  Time-resolved scanning Kerr microscopy (TRSKM) is a powerful technique for
the investigation of picosecond magnetization dynamics at sub-micron length
scales by means of the magneto-optical Kerr effect (MOKE). The spatial
resolution of conventional (focused) Kerr microscopy using a microscope
objective lens is determined by the optical diffraction limit so that the
nanoscale character of the magnetization dynamics is lost. Here we present a
platform to overcome this limitation by means of a near-field TRSKM that
incorporates an atomic force microscope (AFM) with optical access to a metallic
AFM probe with a nanoscale aperture at its tip. We demonstrate the near-field
capability of the instrument through the comparison of time-resolved polar Kerr
images of magnetization dynamics within a microscale NiFe rectangle acquired
using both near-field and focused TRSKM techniques at a wavelength of 800 nm.
The flux-closure domain state of the in-plane equilibrium magnetization
provided the maximum possible dynamic polar Kerr contrast across the central
domain wall, and enabled an assessment of the magneto-optical spatial
resolution of each technique. Line profiles extracted from the Kerr images
demonstrate that the near-field spatial resolution was enhanced with respect to
that of the focused Kerr images. Furthermore, the near-field polar Kerr signal
(~1 mdeg) was more than half that of the focused Kerr signal, despite the
potential loss of probe light due to internal reflections within the AFM tip.
We have confirmed the near-field operation by exploring the influence of the
tip-sample separation, and have determined the spatial resolution to be ~550 nm
for an aperture with a sub-wavelength diameter of 400 nm. The spatial
resolution of the near-field TRSKM was in good agreement with finite element
modelling of the aperture...
