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Time-Resolved Imaging of Magnetisation Dynamics in Nanoscale Magnonic Structures
Davison, Toby Charles
Thesis or dissertation
University of Exeter
In this thesis the results of several different experimental techniques are presented. Time-resolved scanning Kerr microscopy and time-resolved optically pumped scanning optical microscopy measurements were made in Exeter on bi-component anti-dot lattices and permalloy films respectively. Magnetic transmission x-ray microscopy measurements were performed on cobalt nanostructures at the Advanced Light Source at the Lawrence Berkeley National Laboratory in Berkeley, California, USA. Time-resolved Kerr microscopy was used to study bi-component 400nm anti-dot lattices with a 1μm lattice constant. At 200 Oe the mode frequencies were obtained using time-resolved measurements. The mode frequencies of the cobalt filled anti-dots (binary sample) are 3 and 4.4 GHz. The 4.4 GHz mode has propagating character; the 3 GHz mode has non-propagating character. The mode frequencies of the air –filled anti-dot arrays (anti-dot reference sample) are 3.84 and 4.72 GHz. The 3.84 GHz mode has propagating character; the 4.72 GHz mode has non-propagating character. The alteration of the internal field by the air-filled anti-dots lowers the propagating mode frequency compared to the binary sample. Scanning Kerr microscopy was used to study the spatial character of the anti-dot modes. By fitting the spatial character the effective damping parameter(s) were determined. The effective damping parameter for the binary sample was 0.023 and 0.044 for the 4.4 and 3.04 GHz modes respectively. The 3.04GHz mode exists through the cobalt filled anti-dots leading to a high effective damping. The effective damping parameters for the anti-dot reference sample 0.026 and 0.028 for the 3.84 and 4.72 GHz modes respectively. Time-resolved optically pumped measurements have been performed on a continuous 20nm permalloy film. This is a new experimental technique developed during my PhD. Early data acquired on the optical microscope is compared with data measured later and attempts are made to explain the discrepancies. With a 500 Oe out-of-plane field initial time resolved signals show an oscillation at 17 GHz, the origin of the oscillation is unknown and is thought to originate from a magnon or phonon contribution. The experiment overlapping sub-micron pump and probe spots makes acquiring consistent time-resolved signals a difficult challenge. Images revealing circular lobe shapes are observed, the origin of these images is not fully understood. Later measurements are compared to the early measurements. In the recent measurements, images of spin waves with a wavelength and frequency of 2.5 μm and 4 GHz respectively are observed. These values are not reconcilable with the wavelengths and frequencies of 1μm and 17GHz seen in the images and time-resolved signals respectively. Recent measurements also revealed a strong dependence on the pump focus position on the measured images. Lastly, magnetic transmission x-ray microscopy measurements are presented on 700nm cobalt anti-dot structures overlaid on continuous permalloy films of thicknesses ranging from 20 to 60nm. The magnetic ground states of the nanostructures are investigated using L3 edge x-ray absorption and x-ray magnetic circular dichroism as a magnetic contrast mechanism. The reversal fields are determined and consistent with hysteresis loops measurements in Exeter. The dipolar fields from the complex shape of the cobalt anti-dots are expected to modulate the magnetic ground state of the permalloy. Reversal of the permalloy occurs suddenly over a consistent field window, starting and finishing between 13 ~ 17 Oe respectively. The reversal process in the cobalt occurs gradually and full saturation is not observed until fields of up to 350Oe.
PhD in Physics