Fig. 1. Femtosecond laser pulse quenching of a magnetic vortex in Py disks.

(A) Sketch of imaging the femtosecond laser pulse–induced change of spin configuration in a ferromagnetic Py disk by 4D EM operated in Lorentz phase mode with a continuous electron beam. The green femtosecond laser pulse (520 nm, 350 fs pulse duration) is focused to 40 μm on the sample. (B) Schematic Lorentz contrast reverse mechanism of a magnetic vortex in a circular Py disk before and after a femtosecond laser pulse excitation due to the change of spin chirality. Because of the opposite Lorentz force of the imaging electrons impinging on the sample, the Lorentz contrast of a vortex core can be either black or white. The inset depicts the typical transient temperature evolutions after a femtosecond laser excitation (see Materials and Methods) in both the Py disk and the silicon nitride substrate (T_C is the Curie point of the Py disk, T_R is the room temperature, and laser fluence is at 12 mJ/cm²). (C) Femtosecond laser pulse–induced variation of a magnetic vortex in circular, square, and regularly triangular Py disks. The right panel of each Fresnel image schematically depicts the corresponding spin configuration. The blue and red dashed lines correspond to the white and black Lorentz contrasts, respectively, while the blue and red dots correspond to the counterclockwise and clockwise vortices, individually. The green dots mark the magnetic antivortex. The same notes are used in all subsequent figures.