4,881 research outputs found
Magnetic domain formation in itinerant metamagnets
We examine the effects of long-range dipolar forces on metamagnetic
transitions and generalize the theory of Condon domains to the case of an
itinerant electron system undergoing a first-order metamagnetic transition. We
demonstrate that within a finite range of the applied field, dipolar
interactions induce a spatial modulation of the magnetization in the form of
stripes or bubbles. Our findings are consistent with recent observations in the
bilayer ruthenate SrRuO.Comment: 4 pages, 3 figures, minor changes, references adde
Open charm hadroproduction and the charm content of the proton
We advocate charmed-hadron inclusive hadroproduction as a laboratory to probe
intrinsic charm (IC) inside the colliding hadrons. Working at next-to-leading
order in the general-mass variable-flavor-number scheme endowed with
non-perturbative fragmentation functions recently extracted from a global fit
to e^+e^- annihilation data from KEKB, CESR, and LEP1, we first assess the
sensitivity of Tevatron data of D^0, D^+, and D^{*+} inclusive production to
the IC parameterizations provided by Pumplin et al. We then argue that similar
data from pp collisions at RHIC would have the potential to discriminate
between different IC models provided they reach out to sufficiently large
values of transverse momentum.Comment: 22 pages, 7 figures; discussion of large-p_T range at the Tevatron
and high-energy mode of RHIC included, 2 figures added; accepted for
publication in Phys. Rev.
Field-Driven Domain-Wall Dynamics in GaMnAs Films with Perpendicular Anisotropy
We combine magneto-optical imaging and a magnetic field pulse technique to
study domain wall dynamics in a ferromagnetic (Ga,Mn)As layer with
perpendicular easy axis. Contrary to ultrathin metallic layers, the depinning
field is found to be smaller than the Walker field, thereby allowing for the
observation of the steady and precessional flow regimes. The domain wall width
and damping parameters are determined self-consistently. The damping, 30 times
larger than the one deduced from ferromagnetic resonance, is shown to
essentially originate from the non-conservation of the magnetization modulus.
An unpredicted damping resonance and a dissipation regime associated with the
existence of horizontal Bloch lines are also revealed
Forbidden oxygen lines at various nucleocentric distances in comets
To study the formation of the [OI] lines - i.e., 5577 A (the green line),
6300 A and 6364 A (the two red lines) - in the coma of comets and to determine
the parent species of the oxygen atoms using the green to red-doublet emission
intensity ratio (G/R ratio) and the lines velocity widths. We acquired at the
ESO VLT high-resolution spectroscopic observations of comets C/2002 T7
(LINEAR), 73P-C/Schwassmann-Wachmann 3, 8P/Tuttle, and, 103P/Hartley 2 when
they were close to the Earth (< 0.6 au). Using the observed spectra, we
determined the intensities and the widths of the three [OI] lines. We have
spatially extracted the spectra in order to achieve the best possible
resolution of about 1-2", i.e., nucleocentric projected distances of 100 to 400
km depending on the geocentric distance of the comet. We have decontaminated
the [OI] green line from C2 lines blends. It is found that the observed G/R
ratio on all four comets varies as a function of nucleocentric projected
distance. This is mainly due to the collisional quenching of O(1S) and O(1D) by
water molecules in the inner coma. The observed green emission line width is
about 2.5 km/s and decreases as the distance from the nucleus increases which
can be explained by the varying contribution of CO2 to the O(1S) production in
the innermost coma. The photodissociation of CO2 molecules seems to produce
O(1S) closer to the nucleus while the water molecule forms all the O(1S) and
O(1D) atoms beyond 1000 km. Thus we conclude that the main parent species
producing O(1S) and O(1D) in the inner coma is not always the same. The
observations have been interpreted in the framework of the
coupled-chemistry-emission model of Bhardwaj & Raghuram (2012) and the upper
limits of CO2 relative abundances are derived from the observed G/R ratios.
Measuring the [OI] lines could indeed provide a new way to determine the CO2
relative abundance in comets.Comment: accepted for publication in A&A, the abstract is shortene
Magnetic anisotropies and magnetization reversal of the CoCrFeAl Heusler compound
Magnetic anisotropies and magnetization reversal properties of the epitaxial
Heusler compound CoCrFeAl (CCFA) deposited on Fe and Cr
buffer layers are studied. Both samples exhibit a growth-induced fourfold
anisotropy, and magnetization reversal occurs through the formation of stripy
domains or 90 degree domains. During rotational magnetometric scans the sample
deposited on Cr exhibits about 2 degree sharp peaks in the angular dependence
of the coercive field, which are oriented along the hard axis directions. These
peaks are a consequence of the specific domain structure appearing in this
particular measurement geometry. A corresponding feature in the sample
deposited on Fe is not observed.Comment: 11 pages, 7 figure
Controlled vortex core switching in a magnetic nanodisk by a rotating field
The switching process of the vortex core in a Permalloy nanodisk affected by
a rotating magnetic field is studied theoretically. A detailed description of
magnetization dynamics is obtained by micromagnetic simulations.Comment: REVTeX, 5 pages, 5 figure
Magnetization reversal and anomalous coercive field temperature dependence in MnAs epilayers grown on GaAs(100) and GaAs(111)B
The magnetic properties of MnAs epilayers have been investigated for two
different substrate orientations: GaAs(100) and GaAs(111). We have analyzed the
magnetization reversal under magnetic field at low temperatures, determining
the anisotropy of the films. The results, based on the shape of the
magnetization loops, suggest a domain movement mechanism for both types of
samples. The temperature dependence of the coercivity of the films has been
also examined, displaying a generic anomalous reentrant behavior at T200 K.
This feature is independent of the substrate orientation and films thickness
and may be associated to the appearance of new pinning centers due to the
nucleation of the -phase at high temperatures.Comment: 9 pages, 7 figure
Barkhausen noise from zigzag domain walls
We investigate the Barkhausen noise in ferromagnetic thin films with zigzag
domain walls. We use a cellular automaton model that describes the motion of a
zigzag domain wall in an impure ferromagnetic quasi-two dimensional sample with
in-plane uniaxial magnetization at zero temperature, driven by an external
magnetic field. The main ingredients of this model are the dipolar spin-spin
interactions and the anisotropy energy. A power law behavior with a cutoff is
found for the probability distributions of size, duration and correlation
length of the Barkhausen avalanches, and the critical exponents are in
agreement with the available experiments. The link between the size and the
duration of the avalanches is analyzed too, and a power law behavior is found
for the average size of an avalanche as a function of its duration.Comment: 11 pages, 12 figure
Dynamics of domain walls in magnetic nanostrips
We express dynamics of domain walls in ferromagnetic nanowires in terms of
collective coordinates generalizing Thiele's steady-state results. For weak
external perturbations the dynamics is dominated by a few soft modes. The
general approach is illustrated on the example of a vortex wall relevant to
recent experiments with flat nanowires. A two-mode approximation gives a
quantitatively accurate description of both the steady viscous motion of the
wall in weak magnetic fields and its oscillatory behavior in moderately high
fields above the Walker breakdown.Comment: 4 pages, update to published versio
A Role for the Kolliker-Fuse Nucleus in Cholinergic Modulation of Breathing at Night During Wakefulness and NREM Sleep
For many years, acetylcholine has been known to contribute to the control of breathing and sleep. To probe further the contributions of cholinergic rostral pontine systems in control of breathing, we designed this study to test the hypothesis that microdialysis (MD) of the muscarinic receptor antagonist atropine into the pontine respiratory group (PRG) would decrease breathing more in animals while awake than while in NREM sleep. In 16 goats, cannulas were bilaterally implanted into rostral pontine tegmental nuclei (n = 3), the lateral (n = 3) or medial (n = 4) parabrachial nuclei, or the Kölliker-Fuse nucleus (KFN; n = 6). After \u3e2 wk of recovery from surgery, the goats were studied during a 45-min period of MD with mock cerebrospinal fluid (mCSF), followed by at least 30 min of recovery and a second 45-min period of MD with atropine. Unilateral and bilateral MD studies were completed during the day and at night. MD of atropine into the KFN at night decreased pulmonary ventilation and breathing frequency and increased inspiratory and expiratory time by 12–14% during both wakefulness and NREM sleep. However, during daytime studies, MD of atropine into the KFN had no effect on these variables. Unilateral and bilateral nighttime MD of atropine into the KFN increased levels of NREM sleep by 63 and 365%, respectively. MD during the day or at night into the other three pontine sites had minimal effects on any variable studied. Finally, compared with MD of mCSF, bilateral MD of atropine decreased levels of acetylcholine and choline in the effluent dialysis fluid. Our data support the concept that the KFN is a significant contributor to cholinergically modulated control of breathing and sleep
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