The use of ultrashort light pulses on the femtosecond timescale enables the manipulation of magnetic order at its fundamental temporal and spatial limits, giving rise to intriguing phenomena such as ultrafast demagnetization, optically induced superdiffusive spin transport [1, 2], and all-optical magnetization switching.[3–5]
This talk presents recent advances in ultrafast magnetism enabled by extreme-ultraviolet (XUV) magneto-optical Kerr effect (MOKE) spectroscopy using high-harmonic generation sources, which provide element-specific access to femtosecond magnetization dynamics. [6] The work clarifies how dielectric properties, multilayer interference, and interface effects shape XUV magneto-optical observables, establishing a framework for quantitative studies of femtomagnetism and all-optical switching phenomena. [3–5]
A central focus is the controversy of the commonly assumed proportionality between magnetic asymmetry and magnetization in transverse MOKE. [7–9] Wave-propagation modelling and measurements at the Fe 𝑀2,3 edge reveal conditions under which interference effects cause strong deviations, including nonmonotonic behavior, and indicate how optimized geometries or high-𝑍 capping layers can restore linearity. [10] Complementary longitudinal and polar XUV-MOKE geometries are introduced, both exhibiting a robust linear response. The polar geometry, in particular, enables ultrafast measurements on out-of-plane magnetized samples, including epitaxial films as well as crystalline and two-dimensional materials that cannot be prepared on XUV-transparent membranes, and yields large asymmetries that significantly expand the range of sample structures accessible to ultrafast spectroscopy
studies [11].
In ongoing work, these capabilities are applied to laser-driven spin reorientation in DyCo [12] as well as fluence-dependent sublattice dynamics in FeNi alloys, providing microscopic insight into element-resolved pathways of ultrafast magnetic processes. [13]

References
[1] D. Rudolf, C. La-O-Vorakiat, M. Battiato, R. Adam, J. M. Shaw, E. Turgut, P. Maldonado,
S. Mathias, P. Grychtol, H. T. Nembach, T. J. Silva, M. Aeschlimann, H. C. Kapteyn, M. M.
Murnane, C. M. Schneider, and P. M. Oppeneer, “Ultrafast magnetization enhancement in
metallic multilayers driven by superdiffusive spin current”, en, Nature Communications 3,
Publisher: Nature Publishing Group, 1037 (2012).
[2] R. Gupta, F. Cosco, R. S. Malik, X. Chen, S. Saha, A. Ghosh, T. Pohlmann, J. R. L. Mardegan,
S. Francoual, R. Stefanuik, J. Söderström, B. Sanyal, O. Karis, P. Svedlindh, P. M. Oppeneer,
and R. Knut, “Element-resolved evidence of superdiffusive spin current arising from ultrafast demagnetization process”, Physical Review B 108, Publisher: American Physical Society (APS), 064427 (2023).
[3] C. D. Stanciu, F. Hansteen, A. V. Kimel, A. Kirilyuk, A. Tsukamoto, A. Itoh, and T. Rasing,
“All-Optical Magnetic Recording with Circularly Polarized Light”, Physical Review Letters 99,
047601 (2007).