7,575 research outputs found
Rate of Evaporation of Manganese, Copper, Tin, Chromium, and Sulphur from Molten Iron under Vacuum
The vacuum melting of Fe-Mn, Fe-Cu, Fe-Sn, Fe-Cr, Fe-S, and Fe-Si-S alloys was carried out at 1600℃ by using a high-frequency induction furnace. The evaporation of Mn, Cu, Sn, Cr, and S follows first-order kinetics and the specific evaporation constant K^s of each of these elements was obtained. The K^s of sulphur in Fe-Si-S alloys increases with the increase of silicon content. For each of these elements in the binary alloys, the rate constant of evaporation and of transport in melt and the ratio of surface concentration to bulk concentration C^s/C^m were obtained. Furthermore, the evaporation coefficient α of each of these elements was obtained both experimentally and theoretically according to the equation derived by Olette. It was confirmed that for each of these elements in the binary alloys the theoretical value of the product α. C^s/C^m is close to the experimental value of α
A theory of ferromagnetism in planar heterostructures of (Mn,III)-V semiconductors
A density functional theory of ferromagnetism in heterostructures of compound
semiconductors doped with magnetic impurities is presented. The variable
functions in the density functional theory are the charge and spin densities of
the itinerant carriers and the charge and localized spins of the impurities.
The theory is applied to study the Curie temperature of planar heterostructures
of III-V semiconductors doped with manganese atoms. The mean-field,
virtual-crystal and effective-mass approximations are adopted to calculate the
electronic structure, including the spin-orbit interaction, and the magnetic
susceptibilities, leading to the Curie temperature. By means of these results,
we attempt to understand the observed dependence of the Curie temperature of
planar -doped ferromagnetic structures on variation of their
properties. We predict a large increase of the Curie Temperature by additional
confinement of the holes in a -doped layer of Mn by a quantum well.Comment: 8 pages, 7 figure
Theory of Magnetic Anisotropy in III_{1-x}Mn_{x}V Ferromagnets
We present a theory of magnetic anisotropy in diluted magnetic semiconductors with carrier-induced
ferromagnetism. The theory is based on four and six band envelope functions
models for the valence band holes and a mean-field treatment of their exchange
interactions with ions. We find that easy-axis reorientations
can occur as a function of temperature, carrier density , and strain. The
magnetic anisotropy in strain-free samples is predicted to have a
hole-density dependence at small , a dependence at large , and
remarkably large values at intermediate densities. An explicit expression,
valid at small , is given for the uniaxial contribution to the magnetic
anisotropy due to unrelaxed epitaxial growth lattice-matching strains. Results
of our numerical simulations are in agreement with magnetic anisotropy
measurements on samples with both compressive and tensile strains. We predict
that decreasing the hole density in current samples will lower the
ferromagnetic transition temperature, but will increase the magnetic anisotropy
energy and the coercivity.Comment: 15 pages, 15 figure
Electron-electron interaction effects on optical excitations in semiconducting single-walled carbon nanotubes
We report correlated-electron calculations of optically excited states in ten
semiconducting single-walled carbon nanotubes with a wide range of diameters.
Optical excitation occurs to excitons whose binding energies decrease with the
increasing nanotube diameter, and are smaller than the binding energy of an
isolated strand of poly-(paraphenylene vinylene). The ratio of the energy of
the second optical exciton polarized along the nanotube axis to that of the
lowest exciton is smaller than the value predicted within single-particle
theory. The experimentally observed weak photoluminescence is an intrinsic
feature of semiconducting nanotubes, and is consequence of dipole-forbidden
excitons occurring below the optical exciton.Comment: 5 pages, 3 figures, To appear in PR
Probing the Magnetized Interstellar Medium Surrounding the Planetary Nebula Sh 2-216
We present 1420 MHz polarization images of a 2.5 X 2.5 degree region around
the planetary nebula (PN) Sh 2-216. The images are taken from the Canadian
Galactic Plane Survey (CGPS). An arc of low polarized intensity appears
prominently in the north-east portion of the visible disk of Sh 2-216,
coincident with the optically identified interaction region between the PN and
the interstellar medium (ISM). The arc contains structural variations down to
the ~1 arcminute resolution limit in both polarized intensity and polarization
angle. Several polarization-angle "knots" appear along the arc. By comparison
of the polarization angles at the centers of the knots and the mean
polarization angle outside Sh 2-216, we estimate the rotation measure (RM)
through the knots to be -43 +/- 10 rad/m^2. Using this estimate for the RM and
an estimate of the electron density in the shell of Sh 2-216, we derive a
line-of-sight magnetic field in the interaction region of 5.0 +/- 2.0 microG.
We believe it more likely the observed magnetic field is interstellar than
stellar, though we cannot completely dismiss the latter possibility. We
interpret our observations via a simple model which describes the ISM magnetic
field around Sh 2-216, and comment on the potential use of old PNe as probes of
the magnetized ISM.Comment: 25 pages, 4 figures. Accepted for publication in the Astrophysical
Journa
Ab-initio transport theory for digital ferromagnetic heterostructures
MnAs/GaAs superlattices, made by -doping GaAs with Mn, are known as
digital ferromagnetic heterostructures. Here we present a theoretical density
functional study of the electronic, magnetic and transport properties of such
heterostructures. In the absence of intrinsic donors these systems show an half
metallic density of states, with an exchange interaction much stronger than
that of a random alloy with the same Mn concentration. {\it Ab initio}
ballistic transport calculations show that the carriers with energies close to
the Fermi energy are strongly confined within a few monolayers around the MnAs
plane. This strong confinement is responsible for the large exchange coupling.
Therefore the system can be described as a two dimensional half metal with
large conductance in the MnAs plane and small conductance in the perpendicular
direction
Observation of the spin-charge thermal isolation of ferromagnetic Ga_{0.94}Mn_{0.06}As by time-resolved magneto-optical measurement
The dynamics of magnetization under femtosecond optical excitation is studied
in a ferromagnetic semiconductor Ga_{0.94}Mn_{0.06}As with a time-resolved
magneto-optical Kerr effect measurement with two color probe beams. The
transient reflectivity change indicates the rapid rise of the carrier
temperature and relaxation to a quasi-thermal equilibrium within 1 ps, while a
very slow rise of the spin temperature of the order of 500ps is observed. This
anomalous behavior originates from the thermal isolation between the charge and
spin systems due to the spin polarization of carriers (holes) contributing to
ferromagnetism. This constitutes experimental proof of the half-metallic nature
of ferromagnetic Ga_{0.94}Mn_{0.06}As arising from double exchange type
mechanism originates from the d-band character of holes
Coordination and chemical effects on the structural, electronic and magnetic properties in Mn pnictides
Simple structures of MnX binary compounds, namely hexagonal NiAs and
zincblende, are studied as a function of the anion (X = Sb, As, P) by means of
the all-electron FLAPW method within local spin density and generalized
gradient approximations. An accurate analysis of the structural, electronic and
magnetic properties reveals that the cubic structure greatly favours the
magnetic alignment in these compounds leading to high magnetic moments and
nearly half-metallic behaviour for MnSb and MnAs. The effect of the anion
chemical species is related to both its size and the possible hybridization
with the Mn states; both contributions are seen to hinder the magnitude of
the magnetic moment for small and light anions. Our results are in very good
agreement with experiment - where available - and show that the generalized
gradient approximation is essential to correctly recover both the equilibrium
volume and magnetic moment.Comment: 18 pages and 4 figures, Latex-file, submitted to Phys.Rev.
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