20 research outputs found
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Dynamics at the crystal-melt interface in a supercooled chalcogenide liquid near the glass transition.
Direct quantitative measurements of nanoscale dynamical processes associated with structural relaxation and crystallization near the glass transition are a major experimental challenge. These type of processes have been primarily treated as macroscopic phenomena within the framework of phenomenological models and bulk experiments. Here, we report x-ray photon correlation spectroscopy measurements of dynamics at the crystal-melt interface during the radiation induced formation of Se nano-crystallites in pure Se and in binary AsSe4 glass-forming liquids near their glass transition temperature. We observe a heterogeneous dynamical behaviour where the intensity correlation functions g2(q, t) exhibits either a compressed or a stretched exponential decay, depending on the size of the Se nano-crystallites. The corresponding relaxation timescale for the AsSe4 liquid increases as the temperature is raised, which can be attributed to changes in the chemical composition of the melt at the crystal-melt interface with the growth of the Se nano-crystallites
Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5-10 GHz frequency range
We present a scanning transmission x-ray microscopy setup combined with a
novel microwave synchronization scheme in order to study high frequency
magnetization dynamics at synchrotron light sources. The sensitivity necessary
to detect small changes of the magnetization on short time scales and nanometer
spatial dimensions is achieved by combination of the developed excitation
mechanism with a single photon counting electronics that is locked to the
synchrotron operation frequency. The required mechanical stability is achieved
by a compact design of the microscope. Our instrument is capable of creating
direct images of dynamical phenomena in the 5-10 GHz range, with 35 nm
resolution. When used together with circularly polarized x-rays, the above
capabilities can be combined to study magnetic phenomena at microwave
frequencies, such as ferromagnetic resonance (FMR) and spin waves. We
demonstrate the capabilities of our technique by presenting phase resolved
images of a 6 GHz nanoscale spin wave generated by a spin torque oscillator, as
well as the uniform ferromagnetic precession with ~0.1 deg amplitude at 9 GHz
in a micrometer-sized cobalt strip.Comment: 9 pages, 7 figure
Domain fluctuations in a ferroelectric low-strain BaTiO3 thin film
A ferroelectric BaTiO3 thin film grown on a NdScO3 substrate was studied using x-ray photon correlation spectroscopy (XPCS) to characterize thermal fluctuations near the a/b to a/c domain structure transformation present in this low-strain material, which is absent in the bulk. XPCS studies provide a direct comparison of the role of domain fluctuations in first- and second-order phase transformations. The a/b to a/c domain transformation is accompanied by a decrease in fluctuation timescales, and an increase in intensity and correlation length. Surprisingly, domain fluctuations are observed up to 25 degrees C above the transformation, concomitant with the growth of a/c domains and coexistence of both domain types. After a small window of stability, as the Curie temperature is approached, a/c domain fluctuations are observed, albeit slower, potentially due to the structural transformation associated with the ferroelectric to paraelectric transformation. The observed time evolution and reconfiguration of domain patterns highlight the role played by phase coexistence and elastic boundary conditions in altering fluctuation timescales in ferroelectric thin films
Phase separation in the non-equilibrium Verwey transition in magnetite
We present equilibrium and out-of-equilibrium studies of the Verwey
transition in magnetite. In the equilibrium optical conductivity, we find a
step-like change at the phase transition for photon energies below about 2 eV.
The possibility of triggering a non-equilibrium transient metallic state in
insulating magnetite by photo excitation was recently demonstrated by an x-ray
study. Here we report a full characterization of the optical properties in the
visible frequency range across the non-equilibrium phase transition. Our
analysis of the spectral features is based on a detailed description of the
equilibrium properties. The out-of-equilibrium optical data bear the initial
electronic response associated to localized photo-excitation, the occurrence of
phase separation, and the transition to a transient metallic phase for
excitation density larger than a critical value. This allows us to identify the
electronic nature of the transient state, to unveil the phase transition
dynamics, and to study the consequences of phase separation on the
reflectivity, suggesting a spectroscopic feature that may be generally linked
to out-of-equilibrium phase separation
Nonuniform switching of the perpendicular magnetization in a spin-torque-driven magnetic nanopillar
Evidence of extreme domain wall speeds under ultrafast optical excitation
Time-resolved ultrafast EUV magnetic scattering was used to test a recent
prediction of >10 km/s domain wall speeds by optically exciting a magnetic
sample with a nanoscale labyrinthine domain pattern. Ultrafast distortion of
the diffraction pattern was observed at markedly different timescales compared
to the magnetization quenching. The diffraction pattern distortion shows a
threshold-dependence with laser fluence, not seen for magnetization quenching,
consistent with a picture of domain wall motion with pinning sites. Supported
by simulations, we show that a speed of 66 km/s for highly curved
domain walls can explain the experimental data. While our data agree with the
prediction of extreme, non-equilibrium wall speeds locally, it differs from the
details of the theory, suggesting that additional mechanisms are required to
fully understand these effects.Comment: 5 pages, 4 figures; Supplemental Material: 8 pages, 9 figure
Stimulated resonant inelastic X-ray scattering in a solid
When materials are exposed to X-ray pulses with sufficiently high intensity, various nonlinear effects can occur. The most fundamental one consists of stimulated electronic decays after resonant absorption of X-rays. Such stimulated decays enhance the number of emitted photons and the emission direction is confined to that of the stimulating incident photons which clone themselves in the process. Here we report the observation of stimulated resonant elastic (REXS) and inelastic (RIXS) X-ray scattering near the cobalt L3 edge in solid Co/Pd multilayer samples. We observe an enhancement of order 106 of the stimulated over the conventional spontaneous RIXS signal into the small acceptance angle of the RIXS spectrometer. We also find that in solids both stimulated REXS and RIXS spectra contain contributions from inelastic electron scattering processes, even for ultrashort 5 fs pulses. Our results reveal the potential and caveats of the development of stimulated RIXS in condensed matter
Symmetry-dependent ultrafast manipulation of nanoscale magnetic domains
Femtosecond optical pumping of magnetic materials has been used to achieve ultrafast switching and recently to nucleate symmetry-broken magnetic states. However, when the magnetic order parameter already presents a broken-symmetry state, such as a domain pattern, the dynamics are poorly understood and consensus remains elusive. Here, we resolve the controversies in the literature by studying the ultrafast response of magnetic domain patterns with varying degrees of translation symmetry with ultrafast x-ray resonant scattering. A data analysis technique is introduced to disentangle the isotropic and anisotropic components of the x-ray scattering. We find that the scattered intensity exhibits a radial shift restricted to the isotropic component, indicating that the far-from-equilibrium magnetization dynamics are intrinsically related to the spatial features of the domain pattern. Our results suggest alternative pathways for the spatiotemporal manipulation of magnetism via far-from-equilibrium dynamics and by carefully tuning the ground-state magnetic textures