963 research outputs found
Concept of a laser-plasma based electron source for sub-10 fs electron diffraction
We propose a new concept of an electron source for ultrafast electron
diffraction with sub-10~fs temporal resolution. Electrons are generated in a
laser-plasma accelerator, able to deliver femtosecond electron bunches at 5 MeV
energy with kHz repetition rate. The possibility of producing this electron
source is demonstrated using Particle-In-Cell simulations. We then use particle
tracking simulations to show that this electron beam can be transported and
manipulated in a realistic beamline, in order to reach parameters suitable for
electron diffraction. The beamline consists of realistic static magnetic optics
and introduces no temporal jitter. We demonstrate numerically that electron
bunches with 5~fs duration and containing 1.5~fC per bunch can be produced,
with a transverse coherence length exceeding 2~nm, as required for electron
diffraction
Current Induced Fingering Instability in Magnetic Domain Walls
The shape instability of magnetic domain walls under current is investigated
in a ferromagnetic (Ga,Mn)(As,P) film with perpendicular anisotropy. Domain
wall motion is driven by the spin transfer torque mechanism. A current density
gradient is found either to stabilize domains with walls perpendicular to
current lines or to produce finger-like patterns, depending on the domain wall
motion direction. The instability mechanism is shown to result from the
non-adiabatic contribution of the spin transfer torque mechanism.Comment: 5 pages, 3 figures + supplementary material
Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy
The time and field dependence of the magnetic domain structure at
magnetization reversal were investigated by Kerr microscopy in interacting
ferromagnetic Co/Pt multilayers with perpendicular anisotropy. Large local
inhomogeneous magnetostatic fields favor mirroring domain structures and domain
decoration by rings of opposite magnetization. The long range nature of these
magnetostatic interactions gives rise to ultra-slow dynamics even in zero
applied field, i.e. it affects the long time domain stability. Due to this
additionnal interaction field, the magnetization reversal under short magnetic
field pulses differs markedly from the well-known slow dynamic behavior.
Namely, in high field, the magnetization of the coupled harder layer has been
observed to reverse more rapidly by domain wall motion than the softer layer
alone.Comment: 42 pages including 17 figures. submitted to JA
Two-dimensional nanosecond electric field mapping based on cell electropermeabilization
Nanosecond, megavolt-per-meter electric pulses cause permeabilization of cells to small molecules, programmed cell death (apoptosis) in tumor cells, and are under evaluation as a treatment for skin cancer. We use nanoelectroporation and fluorescence imaging to construct two-dimensional maps of the electric field associated with delivery of 15 ns, 10 kV pulses to monolayers of the human prostate cancer cell line PC3 from three different electrode configurations: single-needle, five-needle, and flat-cut coaxial cable. Influx of the normally impermeant fluorescent dye YO-PRO-1 serves as a sensitive indicator of membrane permeabilization. The level of fluorescence emission after pulse exposure is proportional to the applied electric field strength. Spatial electric field distributions were compared in a plane normal to the center axis and 15-20 μm from the tip of the center electrode. Measurement results agree well with models for the three electrode arrangements evaluated in this study. This live-cell method for measuring a nanosecond pulsed electric field distribution provides an operationally meaningful calibration of electrode designs for biological applications and permits visualization of the relative sensitivities of different cell types to nanoelectropulse stimulation. PACS Codes: 87.85.M
Magnetic patterning of (Ga,Mn)As by hydrogen passivation
We present an original method to magnetically pattern thin layers of
(Ga,Mn)As. It relies on local hydrogen passivation to significantly lower the
hole density, and thereby locally suppress the carrier-mediated ferromagnetic
phase. The sample surface is thus maintained continuous, and the minimal
structure size is of about 200 nm. In micron-sized ferromagnetic dots
fabricated by hydrogen passivation on perpendicularly magnetized layers, the
switching fields can be maintained closer to the continuous film coercivity,
compared to dots made by usual dry etch techniques
Life Cycle of an Electropore: A Molecular Dynamics Investigation of the Electroporation of Heterogeneous Lipid Bilayers (PC:PS) In the Presence of Calcium Ions
Magnetic properties and domain structure of (Ga,Mn)As films with perpendicular anisotropy
The ferromagnetism of a thin GaMnAs layer with a perpendicular easy
anisotropy axis is investigated by means of several techniques, that yield a
consistent set of data on the magnetic properties and the domain structure of
this diluted ferromagnetic semiconductor. The magnetic layer was grown under
tensile strain on a relaxed GaInAs buffer layer using a procedure that limits
the density of threading dislocations. Magnetometry, magneto-transport and
polar magneto-optical Kerr effect (PMOKE) measurements reveal the high quality
of this layer, in particular through its high Curie temperature (130 K) and
well-defined magnetic anisotropy. We show that magnetization reversal is
initiated from a limited number of nucleation centers and develops by easy
domain wall propagation. Furthermore, MOKE microscopy allowed us to
characterize in detail the magnetic domain structure. In particular we show
that domain shape and wall motion are very sensitive to some defects, which
prevents a periodic arrangement of the domains. We ascribed these defects to
threading dislocations emerging in the magnetic layer, inherent to the growth
mode on a relaxed buffer
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