3,872 research outputs found
Significance of interface anisotropy in laser induced magnetization precession in ferromagnetic metal films
Laser induced ultrafast demagnetization in ferromagnetic metals was
discovered almost 20 years ago, but currently there is still lack of consensus
on the microscopic mechanism responsible for the corresponding transfer of
angular momentum and energy between electron, lattice and spin subsystems. A
distinct, but intrinsically correlated phenomenon occurring on a longer
timescale is the magnetization precession after the ultrafast demagnetization
process, if a magnetic field is applied to tilt the magnetization vector away
from its easy direction, which can be attributed to the change of anisotropy
after laser heating. In an in-plane magnetized Pt/Co/Pt thin film with
perpendicular interface anisotropy, we found excellent agreement between
theoretical prediction with plausible parameters and experimental data measured
using time resolved magneto-optical Kerr effect. This agreement confirms that
the time evolution of the anisotropy field, which is driven by the interaction
between electrons and phonons, determines the magnetization precession
completely. A detailed analysis shows that, even though the whole sample is
magnetized in-plane, the dynamic interface anisotropy field dictates the
initial phase of the magnetization precession, highlighting the significance of
the interface anisotropy field in laser induced magnetization precession.Comment: 11 pages, 2 figure
Gravitational Lensing & Stellar Dynamics
Strong gravitational lensing and stellar dynamics provide two complementary
and orthogonal constraints on the density profiles of galaxies. Based on
spherically symmetric, scale-free, mass models, it is shown that the
combination of both techniques is powerful in breaking the mass-sheet and
mass-anisotropy degeneracies. Second, observational results are presented from
the Lenses Structure & Dynamics (LSD) Survey and the Sloan Lens ACS (SLACS)
Survey collaborations to illustrate this new methodology in constraining the
dark and stellar density profiles, and mass structure, of early-type galaxies
to redshifts of unity.Comment: 6 pages, 2 figures; Invited contribution in the Proceedings of XXIst
IAP Colloquium, "Mass Profiles & Shapes of Cosmological Structures" (Paris,
4-9 July 2005), eds G. A. Mamon, F. Combes, C. Deffayet, B. Fort (Paris: EDP
Sciences
Racetrack memory based on in-plane-field controlled domain-wall pinning
Magnetic domain wall motion could be the key to the next generation of data storage devices, shift registers without mechanically moving parts. Various concepts of such so-called ‘racetrack memories’ have been developed, but they are usually plagued by the need for high current densities or complex geometrical requirements. We introduce a new device concept, based on the interfacial Dzyaloshinskii-Moriya interaction (DMI), of which the importance in magnetic thin films was recently discovered. In this device the domain walls are moved solely by magnetic fields. Unidirectionality is created utilizing the recent observation that the strength with which a domain wall is pinned at an anisotropy barrier depends on the direction of the in-plane field due to the chiral nature of DMI. We demonstrate proof-of-principle experiments to verify that unidirectional domain-wall motion is achieved and investigate several material stacks for this novel device including a detailed analysis of device performance for consecutive pinning and depinning processes
Spin accumulation and dynamics in inversion-symmetric van der Waals crystals
Inversion symmetric materials are forbidden to show an overall spin texture
in their band structure in the presence of time-reversal symmetry. However, in
van der Waals materials which lack inversion symmetry within a single layer, it
has been proposed that a layer-dependent spin texture can arise leading to a
coupled spin-layer degree of freedom. Here we use time-resolved Kerr rotation
in inversion symmetric WSe and MoSe bulk crystals to study this
spin-layer polarization and unveil its dynamics. Our measurements show that the
spin-layer relaxation time in WSe is limited by phonon-scattering at high
temperatures and that the inter-layer hopping can be tunned by a small in-plane
magnetic field at low temperatures, enhancing the relaxation rates. We find a
significantly lower lifetime for MoSe which agrees with theoretical
expectations of a spin-layer polarization stabilized by the larger spin-orbit
coupling in WSe
Gravitational Microlensing Near Caustics I: Folds
We study the local behavior of gravitational lensing near fold catastrophes.
Using a generic form for the lensing map near a fold, we determine the
observable properties of the lensed images, focusing on the case when the
individual images are unresolved, i.e., microlensing. Allowing for images not
associated with the fold, we derive analytic expressions for the photometric
and astrometric behavior near a generic fold caustic. We show how this form
reduces to the more familiar linear caustic, which lenses a nearby source into
two images which have equal magnification, opposite parity, and are equidistant
from the critical curve. In this case, the simplicity and high degree of
symmetry allows for the derivation of semi-analytic expressions for the
photometric and astrometric deviations in the presence of finite sources with
arbitrary surface brightness profiles. We use our results to derive some basic
properties of astrometric microlensing near folds, in particular we predict for
finite sources with uniform and limb darkening profiles, the detailed shape of
the astrometric curve as the source crosses a fold. We find that the
astrometric effects of limb darkening will be difficult to detect with the
currently planned accuracy of the Space Interferometry Mission. We verify our
results by numerically calculating the expected astrometric shift for the
photometrically well-covered Galactic binary lensing event OGLE-1999-BUL-23,
finding excellent agreement with our analytic expressions. Our results can be
applied to any lensing system with fold caustics, including Galactic binary
lenses and quasar microlensing.Comment: 37 pages, 7 figures. Revised version includes an expanded discussion
of applications. Accepted to ApJ, to appear in the August 1, 2002 issue
(v574
Femtosecond Demagnetization and Hot Hole Relaxation in Ferromagnetic GaMnAs
We have studied ultrafast photoinduced demagnetization in GaMnAs via
two-color time-resolved magneto-optical Kerr spectroscopy. Below-bandgap
midinfrared pump pulses strongly excite the valence band, while near-infrared
probe pulses reveal sub-picosecond demagnetization that is followed by an
ultrafast (1 ps) partial recovery of the Kerr signal. Through comparison
with InMnAs, we attribute the signal recovery to an ultrafast energy relaxation
of holes. We propose that the dynamical polarization of holes through -
scattering is the source of the observed probe signal. These results support
the physical picture of femtosecond demagnetization proposed earlier for
InMnAs, identifying the critical roles of both energy and spin relaxation of
hot holes.Comment: 7 pages, 6 figure
Online, interactive user guidance for high-dimensional, constrained motion planning
We consider the problem of planning a collision-free path for a
high-dimensional robot. Specifically, we suggest a planning framework where a
motion-planning algorithm can obtain guidance from a user. In contrast to
existing approaches that try to speed up planning by incorporating experiences
or demonstrations ahead of planning, we suggest to seek user guidance only when
the planner identifies that it ceases to make significant progress towards the
goal. Guidance is provided in the form of an intermediate configuration
, which is used to bias the planner to go through . We
demonstrate our approach for the case where the planning algorithm is
Multi-Heuristic A* (MHA*) and the robot is a 34-DOF humanoid. We show that our
approach allows to compute highly-constrained paths with little domain
knowledge. Without our approach, solving such problems requires
carefully-crafting domain-dependent heuristics
Influence of laser-excited electron distributions on the x-ray magnetic circular dichroism spectra: Implications for femtosecond demagnetization in Ni
In pump-probe experiments an intensive laser pulse creates non-equilibrium
excited electron distributions in the first few hundred femtoseconds after the
pulse. The influence of non-equilibrium electron distributions caused by a pump
laser on the apparent X-ray magnetic circular dichroism (XMCD) signal of Ni is
investigated theoretically here for the first time, considering electron
distributions immediately after the pulse as well as thermalized ones, that are
not in equilibrium with the lattice or spin systems. The XMCD signal is shown
not to be simply proportional to the spin momentum in these situations. The
computed spectra are compared to recent pump-probe XMCD experiments on Ni. We
find that the majority of experimentally observed features considered to be a
proof of ultrafast spin momentum transfer to the lattice can alternatively be
attributed to non-equilibrium electron distributions. Furthermore, we find the
XMCD sum rules for the atomic spin and orbital magnetic moment to remain valid,
even for the laser induced non-equilibrium electron distributions.Comment: 6 pages, 3 figure
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