Ultrafast Dynamics

In the frame of this research project we study ultrafast phenomena using modern free electron x-ray sources. Our research portfolio includes investigations of ulratrafast demagnetization [1], quenching of the resonant magnetic scattering signal [2], all optical switching, and development of novel and science enabling split-and-delay X-ray optics [3,4,5]. Our experiments are performed at various free-electron laser (FEL) facilities including LCLS, SACLA, FERMI and FLASH. We have developed an end-station for pump–probe small-angle X-ray scattering experiments at FLASH [6]. Static characterization of our sample systems via small angle X-ray scattering and Fourier transform holography is performed at 3rd generation storage rings.


Ultrafast demagnetization on the nanoscale - Investigating the role of ultrafast spin currents

Magnetic systems show the fascinating physical phenomenon of ultrafast demagnetization when optically excited by ultrashort infrared light pulses occuring on timescales down to 100 femtoseconds, defining the fastest time scales possible for future nanoscale magnetic storage devices. Although ultrafast demagnetization was discovered already in 1996 and was intensively studied in the last two decades, many open questions still persist. One important question is to understand how ultrafast demagnetization happens on the nanoscale.

In order to tackle this question we studied ultrafast demagnetization of a magnetic maze domain pattern (Fig. 1(a), inset) by means of magnetic small angle X-ray scattering (mSAXS) [1,6]. In the case of a maze domain pattern mSAXS yields a scattering ring on the detector. The intensity of the ring corresponds to the absolute square of the magnetization of the domains, while the radius reflects the domain size and the dimension of the domain wall, i.e. the region where the magnetization tilts from one domain to the other on typical length scales of 10 nm (transition region between black and yellow domains in Fig. 1(a), inset). The scattering results show ultrafast demagnetization (decreasing intensity of the ring over time delay, Fig. 1(b)) as expected. In addition, the ring diameter vs time delay decreases, which can be explained with a broadening of the domain wall. The latter is a consequence of optically excited electrons (super-diffusive currents) in the material that scatter at the domain wall. Our finding shows that super-diffusive currents seem to play a decisive role in ultrafast demagnetization.

Fig. 1: (a) The magnetic sample with a maze domain pattern is pumped by an optical laser and probed by a delayed X-ray pulse produced by a free-electron laser. The mSAXS pattern is recorded on an IR-protected CCD camera while the intense, directly transmitted radiation is blocked by a beamstop. The inset shows a typical magnetic force micrograph of the sample in the probed maze-domain state. The scale bar in the micrograph corresponds to 2 μm distance. (b) We observe a decrease in the intensity and a shift of the peak position Δqpeak, when comparing unpumped (blue) and pumped (green) scattering curves (pump fluence: 14.2 mJ/cm², time delay: 1.3 ps).

References

[1] B. Pfau et al. Nat. Commun. 3, 1100 (2012).
[6] L. Müller et al. Rev. Sci. Instrum. 84, 013906 (2013).


Quenching of the resonant magnetic X-ray scattering signal - Manipulating magnetism using intense and ultrashort X-ray pulses

With the advent of free-electron lasers (FELs), new opportunities have emerged for studying dynamics in matter on ultra-fast time and ultra-short length scales simultaneously. Studying magnetization dynamics, like e.g. ultrafast demagnetization or all-optical switching, rely on achieving magnetic scattering contrast through the X-ray magnetic circular dichroism (XMCD) effect by tuning the incident X-ray photon energy resonantly to one of the dichroic M or L absorption edges of the magnetic element.

Up to now only little attention has been given to the possible influence of the intense FEL pulses on the observed dynamics and the FEL pulse is commonly considered as a pure probe only. Therefore, we investigated the scattering signal at low and high X-ray fluences (Fig. 2). At extreme FEL fluences, we discovered a quenching of the resonant magnetic scattering signal revealing that the FEL radiation does not only act as a probe but also strongly interacts with the sample, effectively altering it already on a time scale shorter than the pulse duration of 70 fs [2]. Hence, the credo ‘diffract before destruct’ is violated. Moreover, our recent results show that the intra-pulse quenching effect already sets in at low fluences where the sample is neither destroyed nor permanently altered by the X-ray radiation defining a relatively low threshold fluence for FEL experiments where the FEL is meant to be a non-invasive probe.

Fig. 2: Scattering images from a stripe domain pattern. A similar total number of incident photons were used for both patterns, however, once within 1000 successive FEL pulses (left) and once in a single pulse (right). The scattered intensity appears to be very different. The region of highest intensity in the high fluence case is a consequence of Coulomb explosion.

References

[2] L. Müller et al. Phys. Rev. Lett. 110, 234801 (2013).


All optical switching (AOS) - Future of magnetic recording

All-optical switching (AOS) describes the infrared laser induced reversal of magnetization orientation in e.g. a thin magnetic film. In 2007, AOS was first discovered in 3d-4f ferrimagnetic compounds like GdFeCo where the different demagnetization times of the magnetic sublattices result in magnetization reversal via a transient ferromagnetic state. Interestingly, AOS was also found for ferromagnetic systems in 2014, while the underlying processes are not well understood, so far.

In order to identify the mechanism of AOS in ferromagnetic systems we plan to follow the transient states during AOS on a femtosecond to nanosecond time-scale combined with nanometer resolution which we achieve by means of magnetic small angle X-ray scattering (mSAXS) experiments at free electron laser facilities like FLASH or FERMI. Fig. 3 shows the top view of magnetic states in a ferromagnetic Co/Pt thin film with magnetization direction pointing parallel to the surface normal (up/down = black/white) particularly revealing optical induced changes of the magnetic state after femtosecond laser radiation.

Fig. 3: Magneto-optical Kerr images of a [Co(0.8 nm)/Pt(1.4 nm)]2 sample in a) the multi-domain magnetic ground state with an average domain size of ~10 µm and b) the single domain state. The gray region at the center was 'switched' by ~100 shots of a circularly polarized IR laser with a pulse length of 50 fs.


Split-and-delay unit - A tool for studying ultrafast dynamics

X-ray free electron laser (XFEL) sources produce ultrashort, coherent and very intense pulses creating excellent conditions for investigation of ultrafast phenomena. However, the time structure of an XFEL source can compromise the applicability of experimental techniques such as X-ray pump/X-ray probe or X-ray photon correlation spectroscopy (XPCS). XFEL facilities provide ultra short (< 100 fs) single pulses at relatively low frequencies (120 Hz at LCLS in USA, 60 Hz at SACLA in Japan).

To fully exploit the potential of XFEL light with the aforementioned techniques we have successfully developed the first hard X-ray split-and-delay line [3,4,5,6,7] and employed it to perform first split-pulse XPCS at hard X-ray FEL [8]. Figure 4 illustrates this approach with the XPCS method. The experimental technique is based on splitting each X-ray pulse into two equal pulses, separating them in time and bringing them back on the primary beam path. The speckle pattern obtained from the sample, which is illuminated by both pulses, is summed on the area detector. The dynamics of the sample are studied by analyzing the contrast in the summed speckle patterns as a function of the applied delay times (i.e. time intervals between the two pulses).

Fig. 4: Illustration of split-and-delay XPCS technique. A single X-ray pulse is split into two equal intensity pulses using the hard X-ray delay line. The pulses propagate collinearly in the sample direction. (Inset): Concept of the hard X-ray delay line. BR-1, BR-2, BR-3, BR-4, BR-5, BR-6, Bragg reflectors; BS: beam splitter, BM: beam mixer. Paths of the upper and lower branch of the delay line are denoted by red and blue lines, respectively.

References

[3] W. Roseker et al. Opt. Lett. 34, 1768 (2009).

[4] W. Roseker et al. J. Synchr. Rad. 18, 481 (2011).

[5] W. Roseker et al. Proc. SPIE 8504, 850401 (2012).

[6] W.Roseker et al., Sci Rep. 10,  5054 (2020)

[7] W.Roseker et al., Rev Sci Instr.  90(4), 045106 (2019)

[8] W.Roseker et al., Nat. Comm. 9 1704 (2018)