Controlling magnetism on ultra-fast time scales and on nanometer lengthscales is a challenge for modern research in magnetism. Means for inducingdynamics on these time scales are femtosecond optical lasers [1] and THzsources [2]. Probing the dynamics on a nanometer length scale is possiblewith free-electron laser sources. We report on a THz-pump–XUV-probescattering experiment on (Co/Pt)n multilayers (n = 8,16) with perpendicularmagnetic anisotropy (PMA) exhibiting a maze domain pattern. An additionalelectromagnetic undulator available at FLASH was used to produce10-cycle linearly polarized THz pulses. The fundamental wavelength wasset to 150 μm and higher harmonics down to 30 μm have been used as apump. The resulting dynamics have been probed on femtosecond time scales[3] by resonant magnetic small-angle scattering at the cobalt M3 edge. For amultilayer with 8-fold repetition we observed that after 200 fs the scatteringintensity is drastically decreased by one order of magnitude (blue curve inFig. 1(a)). This change is fast compared to the duration of the THz pumppulse, which is about 6 ps long. Besides, a shift of the scattering peak positionto lower Q-values by 14% occurred (red curve in Fig. 1(a)). The latteris similar to what was found when using NIR pumping [4]. However, herewe observed an onset of the peak shift delayed by about 100 fs with respectto the reduction in scattering intensity and a different shape of both signals.Such subtle differences were impossible to resolve in the previous experimentsusing NIR-pump pulses due to the larger temporal jitter (> 100fs).Interestingly, the response is found to be much weaker (scattering intensity)or not resolvable (peak shift) in case of a Co/Pt multilayer with 16-foldrepetition (Fig. 1(b)). The major difference is the PMA of both samples.While the 16-fold multilayer has a strong PMA (K1,eff = 200kJ/m3) the 8-foldmultilayer has an almost vanishing PMA (K1,eff = 30kJ/m3), so that it is muchmore susceptible to magnetic fields and hence the THz magnetic field cancause a significant tilting of the magnetization.[1] E. Beaurepaire, J. Merle, A. Daunois, and J. Bigot, Phys. Rev. Lett. 76,4250 (1996). [2] C. Vicario, C. Ruchert, F. Ardana-Lamas, P.M. Derlet, etal. Nat. Photon. 7, 720 (2013). [3] F. Tavella, N. Stojanovic, G. Geloni, andM. Gensch, Nat Photon. 5, 162 (2011). [4] B. Pfau, S. Schaffert, L. Müller,C. Gutt, A. Al-Shemmary, et al., Nat. Commun. 3, 1100 (2012)
