X-ray absorption spectroscopy (XAS) and the directly linked X-ray reflectivity near absorption edges yield a wealth of specific information on the electronic structure around the resonantly addressed element. Observing the dynamic response of complex materials to optical excitations in pump–probe experiments requires high sensitivity to small changes in the spectra which in turn necessitates the brilliance of free electron laser (FEL) pulses. However, due to the fluctuating spectral content of pulses generated by self-amplified spontaneous emission (SASE), FEL experiments often struggle to reach the full sensitivity and time-resolution that FELs can in principle enable. Here, we implement a setup which solves two common challenges in this type of spectroscopy using FELs: First, we achieve a high spectral resolution by using a spectrometer downstream of the sample instead of a monochromator upstream of the sample. Thus, the full FEL bandwidth contributes to the measurement at the same time, and the FEL pulse duration is not elongated by a monochromator. Second, the FEL beam is divided into identical copies by a transmission grating beam splitter so that two spectra from separate spots on the sample (or from the sample and known reference) can be recorded in-parallel with the same spectrometer, enabling a spectrally resolved intensity normalization of pulse fluctuations in pump–probe scenarios. We analyze the capabilities of this setup around the oxygen K- and nickel L-edges recorded with third harmonic radiation of the free electron laser in Hamburg (FLASH), demonstrating the capability for pump–probe measurements with sensitivity to reflectivity changes on the per mill level.
Magnetic nanoparticles such as FePt in the L1
0
phase are the bedrock of our current data storage technology. As the grains become smaller to keep up with technological demands, the superparamagnetic limit calls for materials with higher magnetocrystalline anisotropy. This, in turn, reduces the magnetic exchange length to just a few nanometers, enabling magnetic structures to be induced within the nanoparticles. Here, we describe the existence of spin-wave solitons, dynamic localized bound states of spin-wave excitations, in FePt nanoparticles. We show with time-resolved x-ray diffraction and micromagnetic modeling that spin-wave solitons of sub–10 nm sizes form out of the demagnetized state following femtosecond laser excitation. The measured soliton spin precession frequency of 0.1 THz positions this system as a platform to develop novel miniature devices.
Ever since its first
observation, the microscopic origin of ultrafast
magnetization dynamics has been actively debated. Even more questions
arise when considering composite materials featuring a combination
of intrinsic and proximity-induced magnetic moments. Currently, it
is unknown whether the specific ultrafast dynamics of different sublattices
in the popular ferromagnets consisting of 3d (Co, Fe) and 4d, 5d (Pd,
Pt) transition metals are playing a crucial role in various effects,
including all-optical magnetization switching. Here we investigate
the element-specific dynamics of Co–Pt alloys on femtosecond
and picosecond time scales using magneto-optical spectroscopy in the
extended ultraviolet (EUV) region. Our results reveal that despite
the proximity-induced nature of the magnetization of Pt atoms, the
two sublattices in the alloy can have different responses to the optical
excitation featuring distinct demagnetization rates. Additionally
we show that it is important to consider the modification of magnetic
anisotropy in opto-magnetic experiments as the vast majority of them
are sensitive only to a single projection of the magnetic moment on
the predefined axis, which may lead to experimental artifacts.
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