title: Surface-governed dynamics of atomic-scale magnetic moments
Creators: name: Arnhold, Lukas
Publisher: name: Universität Stuttgart
Year: 2025
Subjects: subject: 530

description: This thesis explores how atomic-scale environments influence the behavior of individual magnetic moments adsorbed on surfaces, focusing on both their static and dynamic properties. Such magnetic systems, from individual atoms to molecules and nanostructures, exhibit properties that are shaped by interactions with their immediate surroundings. Their energy level diagrams change depending on the symmetry of their binding site, but also their stochastic dynamics and the respective scattering mechanisms are extremely sensitive to variations in the environment. Here, we use time-resolved scanning tunneling microscopy (STM) to address these individual atomic-scale magnets with picometer spatial resolution and uncover how their behavior is governed by the properties of the surface they are adsorbed on. We employ multiplet calculations and rate equation modeling to gain insights into the inner workings of an atom's magnetism. Combining experimental and theoretical approaches, we reveal how surfaces impose magnetic anisotropy, change electronic properties, and define the spin and orbital properties of these magnetic systems. We study the influence of four different rotational symmetries of the magnetic moments environment: 4-fold, 3-fold, 2-fold, and asymmetric. Further, we compare how the symmetries affect magnetic systems with different numbers of electrons. Fe atoms on bilayer MgO have 6 electrons in the 3$d$ shell. The orbital momentum of the atoms mediate magnetic anisotropy, induced by the 4-fold symmetric surface geometry, which stabilizes the magnetic states. The energy separation of the eigenstates enables distinct regimes of scattering with the local environment. Fe atoms are efficiently excited by spin-flip interactions with the tunneling current, while the relaxation of magnetization preferably occurs by tunneling of magnetization through the anisotropy barrier. The scattering regimes are determined by the coupling strengths of the magnetic moment to the surface and the STM tip. In contrast, Ti atoms on MgO have only one electron in the 3$d$ shell but adsorb on two different binding sites. Ti atoms adsorbed on 4-fold symmetric binding sites experience out-of-plane anisotropy, and we can observe and assign transitions that rotate the atoms electron spin momentum and orbital momentum relative to each other. Distinctively, atoms adsorbed on 2-fold symmetric binding sites exhibit quenched orbital momentum, resulting in an in-plane anisotropy and transitions of the magnetic moment that are highly sensitive to environmental scattering contributions. These findings highlight how the surface geometry can lead to contrasting behaviors. Molecular systems, such as CpTicot on Pb nanoislands, demonstrate the potential of protecting a magnetic moment by ligand encapsulation. In this case, the local geometry leads to an asymmetric environment for the Ti spin center. The ligands stabilize the Ti atom's single valence electron in a non-bonding orbital that interacts with the superconducting substrate through a rare, spatially-dependent hybridization process, providing valuable insights into local spin-superconductor coupling. Lastly, we implement 3-fold symmetric ZnO films as a promising substrate for stabilizing magnetic moments in single atoms. Self-assembled Co-Co and Co-H nanostructures on the surface reveal how the symmetry of orbital momentum and chemical bonding modifies the magnetic anisotropy. For, a Co atom adsorbed on the hollow site of ZnO experiences a single in-plane anisotropy component by the crystal field. Adsorbing a single hydrogen on this Co atom contributes a dominant charge contributions above the atom, changing the anisotropy axis to the out-of-plane orientation. This investigation adds valuable understanding for quantum state engineering on the atomic scale. Through a systematic exploration of surface symmetries and atomic electron configurations in both experiment and theory, this thesis deepens the understanding of how atomic-scale environments condition magnetic moments. These findings advance the description of atomic-scale magnetic dynamics and explicitly introduce the orbital momentum as a valuable design resource for such quantum systems.