Time-Resolved Magneto-Optical Investigations of Picosecond Magnetisation Dynamics in Arrays of Non-Ellipsoidal Ferromagnetic Nano-Elements
Keatley, Paul Steven
Thesis or dissertation
University of Exeter
In this thesis the results of magneto-optical experiments will be presented. The experiments were performed on micro-arrays of square nanomagnets in order to characterise the static and time-dependent behaviour of the nanomagnets. The static behaviour was investigated in vector-resolved scanning Kerr microscopy experiments, while the time-dependent behaviour was investigated in time-resolved scanning Kerr microscopy experiments. In the latter so-called pump-probe experiments, magnetisation dynamics were induced by exciting the sample magnetisation with a pulsed magnetic field (pump). The magnetisation dynamics were then detected using the magnetooptical polar Kerr effect (probe). The longitudinal Kerr effect was utilised in the vectorresolved scanning Kerr microscope in order to measure the in-plane components of the static magnetisation. The experimental set-up and methodology of the vector- and timeresolved scanning Kerr microscopy experiments will be discussed in detail, in particular, the detection technique that allows three components of the vector magnetisation to be measured simultaneously. Since the spatial resolution of the magneto-optical probe was insufficient to resolve the spatial character of the magnetisation dynamics within individual nanomagnets, micromagnetic simulations were used to gain insight into the character of the excited modes. Extensive testing of different micromagnetic models was carried out in order to investigate the effect of the different models on the simulated dynamics. The results of measurements carried out on the arrays of square nanomagnets revealed that the static and time-dependent behaviour of the magnetisation became more complicated as the size of the nanomagnets was reduced. In particular, similar hysteresis loops were acquired when the elements were magnetised along the uniaxial anisotropy easy and hard axes, while fast Fourier transform spectra of time-resolved signals revealed that the character of the magnetisation dynamics changed significantly as the element size and/or applied magnetic field were reduced. Interpretation of the experimental results using micromagnetic simulations revealed that the elements had a non-uniform single domain ground state magnetisation. When the field was applied along either edge of the square elements and reversed, the magnetisation was found to switch via a series of metastable non-uniform single domain states. Furthermore, the increasing non-uniformity of the single domain ground state as the element size and/or applied field were reduced lead to significant changes in the mode character excited within the elements. Comparison of 2 experimental spectra with simulated spectra and Fourier images of the dynamic magnetisation revealed that as the element size and/or applied field were reduced, the mode character changed from one that occupied the majority of the volume of the element, to several modes that were localised near to the edges of the element that were perpendicular to the applied field. Furthermore, deviation of the direction of the wavevector of the dynamic magnetisation from the direction of the static magnetisation was found to lead to a dynamic configurational anisotropy within nanomagnets. Following the presentation of the experimental results, the recent developments for future experimental work are presented with the aim to study precessional switching in an isolated nanomagnet. The results obtained in the experiments presented in this thesis are expected to lead to a better understanding of the non-uniform magnetisation dynamics in square nanomagnets, which have application in future magnetic data storage technologies.
Hicken, Robert J