Mohamed F. Elhanoty: Exploring Microscopic Mechanisms of Ultrafast Magnetization Dynamics: Bridging Experimental Observables with Quantum Mechanical Insights through Ab Initio Methods

Abstract

The optical manipulation of magnetic materials using ultrashort laser pulses offers a promising avenue for advancing spintronics and magnetic data storage technologies. However, probing and disentangling the microscopic mechanisms driving magnetization changes remains challenging due to the complex interplay of multiple degrees of freedom at femtosecond timescales and Ångstrom length scales. In this thesis, we employ a combination of fully ab initio time-dependent density functional theory (TDDFT) and linear response theory to investigate the real-time dynamics of spin and electronic excitations in magnetic materials. The study focuses on magnetization dynamics and their various spectroscopic fingerprints in typical pump-probe experiments for a wide range of materials, including simple transition metal magnets, hybrid Stoner-Heisenberg alloys, and Heusler alloys such as Co2MnGa and Co2MnGe. We calculate time-resolved changes in magnetization and magneto-optical responses from first principles, enabling direct comparisons between experimental observations and theoretical predictions. 

Our results identify spin flips mediated by spin-orbit coupling as the primary demagnetization mechanism in simple magnetic elements, while optical intersite spin transfer (OISTR) emerges as a key process in multicomponent magnetic alloys under laser excitation. We also demonstrate the efficient control of rigid magnetic moments in Heisenberg magnets through the OISTR mechanism on femtosecond timescales. Furthermore, by engineering valence bands via elemental substitution, we reveal how band modulation, spin lifetimes, and crystalline disorder influence transient signals measured at specific probing energies. 

We investigate discrepancies in the energy-dependent signatures of microscopic processes at various probing energies, highlighting the importance of accurately interpreting transient signals in magneto-optical experiments. Our findings reveal that induced magnetization changes are significantly smaller than those inferred from experimental measurements, indicating a complex interplay of ultrafast microscopic processes. These observations exhibit diverse energy-dependent signatures, driven by the competition between ultrafast microscopic processes induced by the pump laser. Moreover, our results highlight the pivotal role of quantum mechanical selection rules in distinguishing genuine pump-induced dynamics from artifacts introduced by specific optical probe choices. 

Finally, this thesis explores the limitations of TDDFT in capturing correlation effects in dynamically evolving systems and emphasizes their significance in interpreting transient absorption spectra of fcc Ni at the L-edge. Collectively, this work provides key insights into the interplay between optical excitations, spin flips, and selection rules, improving the interpretation of pump-probe experimental measurements and advancing the theoretical framework for ultrafast magnetization dynamics.