25 research outputs found
Achieving diffraction-limited resolution in soft-X-ray Fourier-transform holography
The spatial resolution of microscopic images acquired via X-ray Fourier-transform holography is limited by the source size of the reference wave and by the numerical aperture of the detector. We analyze the interplay between both influences and show how they are matched in practice. We further identify, how high spatial frequencies translate to imaging artifacts in holographic reconstructions where mainly the reference beam limits the spatial resolution. As a solution, three methods are introduced based on numerical post-processing of the reconstruction. The methods comprise apodization of the hologram, refocusing via wave propagation, and deconvolution using the transfer function of the imaging system. In particular for the latter two, we demonstrate that image details smaller than the source size of the reference beam can be recovered up to the diffraction limit of the hologram. Our findings motivate the intentional application of a large reference-wave source enhancing the image contrast in applications with low photon numbers such as single-shot experiments at free-electron lasers or imaging at laboratory sources.BMBF, 05K10KTB, Verbundprojekt: FSP 301 - FLASH: Nanoskopische Systeme. Teilprojekt 1.1: Universelle Experimentierkammer fĂŒr Streuexperimente mit kohĂ€renten Femtosekunden-Röntgenpulsen Multi Purpose Coherent Scattering Chamber for FLASH and XFEL 'MPscatt
Experimental evaluation of signal-to-noise in spectro-holography via modified uniformly redundant arrays in the soft x-ray and extreme ultraviolet spectral regime
We present dichroic x-ray lensless magnetic imaging by Fourier transform holography with an extended reference scheme via a modified uniformly redundant array (mURA). Holographic images of magnetic domains simultaneously generated by a single pinhole reference as well as by a mURA reference are compared with respect to the signal-to-noise ratio (SNR) as a function of exposure time. We apply this approach for spectro-holographic imaging of ferromagnetic domain patterns in Co/Pt multilayer films. Soft x-rays with wavelengths of 1.59 nm (Co L 3 absorption edge) and 20.8 nm (Co M 2,3 absorption edges) are used for image formation and to generate contrast via x-ray magnetic circular dichroism. For a given exposure time, the mURA-based holography allows to decouple the reconstruction SNR from the spatial resolution. For 1.59 nm wavelength, the reconstruction via the extended reference scheme shows no significant loss of spatial resolution compared to the single pinhole reference. In contrast, at 20.8 nm wavelength the single pinhole reveals some very intricate features which are lost in the image generated by the mURA, although overall a high-quality image is generated. The SNR-advantage of the mURA scheme is most notable when the hologram has to be encoded with few photons, while errors associated with the increased complexity of the reconstruction process reduce the advantage for high-photon-number experiments.BMBF, 05K13KT3, Verbundprojekt 05K2013 - DynaMaX: Messplatz fĂŒr ultraschnelle Dynamik bei BESSY II. Teilprojekt
Materials Informatics for Dark Matter Detection
Dark Matter particles are commonly assumed to be weakly interacting massive
particles (WIMPs) with a mass in the GeV to TeV range. However, recent interest
has shifted towards lighter WIMPs, which are more difficult to probe
experimentally. A detection of sub-GeV WIMPs would require the use of small gap
materials in sensors. Using recent estimates of the WIMP mass, we identify the
relevant target space towards small gap materials (100-10 meV). Dirac
Materials, a class of small- or zero-gap materials, emerge as natural
candidates for sensors for Dark Matter detection. We propose the use of
informatics tools to rapidly assay materials band structures to search for
small gap semiconductors and semimetals, rather than focusing on a few
preselected compounds. As a specific example of the proposed strategy, we use
the organic materials database (omdb.diracmaterials.org) to identify organic
candidates for sensors: the narrow band gap semiconductors BNQ-TTF and DEBTTT
with gaps of 40 and 38 meV, and the Dirac-line semimetal (BEDT-TTF)Br
which exhibits a tiny gap of 50 meV when spin-orbit coupling is
included. We outline a novel and powerful approach to search for dark matter
detection sensor materials by means of a rapid assay of materials using
informatics tools.Comment: 5 pages, 3 figure
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Experimental evaluation of signal-to-noise in spectro-holography via modified uniformly redundant arrays in the soft x-ray and extreme ultraviolet spectral regime
We present dichroic x-ray lensless magnetic imaging by Fourier transform holography with an extended reference scheme via a modified uniformly redundant array (mURA). Holographic images of magnetic domains simultaneously generated by a single pinhole reference as well as by a mURA reference are compared with respect to the signal-to-noise ratio (SNR) as a function of exposure time. We apply this approach for spectro-holographic imaging of ferromagnetic domain patterns in Co/Pt multilayer films. Soft x-rays with wavelengths of 1.59 nm (Co L3 absorption edge) and 20.8 nm (Co M2,3 absorption edges) are used for image formation and to generate contrast via x-ray magnetic circular dichroism. For a given exposure time, the mURA-based holography allows to decouple the reconstruction SNR from the spatial resolution. For 1.59 nm wavelength, the reconstruction via the extended reference scheme shows no significant loss of spatial resolution compared to the single pinhole reference. In contrast, at 20.8 nm wavelength the single pinhole reveals some very intricate features which are lost in the image generated by the mURA, although overall a high-quality image is generated. The SNR-advantage of the mURA scheme is most notable when the hologram has to be encoded with few photons, while errors associated with the increased complexity of the reconstruction process reduce the advantage for high-photon-number experiments
Irreversible transformation of ferromagnetic ordered stripe domains in single-shot IR pump - resonant X-ray scattering probe experiments
The evolution of a magnetic domain structure upon excitation by an intense,
femtosecond Infra-Red (IR) laser pulse has been investigated using single-shot
based time-resolved resonant X-ray scattering at the X-ray Free Electron laser
LCLS. A well-ordered stripe domain pattern as present in a thin CoPd alloy film
has been used as prototype magnetic domain structure for this study. The
fluence of the IR laser pump pulse was sufficient to lead to an almost complete
quenching of the magnetization within the ultrafast demagnetization process
taking place within the first few hundreds of femtoseconds following the IR
laser pump pulse excitation. On longer time scales this excitation gave rise to
subsequent irreversible transformations of the magnetic domain structure. Under
our specific experimental conditions, it took about 2 nanoseconds before the
magnetization started to recover. After about 5 nanoseconds the previously
ordered stripe domain structure had evolved into a disordered labyrinth domain
structure. Surprisingly, we observe after about 7 nanoseconds the occurrence of
a partially ordered stripe domain structure reoriented into a novel direction.
It is this domain structure in which the sample's magnetization stabilizes as
revealed by scattering patterns recorded long after the initial pump-probe
cycle. Using micro-magnetic simulations we can explain this observation based
on changes of the magnetic anisotropy going along with heat dissipation in the
film.Comment: 16 pages, 6 figure
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Ultrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges
We systematically study the fluence dependence of the resonant scattering cross-section from magnetic domains in Co/Pd-based multilayers. Samples are probed with single extreme ultraviolet (XUV) pulses of femtosecond duration tuned to the Co M3,2 absorption resonances using the FERMI@Elettra free-electron laser. We report quantitative data over 3 orders of magnitude in fluence, covering 16ââmJ/cm2/pulse to 10â000ââmJ/cm2/pulse with pulse lengths of 70 fs and 120 fs. A progressive quenching of the diffraction cross-section with fluence is observed. Compression of the same pulse energy into a shorter pulseâimplying an increased XUV peak electric fieldâresults in a reduced quenching of the resonant diffraction at the Co M3,2 edge. We conclude that the quenching effect observed for resonant scattering involving the short-lived Co 3p core vacancies is noncoherent in nature. This finding is in contrast to previous reports investigating resonant scattering involving the longer-lived Co 2p states, where stimulated emission has been found to be important. A phenomenological model based on XUV-induced ultrafast demagnetization is able to reproduce our entire set of experimental data and is found to be consistent with independent magneto-optical measurements of the demagnetization dynamics on the same samples
High-resolution magnetic-domain imaging by Fourier transform holography at 21Â nm wavelength
Exploiting x-ray magnetic circular dichroism at the L-edges of 3d transition metals, Fourier transform holography has become a standard technique to investigate magnetic samples with sub-100 nm spatial resolution. Here, magnetic imaging in the 21 nm wavelength regime using M-edge circular dichroism is demonstrated. Ultrafast pulses in this wavelength regime are increasingly available from both laser- and accelerator-driven soft x-ray sources. We explain the adaptations concerning sample preparation and data evaluation compared to conventional holography in the 1 nm wavelength range. We find the correction of the Fourier transform hologram to in-plane Fourier components to be critical for high-quality reconstruction and demonstrate 70 nm spatial resolution in magnetization imaging with this approach.BMBF, 05K10KTB, Verbundprojekt: FSP 301 - FLASH: Nanoskopische Systeme. Teilprojekt 1.1: Universelle Experimentierkammer fĂŒr Streuexperimente mit kohĂ€renten Femtosekunden-Röntgenpulsen Multi Purpose Coherent Scattering Chamber for FLASH and XFEL 'MPscatt'
Field-free deterministic ultra fast creation of skyrmions by spin orbit torques
Magnetic skyrmions are currently the most promising option to realize
current-driven magnetic shift registers. A variety of concepts to create
skyrmions were proposed and demonstrated. However, none of the reported
experiments show controlled creation of single skyrmions using integrated
designs. Here, we demonstrate that skyrmions can be generated deterministically
on subnanosecond timescales in magnetic racetracks at artificial or natural
defects using spin orbit torque (SOT) pulses. The mechanism is largely similar
to SOT-induced switching of uniformly magnetized elements, but due to the
effect of the Dzyaloshinskii-Moriya interaction (DMI), external fields are not
required. Our observations provide a simple and reliable means for skyrmion
writing that can be readily integrated into racetrack devices
Ultrafast demagnetization dominates fluence dependence of magnetic scattering at Co M edges
We systematically study the fluence dependence of the resonant scattering cross-section from magnetic domains in Co/Pd-based multilayers. Samples are probed with single extreme ultraviolet (XUV) pulses of femtosecond duration tuned to the Co M3,2 absorption resonances using the FERMI@Elettra free-electron laser. We report quantitative data over 3 orders of magnitude in fluence, covering 16ââmJ/cm2/pulse to 10â000ââmJ/cm2/pulse with pulse lengths of 70 fs and 120 fs. A progressive quenching of the diffraction cross-section with fluence is observed. Compression of the same pulse energy into a shorter pulseâimplying an increased XUV peak electric fieldâresults in a reduced quenching of the resonant diffraction at the Co M3,2 edge. We conclude that the quenching effect observed for resonant scattering involving the short-lived Co 3p core vacancies is noncoherent in nature. This finding is in contrast to previous reports investigating resonant scattering involving the longer-lived Co 2p states, where stimulated emission has been found to be important. A phenomenological model based on XUV-induced ultrafast demagnetization is able to reproduce our entire set of experimental data and is found to be consistent with independent magneto-optical measurements of the demagnetization dynamics on the same samples