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
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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
