gradient Approximation (GGA) kernel by climbing up the Jacob’s ladder of functionals. Then the performance of the GGA kernel is studied by calculating the magnon spectra for ferromagnets and Heuslers. Results show that the GGA kernel generally worsens the spin-excitation spectra by overestimating the magnon energies. However, at the Brillouin Zone (BZ) boundary, suppression of the spin wave intensity is consistent with experimental findings. In the second part of this thesis the power of real-time TDDFT is demonstrated by simulating multiple magnon modes in multi-sublattice alloys, where these modes can be vi- sualized in real space and their response to external pulses can be investigated in real-time. The results in the case of two magnetic sublattice alloy suggests the existence of element specific decoupled magnon modes along with the coupled modes. These decoupled modes in- dicate that the constituent elements in the alloy respond at different timescales. Using these studies three distinct ways of ultrafast laser pulse control of magnon modes is demonstrated: (1) element selective destruction of magnon modes, (2) creation of a transient non-collinear state by destruction of specific magnon modes, (3) renormalization of the optical magnon frequency, where we found a linear dependence between the laser intensity and the decrease of the magnon frequency.Niederenergetische Anregungen (