Martin Pavelka: Take It for a Spin: Experiments on Ultrafast Magnetization Dynamics in Ferromagnetic Metals and Insulators

Abstract

The study of ferromagnetic dynamics on the femtosecond timescale constitutes a central pillar of ultrafast science, investigating pathways to future THz technologies, such as all-optical magnetization switching. Yet, the ultimate microscopic channels facilitating angular momentum transfer, whether mediated by spin-orbit coupling, ultrafast magnon generation, or superdiffusive spin currents, remain a subject of an intense debate. Additionally, the emerging field of magnonics seeks to utilize spin waves as information carriers, looking to apply novel materials with strong magnon-phonon coupling, such as the recently discovered two-dimensional van der Waals magnets. This thesis investigates these ultrafast dynamics in representative materials from the two fundamental categories: ferromagnetic metals and insulators.

First, we analyzed element-specific spin and charge dynamics in a photoexcited metallic Co50Pt50 alloy using time-resolved X-ray magnetic circular dichroism and absorption spectroscopy at the CoL3-edge. These experiments utilized the new helical afterburner undulator at the FLASH free-electron laser (Hamburg, Germany) to generate femtosecond soft X-ray pulses with near 100% circular polarization. We identified spin-orbit coupling as the dominant microscopic parameter driving demagnetization. Comparisons with previous measurements on CoPd revealed that the 3d/5dCoPt alloy demonstrates significantly higher demagnetization efficiency, requiring reduced electronic heating to achieve comparable magnetic quenching due to enhanced spin-flip probabilities associated with the higher spin-orbit coupling of Pt.

Second, we examined phonon and spin dynamics in exciton-driven insulating CrI3 flakes using time-resolved magneto-optical Kerr effect and reflectivity measurements. By selectively pumping its A- and B-excitons, we tracked coherent and incoherent dynamics across femtosecond, picosecond and nanosecond timescales. We detected coherent optical phonon modes at 2.4 THz and 3.9 THz, corresponding to Cr-I bond bending and stretching, which we found to modulate magnetism via exchange interaction modifications. This lattice-spin coupling is tunable by excitation density and exciton choice. While the A-exciton drives uniform phonon-driven demagnetization and localized spin oscillations, the low-frequency mode excited by the B-exciton exhibits anomalous behavior, potentially signaling a topological radial magnon mode at 2.4 THz carrying orbital angular momentum. Quantitatively distinct demagnetization behaviors were also observed for each exciton in the incoherent regime.