TRR 360 Seminar:

Imaging Magnetic Dynamics with In-situ and Ultrafast Lorentz Microscopy

Sascha Schäfer

Imaging Magnetic Dynamics with In-situ and Ultrafast Lorentz Microscopy

Prof. Dr. Sascha Schäfer

Department of Physics, University of Regensburg, Regensburg, Germany
Regensburg Center for Ultrafast Nanoscopy (RUN), Regensburg, Germany

The interaction of magnetic materials with ultrashort light pulses provides intriguing glimpses into the inner workings of the magnetic couplings within a material but also offers an important experimental handle to locally manipulate magnetic textures and phases. Whereas many studies in the field of ultrafast magnetism have utilized various optical pump-probe methodologies, these approaches often lack the required spatial resolution for mapping the nanoscale details of magnetic textures. Here, I will present two methodologies to address ultrafast nanoscale magnetic dynamics in Lorentz microscopy.

In the first part of the talk, I will discuss how in-situ Lorentz microscopy with femtosecond optical excitation [1] can unravel nonlinearities in the magnetic response upon light excitation, focusing on the example of light-induced switching processes in the magnetic texture of Fe₃Sn₂ thin films [2]. This material hosts dipolar skyrmions [3] which are composed of chiral Bloch-like domain walls in the interior of the film and chiral Néel-like caps at each film surface. Femtosecond light pulses allow for the stochastic switching of the Néel cap chirality. The analysis of switching probabilities yields insights into the importance of interactions within and in-between skyrmions.   

In the second part of the talk, I will discuss the current state of ultrafast transmission electron microscopy [4], specifically ultrafast Lorentz microscopy [6], in which femtosecond or picosecond electron pulses enable the stroboscopic imaging of optically or current-driven magnetic dynamics. I will highlight the impact of novel femtosecond electron sources, including radiofrequency beam choppers [7] and laser-driven cold-field emitter sources [8], for different magnetic imaging techniques in electron microscopy, as well as emerging possibilities due to fast electron detectors [9].

[1] T. Eggebrecht; M. Möller et al., Phys. Rev. Lett. 118, 097203 (2017).
[2] A Kovács, J. T. Weber et al., Comm. Mat. 6, 223, (2025).
[3] L. Kong et al., Phys. Rev. B 107, 174425 (2023).
[4] Feist et al., Ultramicroscopy 176, 63 (2017).
[5] Feist et al., Nature 521, 200 (2015).
[6] N. Rubiano da Silva et al., Phys. Rev. X 8, 031052 (2018).
[7] C. Liu et al., Nat. Mat. 24, 406 (2025).
[8] A. Schröder et al., Ultramicroscopy 275, 114158 (2025).
[9] A. Schröder et al., Ultramicroscopy 256, 113881 (2024).