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Lawrence Berkeley National Laboratory

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Rajesh Chopdekar

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last update: 03/30/2020

Tutorial - Time-resolved PEEM

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.
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.
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.
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.
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.
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.