5,816 research outputs found
Domain structure of epitaxial Co films with perpendicular anisotropy
Epitaxial hcp Cobalt films with pronounced c-axis texture have been prepared
by pulsed lased deposition (PLD) either directly onto Al2O3 (0001) single
crystal substrates or with an intermediate Ruthenium buffer layer. The crystal
structure and epitaxial growth relation was studied by XRD, pole figure
measurements and reciprocal space mapping. Detailed VSM analysis shows that the
perpendicular anisotropy of these highly textured Co films reaches the
magnetocrystalline anisotropy of hcp-Co single crystal material. Films were
prepared with thickness t of 20 nm < t < 100 nm to study the crossover from
in-plane magnetization to out-of-plane magnetization in detail. The analysis of
the periodic domain pattern observed by magnetic force microscopy allows to
determine the critical minimum thickness below which the domains adopt a pure
in-plane orientation. Above the critical thickness the width of the stripe
domains is evaluated as a function of the film thickness and compared with
domain theory. Especially the discrepancies at smallest film thicknesses show
that the system is in an intermediate state between in-plane and out-of-plane
domains, which is not described by existing analytical domain models
The alpha effect with imposed and dynamo-generated magnetic fields
Estimates for the nonlinear alpha effect in helical turbulence with an
applied magnetic field are presented using two different approaches: the
imposed-field method where the electromotive force owing to the applied field
is used, and the test-field method where separate evolution equations are
solved for a set of different test fields. Both approaches agree for stronger
fields, but there are apparent discrepancies for weaker fields that can be
explained by the influence of dynamo-generated magnetic fields on the scale of
the domain that are referred to as meso-scale magnetic fields. Examples are
discussed where these meso-scale fields can lead to both drastically
overestimated and underestimated values of alpha compared with the kinematic
case. It is demonstrated that the kinematic value can be recovered by resetting
the fluctuating magnetic field to zero in regular time intervals. It is
concluded that this is the preferred technique both for the imposed-field and
the test-field methods.Comment: 10 pages, 8 figures, published versio
Dynamo generated field emergence through recurrent plasmoid ejections
Magnetic buoyancy is believed to drive the transport of magnetic flux tubes
from the convection zone to the surface of the Sun. The magnetic fields form
twisted loop-like structures in the solar atmosphere. In this paper we use
helical forcing to produce a large-scale dynamo-generated magnetic field, which
rises even without magnetic buoyancy. A two layer system is used as
computational domain where the upper part represents the solar atmosphere.
Here, the evolution of the magnetic field is solved with the stress--and--relax
method. Below this region a magnetic field is produced by a helical forcing
function in the momentum equation, which leads to dynamo action. We find
twisted magnetic fields emerging frequently to the outer layer, forming
arch-like structures. In addition, recurrent plasmoid ejections can be found by
looking at space--time diagrams of the magnetic field. Recent simulations in
spherical coordinates show similar results.Comment: 4 pages, 8 figures, To appear in the proceedings of the IAU273
"Physics of Sun and Star Spots
Small-scale dynamos in simulations of stratified turbulent convection
Small-scale dynamo action is often held responsible for the generation of
quiet-Sun magnetic fields. We aim to determine the excitation conditions and
saturation level of small-scale dynamos in non-rotating turbulent convection at
low magnetic Prandtl numbers. We use high resolution direct numerical
simulations of weakly stratified turbulent convection. We find that the
critical magnetic Reynolds number for dynamo excitation increases as the
magnetic Prandtl number is decreased, which might suggest that small-scale
dynamo action is not automatically evident in bodies with small magnetic
Prandtl numbers as the Sun. As a function of the magnetic Reynolds number
(), the growth rate of the dynamo is consistent with an scaling. No evidence for a logarithmic increase of the growth rate
with is found.Comment: 6 pages, 5 figures, submitted to Astron. Nach
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