2,391 research outputs found
Magnetic Properties of Epitaxial and Polycrystalline Fe/Si Multilayers
Fe/Si multilayers with antiferromagnetic interlayer coupling have been grown
via ion-beam sputtering on both glass and single-crystal substrates. High-angle
x-ray diffraction measurements show that both sets of films have narrow Fe
peaks, implying a large crystallite size and crystalline iron silicide spacer
layers. Low-angle x-ray diffraction measurements show that films grown on glass
have rougher interfaces than those grown on single-crystal substrates. The
multilayers grown on glass have a larger remanent magnetization than the
multilayers grown on single-crystal substrates. The observation of
magnetocrystalline anisotropy in hysteresis loops and peaks in x-ray
diffraction demonstrates that the films grown on MgO and Ge are epitaxial. The
smaller remanent magnetization in Fe/Si multilayers with better layering
suggests that the remanence is not an intrinsic property.Comment: 9 pages, RevTex, 4 figures available by fax. Send email to
[email protected] for more info. Submitted to '95 MMM proceeding
Periodic chiral magnetic domains in single-crystal nickel nanowires
We report on experimental and computational investigations of the domain
structure of ~0.2 x 0.2 x 8 {\mu}m single-crystal Ni nanowires (NWs). The Ni
NWs were grown by a thermal chemical vapor deposition technique that results in
highly-oriented single-crystal structures on amorphous SiOx coated Si
substrates. Magnetoresistance measurements of the Ni NWs suggest the average
magnetization points largely off the NW long axis at zero field. X-ray
photoemission electron microscopy images show a well-defined periodic
magnetization pattern along the surface of the nanowires with a period of
{\lambda} = 250 nm. Finite element micromagnetic simulations reveal that an
oscillatory magnetization configuration with a period closely matching
experimental observation ({\lambda} = 240 nm) is obtainable at remanence. This
magnetization configuration involves a periodic array of alternating chirality
vortex domains distributed along the length of the NW. Vortex formation is
attributable to the cubic anisotropy of the single crystal Ni NW system and its
reduced structural dimensions. The periodic alternating chirality vortex state
is a topologically protected metastable state, analogous to an array of
360{\deg} domain walls in a thin strip. Simulations show that other remanent
states are also possible, depending on the field history. Effects of material
properties and strain on the vortex pattern are investigated. It is shown that
at reduced cubic anisotropy vortices are no longer stable, while negative
uniaxial anisotropy and magnetoelastic effects in the presence of compressive
biaxial strain contribute to vortex formation.Comment: 15 pages, 11 figure
Modeling Ultraviolet Wind Line Variability in Massive Hot Stars
We model the detailed time-evolution of Discrete Absorption Components (DACs)
observed in P Cygni profiles of the Si IV lam1400 resonance doublet lines of
the fast-rotating supergiant HD 64760 (B0.5 Ib). We adopt the common assumption
that the DACs are caused by Co-rotating Interaction Regions (CIRs) in the
stellar wind. We perform 3D radiative transfer calculations with hydrodynamic
models of the stellar wind that incorporate these large-scale density- and
velocity-structures. We develop the 3D transfer code Wind3D to investigate the
physical properties of CIRs with detailed fits to the DAC shape and morphology.
The CIRs are caused by irregularities on the stellar surface that change the
radiative force in the stellar wind. In our hydrodynamic model we approximate
these irregularities by circular symmetric spots on the stellar surface. We use
the Zeus3D code to model the stellar wind and the CIRs, limited to the
equatorial plane. We constrain the properties of large-scale wind structures
with detailed fits to DACs observed in HD 64760. A model with two spots of
unequal brightness and size on opposite sides of the equator, with opening
angles of 20 +/- 5 degr and 30 +/- 5 degr diameter, and that are 20 +/- 5 % and
8 +/- 5 % brighter than the stellar surface, respectively, provides the best
fit to the observed DACs. The recurrence time of the DACs compared to the
estimated rotational period corresponds to spot velocities that are 5 times
slower than the rotational velocity. The mass-loss rate of the structured wind
model for HD 64760 does not exceed the rate of the spherically symmetric smooth
wind model by more than 1 %. The fact that DACs are observed in a large number
of hot stars constrains the clumping that can be present in their winds, as
substantial amounts of clumping would tend to destroy the CIRs.Comment: 58 pages, 16 figures, 1 animation. Accepted for publication in The
Astrophysical Journal, Main Journal. More information and animations are
available at http://alobel.freeshell.org/hotstars.htm
Properties of Galactic early-type O-supergiants: A combined FUV-UV and optical analysis
We aim to constrain the properties and evolutionary status of early and
mid-spectral type supergiants (from O4 to O7.5). These posses the highest
mass-loss rates among the O stars, and exhibit conspicuous wind profiles. Using
the non-LTE wind code CMFGEN, we simultaneously analyzed the FUV-UV and optical
spectral range to determine the photospheric properties and wind parameters. We
derived effective temperatures, luminosities, surface gravities, surface
abundances, mass-loss rates, wind terminal velocities, and clumping filling
factors. The supergiants define a very clear evolutionary sequence, in terms of
ages and masses, from younger and more massive stars to older stars with lower
initial masses. O4 supergiants cluster around the 3 Myr isochrone and are more
massive than 60 Msun, while the O5 to O7.5 stars have masses in the range 50 -
40 Msun and are 4 +/- 0.3 Myr old. The surface chemical composition is typical
of evolved O supergiants (nitrogen-rich, carbon- and oxygen-poor). While the
observed ranges of carbon and nitrogen mass-fractions are compatible with those
expected from evolutionary models for the measured stellar masses, the N/C
ratios as a function of age are inconsistent with the theoretical predictions
for the four earliest (O4 spectral type) stars of the sample. We question the
efficiency of rotational mixing as a function of age for these stars and
suggest that another mechanism may be needed to explain the observed abundance
patterns. Mass-loss rates derived with clumped-models range within a factor of
three of the theoretical mass-loss rates. The corresponding volume-filling
factors associated with small-scale clumping are 0.05 +/- 0.02. Clumping is
found to start close to the photosphere for all but three stars, two of which
are fast rotators.Comment: 30 pages, 11 figures, accepted for publication in A&A 201
{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers
The magnetization distribution, its energetic characterization by the
interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt
multilayers are investigated using density functional theory combined with the
direct method to determine phonon frequencies. It is found that ferromagnetic
order between consecutive Fe layers is favoured, with the enhanced magnetic
moments at the interface. The bilinear and biquadratic coupling coefficients
between Fe layers are shown to saturate fast with increasing thickness of
nonmagnetic Pt layers which separate them. The phonon calculations demonstrate
a rather strong dependence of partial iron phonon densities of states on the
actual position of Fe monolayer in the multilayer structure.Comment: 7 pages, 8 figure
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