226 research outputs found
Magnetic and structural properties of GeMn films: precipitation of intermetallic nanomagnets
We present a comprehensive study relating the nanostructure of Ge_0.95Mn_0.05
films to their magnetic properties. The formation of ferromagnetic nanometer
sized inclusions in a defect free Ge matrix fabricated by low temperature
molecular beam epitaxy is observed down to substrate temperatures T_S as low as
70 deg. Celsius. A combined transmission electron microscopy (TEM) and electron
energy-loss spectroscopy (EELS) analysis of the films identifies the inclusions
as precipitates of the ferromagnetic compound Mn_5Ge_3. The volume and amount
of these precipitates decreases with decreasing T_S. Magnetometry of the films
containing precipitates reveals distinct temperature ranges: Between the
characteristic ferromagnetic transition temperature of Mn_5Ge_3 at
approximately room temperature and a lower, T_S dependent blocking temperature
T_B the magnetic properties are dominated by superparamagnetism of the Mn_5Ge_3
precipitates. Below T_B, the magnetic signature of ferromagnetic precipitates
with blocked magnetic moments is observed. At the lowest temperatures, the
films show features characteristic for a metastable state.Comment: accepted for publication in Phys. Rev. B 74 (01.12.2006). High
resolution images ibide
Long-lived magnons throughout the Brillouin zone of the strong-leg spin ladder (CHN)CuBr
Inelastic neutron scattering is used to measure spin excitations in fully
deuterated single crystal samples of the strong-leg antiferromagnetic S=1/2
spin ladder compound (CHN)CuBr. Sharp resolution-limited
magnons are observed across the entire one-dimensional Brillouin zone. The
results validate the previously proposed {\it symmetric} spin ladder model and
provide a reliable estimate of the relevant exchange interactions.Comment: 5 pages, 5 figure
Rotating skyrmion lattices by spin torques and field or temperature gradients
Chiral magnets like MnSi form lattices of skyrmions, i.e. magnetic whirls,
which react sensitively to small electric currents j above a critical current
density jc. The interplay of these currents with tiny gradients of either the
magnetic field or the temperature can induce a rotation of the magnetic pattern
for j>jc. Either a rotation by a finite angle of up to 15 degree or -- for
larger gradients -- a continuous rotation with a finite angular velocity is
induced. We use Landau-Lifshitz-Gilbert equations extended by extra damping
terms in combination with a phenomenological treatment of pinning forces to
develop a theory of the relevant rotational torques. Experimental neutron
scattering data on the angular distribution of skyrmion lattices suggests that
continuously rotating domains are easy to obtain in the presence of remarkably
small currents and temperature gradients.Comment: 12 pages, 10 figure
The new small-angle neutron scattering instrument SANS-1 at MLZ—characterization and first results
AbstractA thorough characterization of the key features of the new small-angle neutron scattering instrument SANS-1 at MLZ, a joint project of Technische Universität München and Helmholtz Zentrum Geesthacht, is presented. Measurements of the neutron beam profile, divergency and flux are given for various positions along the instrument including the sample position, and agree well with Monte Carlo simulations of SANS-1 using the program McStas. Secondly, the polarization option of SANS-1 is characterized for a broad wavelength band. A key feature of SANS-1 is the large accessible Q-range facilitated by the sideways movement of the detector. Particular attention is hence paid to the effects that arise due to large scattering angles on the detector where a standard cos3 solid angle correction is no longer applicable. Finally the performance of the instrument is characterized by a set of standard samples
Shape and orientation of stellar velocity ellipsoids in spiral galaxies
We present a numerical study of the properties of the stellar velocity
distribution in stellar discs which have developed a saturated, two-armed
spiral structure. We follow the growth of the spiral structure deeply into the
non-linear regime by solving the Boltzmann moment equations up to second order.
By adopting the thin-disc approximation, we restrict our study of the stellar
velocity distribution to the plane of the stellar disc. We find that the outer
(convex) edges of stellar spiral arms are characterized by peculiar properties
of the stellar velocity ellipsoids, which make them distinct from most other
galactic regions. In particular, the ratio \sigma_1:\sigma_2 of the smallest
versus largest principal axes of the stellar velocity ellipsoid can become
abnormally small (as compared to the rest of the disc) near the outer edges of
spiral arms. Moreover, the epicycle approximation fails to reproduce the ratio
of the tangential versus radial velocity dispersions in these regions. These
peculiar properties of the stellar velocity distribution are caused by
large-scale non-circular motions of stars, which in turn are triggered by the
non-axisymmetric gravitational field of stellar spiral arms. The magnitude of
the vertex deviation appears to correlate globally with the amplitude of the
spiral stellar density perturbations. However, locally there is no simple
correlation between the vertex deviation and the density perturbations.
(Abstract abridged).Comment: 16 pages, 18 figures, Accepted for publication in MNRA
Orbital textures and charge density waves in transition metal dichalcogenides
Low-dimensional electron systems, as realized naturally in graphene or
created artificially at the interfaces of heterostructures, exhibit a variety
of fascinating quantum phenomena with great prospects for future applications.
Once electrons are confined to low dimensions, they also tend to spontaneously
break the symmetry of the underlying nuclear lattice by forming so-called
density waves; a state of matter that currently attracts enormous attention
because of its relation to various unconventional electronic properties. In
this study we reveal a remarkable and surprising feature of charge density
waves (CDWs), namely their intimate relation to orbital order. For the
prototypical material 1T-TaS2 we not only show that the CDW within the
two-dimensional TaS2-layers involves previously unidentified orbital textures
of great complexity. We also demonstrate that two metastable stackings of the
orbitally ordered layers allow to manipulate salient features of the electronic
structure. Indeed, these orbital effects enable to switch the properties of
1T-TaS2 nanostructures from metallic to semiconducting with technologically
pertinent gaps of the order of 200 meV. This new type of orbitronics is
especially relevant for the ongoing development of novel, miniaturized and
ultra-fast devices based on layered transition metal dichalcogenides
Implementation of aerosol–cloud interactions in the regional atmosphere–aerosol model COSMO-MUSCAT(5.0) and evaluation using satellite data
The regional atmospheric model Consortium for Small-scale Modeling
(COSMO) coupled to the Multi-Scale Chemistry Aerosol Transport model (MUSCAT)
is extended in this work to represent aerosol–cloud interactions. Previously,
only one-way interactions (scavenging of aerosol and in-cloud chemistry) and
aerosol–radiation interactions were included in this model. The new version
allows for a microphysical aerosol effect on clouds. For this, we use the
optional two-moment cloud microphysical scheme in COSMO and the
online-computed aerosol information for cloud condensation nuclei concentrations
(Cccn), replacing the constant Cccn profile. In the
radiation scheme, we have implemented a droplet-size-dependent cloud optical
depth, allowing now for aerosol–cloud–radiation interactions. To evaluate the
models with satellite data, the Cloud Feedback Model Intercomparison Project
Observation Simulator Package (COSP) has been implemented. A case study has
been carried out to understand the effects of the modifications, where the
modified modeling system is applied over the European domain with a
horizontal resolution of 0.25°  ×  0.25°. To reduce the
complexity in aerosol–cloud interactions, only warm-phase clouds are
considered. We found that the online-coupled aerosol introduces significant
changes for some cloud microphysical properties. The cloud effective radius
shows an increase of 9.5 %, and the cloud droplet number concentration is
reduced by 21.5 %
Skyrmion Hall Effect Revealed by Direct Time-Resolved X-Ray Microscopy
Magnetic skyrmions are highly promising candidates for future spintronic
applications such as skyrmion racetrack memories and logic devices. They
exhibit exotic and complex dynamics governed by topology and are less
influenced by defects, such as edge roughness, than conventionally used domain
walls. In particular, their finite topological charge leads to a predicted
"skyrmion Hall effect", in which current-driven skyrmions acquire a transverse
velocity component analogous to charged particles in the conventional Hall
effect. Here, we present nanoscale pump-probe imaging that for the first time
reveals the real-time dynamics of skyrmions driven by current-induced spin
orbit torque (SOT). We find that skyrmions move at a well-defined angle
{\Theta}_{SH} that can exceed 30{\deg} with respect to the current flow, but in
contrast to theoretical expectations, {\Theta}_{SH} increases linearly with
velocity up to at least 100 m/s. We explain our observation based on internal
mode excitations in combination with a field-like SOT, showing that one must go
beyond the usual rigid skyrmion description to unravel the dynamics.Comment: pdf document arxiv_v1.1. 24 pages (incl. 9 figures and supplementary
information
Spiral spin-liquid and the emergence of a vortex-like state in MnScS
Spirals and helices are common motifs of long-range order in magnetic solids,
and they may also be organized into more complex emergent structures such as
magnetic skyrmions and vortices. A new type of spiral state, the spiral
spin-liquid, in which spins fluctuate collectively as spirals, has recently
been predicted to exist. Here, using neutron scattering techniques, we
experimentally prove the existence of a spiral spin-liquid in MnScS by
directly observing the 'spiral surface' - a continuous surface of spiral
propagation vectors in reciprocal space. We elucidate the multi-step ordering
behavior of the spiral spin-liquid, and discover a vortex-like triple-q phase
on application of a magnetic field. Our results prove the effectiveness of the
- Hamiltonian on the diamond lattice as a model for the spiral
spin-liquid state in MnScS, and also demonstrate a new way to realize a
magnetic vortex lattice.Comment: 10 pages, 11 figure
Low-Velocity Streams in the Solar Neighborhood Caused by the Galactic Bar
We find that a steady state bar induces transient features at low velocities
in the solar neighborhood velocity distribution due to the initial response of
the disc, following the formation of the bar. We associate these velocity
streams with two quasi-periodic orbital families, librating around the stable
x_1(1) and x_1(2) orbits near the bar's outer Lindblad resonance (OLR). In a
reference frame moving with the bar, these otherwise stationary orbits precess
on a timescale dependent on the strength of the bar, consistent with
predictions from a simple Hamiltonian model for the resonance. This behavior
allows the two orbital families to reach the solar neighborhood and manifest
themselves as clumps in the u-v plane moving away from (x_1(2)), and toward
(x_1(1)) the Galactic center. Depending on the bar parameters and time since
its formation, this model is consistent with the Pleiades and Coma Berenices,
or Pleiades and Sirius moving groups seen in the Hipparcos stellar velocity
distribution, if the Milky Way bar angle is 30<phi_0<45 [deg] and its pattern
speed is Omega_b/Omega_0=1.82\pm 0.07, where Omega_0 is the angular velocity of
the local standard of rest (LSR). Since the process is recurrent, we can
achieve a good match about every six LSR rotations. However, to be consistent
with the fraction of stars in the Pleiades, we estimate that the Milky Way bar
formed ~2 Gyr ago. This model argues against a common dynamical origin for the
Hyades and Pleiades moving groups.Comment: 9 pages, 8 figures, Accepted for publication in MNRAS with minor
revision
- …