Lorentz phase imaging of femtosecond laser pulse–induced magnetic structures

To image the femtosecond laser quenching–induced changes of magnetic structure in the Py disks, we performed out-of-focus (over-focus with a defocus value of about 300 μm) Fresnel phase imaging in 4D EM (5861) operated under Lorentz-mode conditions (62, 63), which enabled in situ femtosecond laser excitation. To obtain high Lorentz contrast, the images were collected using the continuous electron beam of the 4D EM rather than pulsed electrons. Linearly polarized green femtosecond laser pulses (520 nm, 40 μm focal spot size, 350 fs pulse duration) were used for excitation, which were generated from infrared femtosecond laser pulses (1040 nm, 350 fs pulse duration) by second harmonic generation. The in-plane circular magnetizations (clockwise or counterclockwise) of the magnetic vortices exerted opposite Lorentz forces on the imaging electrons, resulting in contrasts or phase shift of the electron beam related to the vortex core, respectively (3941). The high-throughput Fresnel phase imaging allowed the investigation of the femtosecond laser quenching–induced magnetization changes at the nanometer scale and the statistical properties of the resulting magnetic structures.

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