1,081 research outputs found
Evolution of ferromagnetic circular dichroism coincident with magnetization and anomalous Hall effect in Co-doped rutile TiO2
Magnetic circular dichroism (MCD) of rutile Ti1-xCoxO2-d is systematically
examined with various x and d to reveal a phase diagram for the appearance of
ferromagnetism at higher carrier concentration and Co content. The phase
diagram exactly matches with that determined from anomalous Hall effect (AHE).
The magnetic field dependence of MCD also shows good coincidence with those of
the magnetization and AHE. The coincidence of these independent measurements
strongly suggests single and intrinsic ferromagnetic origin.Comment: 9 pages, 4 figure
Role of charge carriers for ferromagnetism in cobalt-doped rutile TiO2
Electric and magnetic properties of a high temperature ferromagnetic oxide
semiconductor, cobalt-doped rutile TiO2, are summarized. The cobalt-doped
rutile TiO2 epitaxial thin films with different electron densities and cobalt
contents were grown on r-sapphire substrates with laser molecular beam epitaxy.
Results of magnetization, magnetic circular dichroism, and anomalous Hall
effect measurements were examined for samples with systematically varied
electron densities and cobalt contents. The samples with high electron
densities and cobalt contents show the high temperature ferromagnetism,
suggesting that charge carriers induce the ferromagnetism.Comment: 14 pages, 12 figure
Bulk and Surface Magnetization of Co atoms in Rutile Ti_[1-x]Co_xO_[2-delta] Thin Films Revealed by X-Ray Magnetic Circular Dichroism
We have studied magnetism in Ti_[1-x]Co_xO_[2-\delta] thin films with various
x and \delta by soft x-ray magnetic circular dichroism (XMCD) measurements at
the Co L_[2,3] absorption edges. The estimated ferromagnetic moment by XMCD was
0.15-0.24 \mu\beta/Co in the surface, while in the bulk it was 0.82-2.25
\mu\beta/Co, which is in the same range as the saturation magnetization of
1.0-1.5 \mu\beta/Co. Theseresults suggest that the intrinsic origin of the
erromagnetism. The smaller moment of Co atom at surface is an indication of a
magnetically dead layer of a few nm thick at the surface of the thin films.Comment: This Paper is accepted in J. of Phys: Conds. Matte
Magnetic oxide semiconductors
Magnetic oxide semiconductors, oxide semiconductors doped with transition
metal elements, are one of the candidates for a high Curie temperature
ferromagnetic semiconductor that is important to realize semiconductor
spintronics at room temperature. We review in this paper recent progress of
researches on various magnetic oxide semiconductors. The magnetization,
magneto-optical effect, and magneto-transport such as anomalous Hall effect are
examined from viewpoint of feasibility to evaluate the ferromagnetism. The
ferromagnetism of Co-doped TiO2 and transition metal-doped ZnO is discussed.Comment: 26 pages, 5 tables, 6 figure
Accretion Disk Illumination in Schwarzschild and Kerr Geometries: Fitting Formulae
We describe the methodology and compute the illumination of geometrically
thin accretion disks around black holes of arbitrary spin parameter exposed
to the radiation of a point-like, isotropic source at arbitrary height above
the disk on its symmetry axis. We then provide analytic fitting formulae for
the illumination as a function of the source height and the black hole
angular momentum . We find that for a source on the disk symmetry axis and
, the main effect of the parameter is allowing the disk to extend
to smaller radii (approaching as ) and thus allow the
illumination of regions of much higher rotational velocity and redshift. We
also compute the illumination profiles for anisotropic emission associated with
the motion of the source relative to the accretion disk and present the
fractions of photons absorbed by the black hole, intercepted by the disk or
escaping to infinity for both isotropic and anisotropic emission for
and . As the anisotropy (of a source approaching the disk) increases
the illumination profile reduces (approximately) to a single power-law, whose
index, , because of absorption of the beamed photons by the black hole,
saturates to a value no higher than . Finally, we compute the
fluorescence Fe line profiles associated with the specific illumination and
compare them among various cases.Comment: 26 pages, 21 b/w figures, accepted for publication in the
Astrophysical Journal as of 4/16/200
A ferromagnetic oxide semiconductor as spin injection electrode in magnetic tunnel junction
A magnetic tunnel junctions composed of room temperature ferromagnetic
semiconductor rutile Ti1-xCoxO2-d and ferromagnetic metal Fe0.1Co0.9 separated
by AlOx barrier showed positive tunneling magnetoresistance (TMR) with a ratio
of ~11 % at 15 K, indicating that Ti1-xCoxO2-d can be used as a spin injection
electrode. The TMR decreased with increasing temperature and vanished above 180
K. TMR action at high temperature is likely prohibited by the inelastic
tunneling conduction due to the low quality of the amorphous barrier layer
and/or the junction interface.Comment: 9 pages, 4 figure
Signature of Carrier-Induced Ferromagnetism in Ti_{1-x}Co_{x}O_{2-delta}: Exchange Interaction Between High-Spin Co 2+ and the Ti 3d Conduction Band
X-ray photoemission spectroscopy measurements were performed on thin-film
samples of rutile Ti_{1-x}Co_{x}O_{2-delta} to reveal the electronic structure.
The Co 2p core level spectra indicate that the Co ions take the high-spin Co 2+
configuration, consistent with substitution on the Ti site. The high spin state
and the shift due to the exchange splitting of the conduction band suggest
strong hybridization between carriers in the Ti 3d t2g band and the t2g states
of the high-spin Co 2+. These observations support the argument that room
temperature ferromagnetism in Ti_{1-x}Co_{x}O_{2-delta} is intrinsic.Comment: 4 pages, 5 figures. Accepted for publication in Physical Review
Letter
Poynting-Robertson effect on black-hole-driven winds
Layers of ionized plasma, in the form of winds ejected from the accretion
disk of Supermassive Black Holes (SMBHs) are frequently observed in Active
Galactic Nuclei (AGNs). Winds with a velocity often exceeding are called
Ultra-Fast-Outflows (UFOs) and thanks to their high power they can play a key
role in the co-evolution between the SMBH and the host galaxy. In order to
construct a proper model of the properties of these winds, it is necessary to
consider special relativistic corrections due to their very high velocities. We
present a derivation of the Poynting-Robertson effect (P-R effect) and apply it
to the description of the dynamics of UFOs. The P-R effect is a special
relativistic correction which breaks the isotropy of the radiation emitted by a
moving particle funneling the radiation in the direction of motion. As a result
of the conservation of the four-momentum, the emitting particles are subjected
to a drag force and decelerate. We provide a derivation of the drag force
caused by the P-R effect starting from general Lorentz transformations and
assuming isotropic emission in the gas reference frame. Then, we derive the
equations to easily implement this drag force in future simulations. Finally,
we apply them in a toy model in which the gas particles move radially under the
influence of the gravitation force, the radiation pressure and the drag due to
the P-R effect. P-R effect plays an important role in determining the velocity
profile of the wind. For a wind launched from (where stands
for the Schwarzschild radius), the asymptotic velocity reached by the wind is
between % and % smaller than the one it would possess if we neglect the
effect. This shows that the P-R effect should be taken into account when
studying the dynamics of high-velocity, photoionized outflows in general.Comment: Accepted for publication on Astronomy & Astrophysics. 7 pages, 4
figure
Ultrafast optical control of magnetization in EuO thin films
All-optical pump-probe detection of magnetization precession has been
performed for ferromagnetic EuO thin films at 10 K. We demonstrate that the
circularly-polarized light can be used to control the magnetization precession
on an ultrafast time scale. This takes place within the 100 fs duration of a
single laser pulse, through combined contribution from two nonthermal
photomagnetic effects, i.e., enhancement of the magnetization and an inverse
Faraday effect. From the magnetic field dependences of the frequency and the
Gilbert damping parameter, the intrinsic Gilbert damping coefficient is
evaluated to be {\alpha} \approx 3\times10^-3.Comment: 5 pages, 3 figures, accepted for publication in Phys. Rev.
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