Terahertz Radiation driven Dynamics of Magnetic Domain Structures probed by coherent Free-Electron Laser Light

The new free-electron laser (FEL) sources provide radiation with unprecedented parameters in terms of ultrashort pulse length, high photon flux, and coherence. These properties make FELs ideal tools for studying ultrafast dynamics in matter on a previously inaccessible level.
Tuning the FEL photon energy resonantly to the magneto-dichroic transition of cobalt at 59.6 eV (equivalent to a wavelength of 20.8 nm) yields magnetic scattering contrast from a thin cobalt/platinum multilayer sample via the X-ray magnetic dichroism effect. Due to their magnetic anisotropy, these samples show domains magnetized in the out-of-plane direction with a typical domain width of 80 nm. For our transmission small-angle X-ray scattering (SAXS) geometry the magnetic scattering signal, therefore, originates from circular dichroism only.
After magnetic scattering of resonantly tuned XUV radiation from magnetic domains systems was proven feasible at FELs, ultrafast demagnetization, discovered initially by Beaurepaire in 1996, was for the first time measured in a mesoscopic magnetic domain systems at FLASH in Hamburg in an IR pump – FEL probe type of experiment. An ultrafast spatial response was found to accompany the demagnetization process.
In a later experimental approach, magnetic domain systems were pumped using THz radiation and probed by SAXS using XUV radiation from the FEL source. The 10-cycle THz pulse is produced by an additional electromagnetic undulator available at FLASH and therefore allows for measurements with minimal time jitter. In our experimental configuration, the THz magnetic field in the sample plane was ~ 20 mT. For samples in their close-to-equilibrium maze-domain configuration no change in form or strength of the scattering pattern was observed. However, when putting the sample in a static magnetic field of a few 10 mT, the magnetic domains are partially aligned along the magnetic field resulting in an anisotropic scattering pattern. We find that, in dependence of the pump-probe delay time, this anisotropy changes on time scales of a few picoseconds (Fig. 2). This experiment shows that THz radiation can affect magnetic domain systems directly which can be of great interest for future FEL experiments concentrating on THz control of magnetism.