3,165 research outputs found
X-ray photoemission characterization of La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} films
The Curie temperature and x-ray photoemission spectra of thin films of
La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} (LCSMO) have been studied as a function
of the Ca/Sr ratio. The films were grown by off-axis cosputtering from
individual targets of La_{0.67}Ca_{0.33}MnO_{3} (LCMO) and
La_{0.67}Sr_{0.33}MnO_{3} (LSMO) onto (100) oriented NdGaO_{3} substrates. The
films grow with a (100) orientation, with no other orientations observed by
x-ray diffraction. For the alloy mixtures, the Curie temperature, T_C, varies
slowly as the Ca/Sr is decreased, remaining 300 K, while for the LCMO
and LSMO films T_C is 260 and 330 K, respectively. The Mn-O valence structure
is composed of two dominant peaks, whose positions undergo a change as the Ca
fraction is decreased. The core lines behave as linear combinations of lines
from pure LCMO and LSMO.Comment: 3 pages, 5 eps figures. To be published in Journal of Applied Physics
(Proceedings of MMM'98
Coronal heating in multiple magnetic threads
Context. Heating the solar corona to several million degrees requires the
conversion of magnetic energy into thermal energy. In this paper, we
investigate whether an unstable magnetic thread within a coronal loop can
destabilise a neighbouring magnetic thread. Aims. By running a series of
simulations, we aim to understand under what conditions the destabilisation of
a single magnetic thread can also trigger a release of energy in a nearby
thread. Methods. The 3D magnetohydrodynamics code, Lare3d, is used to simulate
the temporal evolution of coronal magnetic fields during a kink instability and
the subsequent relaxation process. We assume that a coronal magnetic loop
consists of non-potential magnetic threads that are initially in an equilibrium
state. Results. The non-linear kink instability in one magnetic thread forms a
helical current sheet and initiates magnetic reconnection. The current sheet
fragments, and magnetic energy is released throughout that thread. We find
that, under certain conditions, this event can destabilise a nearby thread,
which is a necessary requirement for starting an avalanche of energy release in
magnetic threads. Conclusions. It is possible to initiate an energy release in
a nearby, non-potential magnetic thread, because the energy released from one
unstable magnetic thread can trigger energy release in nearby threads, provided
that the nearby structures are close to marginal stability
Characterization of transport and magnetic properties in thin film La(0.67)(Ca(x)Sr(1-x))(0.33)MnO(3) mixtures
We have grown thin films of (100) oriented
La_{0.67}(Ca_{x}Sr_{1-x})_{0.33}MnO_{3} on (100) NdGaO_{3} substrates by
off-axis sputtering. We have looked at the changes in the resistivity and
magnetoresistance of the samples as the Ca/Sr ratio was varied. We find that as
the calcium fraction is decreased, the lattice match to the substrate
decreases, and the films become more disordered, as observed in transport
measurements and the variation in Curie and peak resistance temperatures. We
find a correlation between the temperature independent and T^2 terms to the low
temperature resistivity. The room temperature magnetoresistance exhibits a
maximum as the peak temperature is increased by the substitution of Sr for Ca,
and a change in the field dependence to the resistivity at room temperature is
observed.Comment: 5 pages, 6 eps figures, to be published in Journal of Applied Physic
Magnetic reconnection during collisionless, stressed, X-point collapse using Particle-in-Cell simulation
Two cases of weakly and strongly stressed X-point collapse were considered.
Here descriptors weakly and strongly refer to 20 % and 124 % unidirectional
spatial compression of the X-point, respectively. In the weakly stressed case,
the reconnection rate, defined as the out-of-plane electric field in the
X-point (the magnetic null) normalised by the product of external magnetic
field and Alfv\'en speeds, peaks at 0.11, with its average over 1.25 Alfv\'en
times being 0.04. Electron energy distribution in the current sheet, at the
high energy end of the spectrum, shows a power law distribution with the index
varying in time, attaining a maximal value of -4.1 at the final simulation time
step (1.25 Alfv\'en times). In the strongly stressed case, magnetic
reconnection peak occurs 3.4 times faster and is more efficient. The peak
reconnection rate now attains value 2.5, with the average reconnection rate
over 1.25 Alfv\'en times being 0.5. The power law energy spectrum for the
electrons in the current sheet attains now a steeper index of -5.5, a value
close to the ones observed in the vicinity of X-type region in the Earth's
magneto-tail. Within about one Alfv\'en time, 2% and 20% of the initial
magnteic energy is converted into heat and accelerated particle energy in the
case of weak and strong stress, respectively. In the both cases, during the
peak of the reconnection, the quadruple out-of-plane magnetic field is
generated, hinting possibly to the Hall regime of the reconnection. These
results strongly suggest the importance of the collionless, stressed X-point
collapse as a possible contributing factor to the solution of the solar coronal
heating problem or more generally, as an efficient mechanism of converting
magnetic energy into heat and super-thermal particle energy.Comment: Final Accepted Version (Physics of Plasmas in Press 2007
Potential barrier lowering and electrical transport at the LaAlO/SrTiO heterointerface
Using a combination of vertical transport measurements across and lateral
transport measurements along the LaAlO/SrTiO heterointerface, we
demonstrate that significant potential barrier lowering and band bending are
the cause of interfacial metallicity. Barrier lowering and enhanced band
bending extends over 2.5 nm into LaAlO as well as SrTiO. We explain
origins of high-temperature carrier saturation, lower carrier concentration,
and higher mobility in the sample with the thinnest LaAlO film on a
SrTiO substrate. Lateral transport results suggest that parasitic
interface scattering centers limit the low-temperature lateral electron
mobility of the metallic channel.Comment: 10 pages, 3 figures, and 1 tabl
The atomic structure of large-angle grain boundaries and in and their transport properties
We present the results of a computer simulation of the atomic structures of
large-angle symmetrical tilt grain boundaries (GBs) (misorientation
angles \q{36.87}{^{\circ}} and \q{53.13}{^{\circ}}),
(misorientation angles \q{22.62}{^{\circ}} and \q{67.38}{^{\circ}}). The
critical strain level criterion (phenomenological criterion)
of Chisholm and Pennycook is applied to the computer simulation data to
estimate the thickness of the nonsuperconducting layer enveloping
the grain boundaries. The is estimated also by a bond-valence-sum
analysis. We propose that the phenomenological criterion is caused by the
change of the bond lengths and valence of atoms in the GB structure on the
atomic level. The macro- and micro- approaches become consistent if the
is greater than in earlier papers. It is predicted that the
symmetrical tilt GB \theta = \q{53.13}{^{\circ}} should demonstrate
a largest critical current across the boundary.Comment: 10 pages, 2 figure
One-Step Processing of Spinel Ferrites via the High-Energy Ball Milling of Binary Oxides
MnZn ferrites have been produced via the high-energy ball milling of binary oxide precursors. The milled ferrites have a nonequilibrium cation site distribution, with an unusually high population of Zn cations on the octahedral sites. The particle size distribution drops precipitously with milling time from 60±1 to ∼14±1 nm at 10 h, but increases to 18.5±1 nm after long durations (20–40 h) concurrent with the formation of nearly pure ferrite. A 1 h anneal at 673 K facilitates a redistribution of cations to their near equilibrium sites. This processing approach circumvents the need for deleterious high-temperature heat treatments that often lead to nonstoichiometries in the resulting ferrites
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