Imaging vortex pinning and gyration by time-resolved and in-situ Lorentz microscopy
von Marcel Möller
Datum der mündl. Prüfung:2022-08-30
Betreuer:Prof. Dr. Claus Ropers
Gutachter:Prof. Dr. Claus Ropers
Gutachter:Prof. Dr. Stefan Mathias
EnglischMagnetism encompasses a large variety of scientifically interesting and industrially valuable phenomena. In particular, nanoscopic magnetic textures—such as vortices, merons and skyrmions—have been in the spotlight of contemporary research, due to various possible applications, such as three-dimensional memory, logic gates or neuromorphic computing. Studying the control of such textures employing electric, magnetic or optical fields, demands instruments with sufficient spatial and temporal resolution. With the development of high-brightness electron sources, ultrafast transmission electron microscopy was established as an essential tool for the study of optically-driven magnetization dynamics. Yet, its potential to probe current- or field-driven dynamics of magnetic textures remained unexplored. In the first publication of this cumulative thesis, I and my coworkers use Lorentz microscopy in the Göttingen Ultrafast Transmission Electron Microscope to image the time-resolved gyration of a vortex in a magnetic nanostructure driven by radio-frequency currents. We demonstrate tracking of the vortex core with a localization precision of ±2 nm with an electron pulse duration < 3 ps at few-minute integration time. In studying the decay of the gyration after the sudden removal of the driving current, we find a transient change in the frequency and damping of the core orbit, attributed to disorder in the sample. The second study combines conventional and time-resolved Lorentz microscopy with bright-field imaging of the magnetic vortex nanostructures to identify the origin of this disorder. A comprehensive study of the time-resolved gyration is enabled by the implementation of a novel photoemission source, reducing the image acquisition time by multiple orders of magnitude. Furthermore, a new method for probing static pinning sites in magnetic nanostructures is developed, that allows for the reconstruction of the underlying pinning potential. The comparisons of the time-resolved and conventional results with bright-field images suggest grain boundaries in the polycrystalline film to be one source of pinning.
Keywords: ultrafast transmission electron microscopy; UTEM; Lorentz microscopy; magnetic vortex; pinning; in-situ TEM; permalloy; magnetism; solid-state physics; ultrafast dynamics