13 research outputs found
The FemtoSpeX facility at BESSY II
The FemtoSpeX facility of the BESSY II storage ring is dedicated to ultrafast optical-pump & soft x-ray probe experiments. Experimental end-stations for experiments in transmission, reflection, and diffraction geometry are available
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Robust Magnetic Order Upon Ultrafast Excitation of an Antiferromagnet
The ultrafast manipulation of magnetic order due to optical excitation is governed by the intricate flow of energy and momentum between the electron, lattice, and spin subsystems. While various models are commonly employed to describe these dynamics, a prominent example being the microscopic three temperature model (M3TM), systematic, quantitative comparisons to both the dynamics of energy flow and magnetic order are scarce. Here, an M3TM was applied to the ultrafast magnetic order dynamics of the layered antiferromagnet GdRh2Si2. The femtosecond dynamics of electronic temperature, surface ferromagnetic order, and bulk antiferromagnetic order were explored at various pump fluences employing time- and angle-resolved photoemission spectroscopy and time-resolved resonant magnetic soft X-ray diffraction, respectively. After optical excitation, both the surface ferromagnetic order and the bulk antiferromagnetic order dynamics exhibit two-step demagnetization behaviors with two similar timescales (<1 ps, ∼10 ps), indicating a strong exchange coupling between localized 4f and itinerant conduction electrons. Despite a good qualitative agreement, the M3TM predicts larger demagnetization than the experimental observation, which can be phenomenologically described by a transient, fluence-dependent increased Néel temperature. The results indicate that effects beyond a mean-field description have to be considered for a quantitative description of ultrafast magnetic order dynamics
a versatile optical pump–soft X-ray probe facility with 100 fs X-ray pulses of variable polarization
Here the major upgrades of the femtoslicing facility at BESSY II (Khan et al.,
2006) are reviewed, giving a tutorial on how elliptical-polarized ultrashort
soft X-ray pulses from electron storage rings are generated at high repetition
rates. Employing a 6 kHz femtosecond-laser system consisting of two amplifiers
that are seeded by one Ti:Sa oscillator, the total average flux of photons of
100 fs duration (FWHM) has been increased by a factor of 120 to up to 106
photons s-1 (0.1% bandwidth)-1 on the sample in the range from 250 to 1400 eV.
Thanks to a new beamline design, a factor of 20 enhanced flux and improvements
of the stability together with the top-up mode of the accelerator have been
achieved. The previously unavoidable problem of increased picosecond-
background at higher repetition rates, caused by `halo' photons, has also been
solved by hopping between different `camshaft' bunches in a dedicated fill
pattern (`3+1 camshaft fill') of the storage ring. In addition to an increased
X-ray performance at variable (linear and elliptical) polarization, the sample
excitation in pump-probe experiments has been considerably extended using an
optical parametric amplifier that supports the range from the near-UV to the
far-IR regime. Dedicated endstations covering ultrafast magnetism experiments
based on time-resolved X-ray circular dichroism have been either upgraded or,
in the case of time-resolved resonant soft X-ray diffraction and reflection,
newly constructed and adapted to femtoslicing requirements. Experiments at low
temperatures down to 6 K and magnetic fields up to 0.5 T are supported. The
FemtoSpeX facility is now operated as a 24 h user facility enabling a new
class of experiments in ultrafast magnetism and in the field of transient
phenomena and phase transitions in solids
Analysis of the halo background in femtosecond slicing experiments
The slicing facility FemtoSpeX at BESSY II offers unique opportunities to study photo-induced dynamics on femtosecond time scales by means of X-ray magnetic circular dichroism, resonant and non-resonant X-ray diffraction, and X-ray absorption spectroscopy experiments in the soft X-ray regime. Besides femtosecond X-ray pulses, slicing sources inherently also produce a so-called `halo' background with a different time structure, polarization and pointing. Here a detailed experimental characterization of the halo radiation is presented, and a method is demonstrated for its correct and unambiguous removal from femtosecond time-resolved data using a special laser triggering scheme as well as analytical models. Examples are given for time-resolved measurements with corresponding halo correction, and errors of the relevant physical quantities caused by either neglecting or by applying a simplified model to describe this background are estimated
Ultrafast laser-induced magneto-optical changes in resonant magnetic x-ray reflectivity
<p>Datasets for the publication "Ultrafast laser-induced magneto-optical changes in resonant magnetic x-ray reflectivity", published in Physical Review B <strong>108</strong>, 054439 (2023).</p><p> </p>
Photoinduced Transient States of Antiferromagnetic Orderings in LaSrFeO and SrFeO Thin Films Observed through Time-resolved Resonant Soft X-ray Scattering
The relationship between the magnetic interaction and photoinduced dynamics
in antiferromagnetic perovskites is investigated in this study. In
LaSrFeO thin films, commensurate spin ordering is
accompanied by charge disproportionation, whereas SrFeO thin films show
incommensurate helical antiferromagnetic spin ordering due to increased
ferromagnetic coupling compared to LaSrFeO. To
understand the photoinduced spin dynamics in these materials, we investigate
the spin ordering through time-resolved resonant soft X-ray scattering. In
LaSrFeO, ultrafast quenching of the magnetic
ordering within 130 fs through a nonthermal process is observed, triggered by
charge transfer between the Fe atoms. We compare this to the photoinduced
dynamics of the helical magnetic ordering of SrFeO. We find that the
change in the magnetic coupling through optically induced charge transfer can
offer an even more efficient channel for spin-order manipulation.Comment: 7 pages, 5 figure
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Measurement of Spin Dynamics in a Layered Nickelate Using X-Ray Photon Correlation Spectroscopy: Evidence for Intrinsic Destabilization of Incommensurate Stripes at Low Temperatures.
We study the temporal stability of stripe-type spin order in a layered nickelate with x-ray photon correlation spectroscopy and observe fluctuations on timescales of tens of minutes over a wide temperature range. These fluctuations show an anomalous temperature dependence: they slow down at intermediate temperatures and speed up on both heating and cooling. This behavior appears to be directly connected with spatial correlations: stripes fluctuate slowly when stripe correlation lengths are large and become faster when spatial correlations decrease. A low-temperature decay of nickelate stripe correlations, reminiscent of what occurs in cuprates as a result of a competition between stripes and superconductivity, hence occurs via loss of both spatial and temporal correlations