Browsing by Author "Chou, Kang Wei"

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Vortex dynamics studied by time-resolved x-ray microscopy
(2007) Chou, Kang Wei; Schütz, Gisela (Prof. Dr.)
The main topic of this work is the study of the magnetization dynamics in thin film, confined magnetic structures with a lateral size in the micron and submicron range, more specifically in structures with a single vortex. The change of the magnetization distribution was investigated under the influence of a time varying magnetic field. Time-resolved measurements, based on a stroboscopic measuring scheme, were therefore implemented into a scanning transmission x-ray microscope (STXM). A time and lateral resolution of about 70 – 100 ps and 30 – 40 nm can be achieved respectively. Initially, a brief introduction is given to the physics of magnetism and the basic concepts of magnetization dynamics, with special emphasis on the dynamic behaviour of magnetic vortex structures. The employed experimental technique is described next. Special attention goes to the experimental setup for time-resolved STXM measurements, where three different excitation types were developed: pulsed, sine and burst excitation. Finally, the experimental results are presented. The static configuration of micron-sized vortex structures was imaged. The in-plane as well as the out-of-plane magnetic contrast were recorded. The in-plane curling magnetization can clearly be observed thanks to the XMCD effect. Beside single layers, stacks of two different ferromagnetic layers (Permalloy & Co) with an insulating Cu layer in between, were also probed separately by taking advantage of the element specificity of the XMCD effect. Complex domain patterns appear due to a coupling between the magnetic layers. The gyrotropic mode in ferromagnetic structures with a single vortex, was investigated intensively. Not only was the gyrotropic motion imaged, providing qualitative information, but quantitative results were derived as well. In the first instance, a differential imaging technique was introduced reducing drastically the signal-to-noise-ratio. The gyrotropic motion was then observed by recording images of the relaxation process after a fast in-plane field pulse excitation. The resonance frequency of the gyrotropic mode was determined as a function of the aspect ratio of the patterns. The excitation of a vortex structure with single bursts, showed that the vortex core was gyrating with a constantly increasing phase shift. Furthermore, the gyrotropic motion was observed using a resonant sine excitation with a frequency close to the gyrotropic resonance frequency. At very low field amplitudes, a linear response was observed for the extent of the gyrotropic motion (~ velocity) as a function of the field amplitude. Non-linear effects occur at slightly higher field amplitudes of the exciting field. The reversal of the out-of-plane vortex core magnetization was performed by changing the amplitude of the alternating magnetic field. The reversal was observed indirectly by investigating the sense of gyration, but also directly by imaging the out-of-plane magnetic contrast. Different field amplitude levels were observed with a fixed vortex core polarization, but both states can appear at lower field amplitudes, showing a complex hysteresis behaviour. A different reversal scheme was introduced as well where the out-of-plane vortex core magnetization could be reversed by applying short in-plane bursts of an alternating magnetic field. Two dimensional micromagnetic simulations indicated that the reversal process is the result of the consecutive creation and annihilation of a vortex-antivortex pair. Finally, the vortex core velocity was determined as a function of the field amplitude of the alternating magnetic field. Clear jumps occur at the corresponding switching thresholds. Additionally, in a specific field amplitude range, a clear discrepancy can be distinguished in the vortex core speed for a core pointing up and down, respectively. The gyrotropic motion was also imaged for a trilayer system composed of two different ferromagnetic layers (Permalloy & Co) with an insulating Cu layer in between. The stack was excited with an alternating magnetic field and due to the coupling between the ferromagnetic layers, different dynamic motions appear in the two layers. The motion in the Co layer deviates from a gyrotropic motion. In conclusion, a short overview is given with the perspectives for future measurements and how it can possibly be improved.