18 research outputs found
Analysis of charge and orbital order in Fe_{3}O_{4} by Fe L_{2,3} resonant x-ray diffraction
To elucidate charge and orbital order below the Verwey transition temperature
TV∼125 K, a thin layer of magnetite partially detwined by growth on the
stepped MgO(001) substrate has been studied by means of soft x-ray diffraction
at the Fe L2,3 resonance. The azimuth angle, incident photon polarization, and
energy dependence of the (0012)c and (001)c reflection intensities have been
measured, and analyzed using a configuration-interaction FeO6 cluster model.
The azimuth dependence of the (0012)c reflection intensities directly
represents the space-group symmetry of the orbital order in the initial state
rather than indirectly through the intermediate-state level shifts caused by
the order-induced lattice distortions. From the analysis of the (0012)c
reflection intensities, the orbital order in the t2g orbitals of B sites below
TV is proved to have a large monoclinic deformation with the value of
Re[Fxy]/Re[Fyz]∼2. This finding contradicts the majority of theories on the
Verwey transition so far proposed. We show that the experimentally observed
resonance spectra cannot be explained by orbital and charge orders obtained
with recent LDA+U and GGA+U band structure calculations but by a complex-
number orbital order with excellent agreement
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
Ultrafast photoinduced phase transitions in complex materials probed by time resolved resonant soft x ray diffraction
Ultrafast and Energy-Efficient Quenching of Spin Order: Antiferromagnetism Beats Ferromagnetism
By comparing femtosecond laser pulse induced ferro- and antiferromagnetic dynamics in one and the same material-metallic dysprosium-we show both to behave fundamentally different. Antiferromagnetic order is considerably faster and much more efficiently reduced by optical excitation than its ferromagnetic counterpart. We assign the fast and extremely efficient process in the antiferromagnet to an interatomic transfer of angular momentum within the spin system. Our findings imply that this angular momentum transfer channel is effective in other magnetic metals with nonparallel spin alignment. They also point out a possible route towards energy-efficient spin manipulation for magnetic devices
Die Sprache der Götter, Helden und Menschen. Zur Aktualität von Giambattista Vico an seinem 350. Geburtstag
Ultrafast and Energy-Efficient Quenching of Spin Order: Antiferromagnetism Beats Ferromagnetism
Direct Visualization of Spatial Inhomogeneity of Spin Stripes Order in La1.72Sr0.28NiO4
In several strongly correlated electron systems, the short range ordering of defects, charge and local lattice distortions are found to show complex inhomogeneous spatial distributions. There is growing evidence that such inhomogeneity plays a fundamental role in unique functionality of quantum complex materials. La1.72Sr0.28NiO4 is a prototypical strongly correlated perovskite showing spin stripes order. In this work we present the spatial distribution of the spin order inhomogeneity by applying micro X-ray diffraction to La1.72Sr0.28NiO4, mapping the spin-density-wave order below the 120 K onset temperature. We find that the spin-density-wave order shows the formation of nanoscale puddles with large spatial fluctuations. The nano-puddle density changes on the microscopic scale forming a multiscale phase separation extending from nanoscale to micron scale with scale-free distribution. Indeed spin-density-wave striped puddles are disconnected by spatial regions with negligible spin-density-wave order. The present work highlights the complex spatial nanoscale phase separation of spin stripes in nickelate perovskites and opens new perspectives of local spin order control by strain
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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