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Tutorial - Time-resolved PEEM
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Pump-probe experiment measure the
average, repeatable dynamics of a system using a first short
pump pulse that starts the dynamics and a second short probe
pulse that measures the state of the system. The dynamics of
the system is determined in time by capturing the state of the
system as function of the delay between pump and probe. Stochastic
processes cannot be measured in a pump-probe style experiment.
The time-resolution is limited by the convoluted length or rise
time of both pulses. The Advanced Light Sources produces X-ray
pulses of 50-80 ps in length. A synchronized laser pulse can
be used as a pump as well as short current pulses from an electronic
pulse generator. The experiments were repeated at 3 MHz rate,
set by the bunch frequency of the ALS in 2-bunch mode. |
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| Magnetization dynamics experiments
usually require a fast magnetic field pulse, which excites a
magnetic mode. In PEEM experiments at the ALS, the field pulse
has been generated using a photoconductive switch, which is
integrated in a RF frequency waveguide. The magnetic sample
resides on top of the waveguide and experiences an in-plane
magnetic field when a current pulse is produced by the photoswitch,
triggered by the laser. The laser can alsodirectly excite the
magnetic sample by an electrnoic or thermal excitation. |
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The pulse profile of the current
pulse through the waveguide is shown on the left. The pulse
was directly measured using TR-PEEM, utilizing the small deflection
of the photoemitted electrons by the electric pulse. The effect
is greatly exaggerated in the picture. The pulse shows a rapid
rise, at or below the temporal resolution of the probe and a
slower decay, limited by carrier recombination and sweep out
in the switch. A pulse length of about 500 ps was measured.
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| A movie of the magnetization dynamics
of CoFe patterrn is shown. The pattern was generated by photolithography
and lift-off to create the waveguide, and subsquent focused
ion beam milling to create the micron-size patterns. The top
image shows the magnetoc domain structure imaged using XMCD
contrast, the botoom shows a gradient image that emhasizes domain
walls. The movie extends over 8 nanoseconds. Domain dynamics
is easily visible at this low magnification in the triangles
on the left and some of the larger squares. The ability to study
a large number of patterns in parallel is very useful since
the acquisition of high-quality images for a multitude of time
points is time-consuming. |
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A higher magnification the gyrotropic
vortex mode, excited by the field pulse, becomes visible. The
vortex core moves on an elliptical trajectory, driven by the
magnetic field pulse. |
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| The vortex trajectories are shown
for a 1x1 µm2, 1.5x1 µm2, and a
2x1 µm2 pattern. During the field pulse the vortex core
accelerates parallel or antiparallel to the field direction and then
gyrates driven by the magnetostatic field around the pattern center.
The direction of the acceleration of the rotation is determined by
the handedness of the vortex. |
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