3,546 research outputs found
Defect related switching field reduction in small magnetic particle arrays
An array of 42 mum square, 3 mum thick garnet particles has been studied. The strong crystalline uniaxial anisotropy of these particles results in the stable remanent state being single domain with magnetization parallel to the film normal. Magneto-optic measurements of individual particles provide distribution statistics for the easy-axis switching field H-sw, and the in-plane hard-axis effective anisotropy field, H-eff, which induces the formation of a metastable stripe domain structure. Both H-sw and H-eff are much smaller than the crystalline anisotropy field. Micromagnetic simulations show that the small H-sw cannot be attributed to shape anisotropy, but is consistent with smooth, localized reductions in the crystalline anisotropy caused by defects in either the particles or the substrate
Meridional flow and differential rotation by gravity darkening in fast rotating solar-type stars
An explanation is presented for the rather strong total surface differential
rotation of the observed very young solar-type stars like AB Dor and PZ Tel.
Due to its rapid rotation a nonuniform energy flux leaves the stellar core so
that the outer convection zone is nonuniformly heated from below. Due to this
`gravity darkening' of the equator a meridional flow is created flowing
equatorwards at the surface and thus accelerating the equatorial rotation. The
effect linearly grows with the normalized pole-equator difference, \epsilon, of
the heat-flux at the bottom of the convection zone. A rotation rate of about 9
h leads to \epsilon=0.1 for a solar-type star. In this case the resulting
equator-pole differences of the angular velocity at the stellar surface,
\delta\Omega, varies from unobservable 0.005/day to the (desired) value of 0.03
day when the dimensionless diffusivity factors and c_\chi vary
between 1 and 0.1 (standard value c_\nu \simeq c_\chi \simeq 0.3, see Table 1.)
In all cases the related temperature differences between pole and equator at
the surface are unobservably small.
The (clockwise) meridional circulation which we obtain flows opposite to the
(counterclockwise) circulation appearing as a byproduct in the \Lambda-theory
of the nonuniform rotation in outer convection zones. The consequences of this
situation for those dynamo theories of stellar activity are discussed which
work with the meridional circulation as the dominant magnetic-advection effect
in latitude to produce the solar-like form of the butterfly diagram.
Key words: Hydrodynamics, Star: rotation, Stars: pre-main sequence, Stellar
activityComment: 4 pages, 3 figures, Astronomy and Astrophysics (subm.
A General Precipitation-Limited L_X-T-R Relation Among Early-Type Galaxies
The relation between X-ray luminosity (L_X) and ambient gas temperature (T)
among massive galactic systems is an important cornerstone of both
observational cosmology and galaxy-evolution modeling. In the most massive
galaxy clusters, the relation is determined primarily by cosmological structure
formation. In less massive systems, it primarily reflects the feedback response
to radiative cooling of circumgalactic gas. Here we present a simple but
powerful model for the L_X-T relation as a function of physical aperture R
within which those measurements are made. The model is based on the
precipitation framework for AGN feedback and assumes that the circumgalactic
medium is precipitation-regulated at small radii and limited by cosmological
structure formation at large radii. We compare this model with many different
data sets and show that it successfully reproduces the slope and upper envelope
of the L_X-T-R relation over the temperature range from ~0.2 keV through >10
keV. Our findings strongly suggest that the feedback mechanisms responsible for
regulating star formation in individual massive galaxies have much in common
with the precipitation-triggered feedback that appears to regulate
galaxy-cluster cores.Comment: Submitted to ApJ, 9 pages, 3 figures (v2 fixes a few small typos
Collective spin waves in arrays of Permalloy nanowires with single-side periodically modulated width
We have experimentally and numerically investigated the dispersion of
collective spin waves prop-agating through arrays of longitudinally magnetized
nanowires with periodically modulated width. Two nanowire arrays with
single-side modulation and different periodicity of modulation were studied and
compared to the nanowires with homogeneous width. The spin-wave dispersion,
meas-ured up to the third Brillouin zone of the reciprocal space, revealed the
presence of two dispersive modes for the width-modulated NWs, whose amplitude
of magnonic band depends on the modula-tion periodicity, and a set of
nondispersive modes at higher frequency. These findings are different from
those observed in homogeneous width NWs where only the lowest mode exhibits
sizeable dis-persion. The measured spin-wave dispersion has been satisfactorily
reproduced by means of dynam-ical matrix method. Results presented in this work
are important in view of the possible realization of frequency tunable magnonic
device
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