123 research outputs found
Al-Substitution Effects on Physical Properties of the Colossal Magnetoresistance Compouns La0.67ca0.33mno3
We present a detailed study of the polycrystalline perovskite manganites
La0.67Ca0.33AlxMn1-xO3 (x = 0, 0.1, 0.15, 0.5) at low temperatures and high
magnetic fields, including electrical resistance, magnetization, ac
susceptibility. The static magnetic susceptibility was also measured up to 1000
K. All the samples show colossal magnetoresistance behavior and the Curie
temperatures decrease with Al doping. The data suggest the presence of
correlated magnetic clusters near by the ferromagnetic transition. This appears
to be a consequence of the structural and magnetic disorder created by the
random distribution of Al atoms.Comment: 13 pages including 5 figure
Injection and detection of spin in a semiconductor by tunneling via interface states
Injection and detection of spin accumulation in a semiconductor having
localized states at the interface is evaluated. Spin transport from a
ferromagnetic contact by sequential, two-step tunneling via interface states is
treated not in itself, but in parallel with direct tunneling. The spin
accumulation induced in the semiconductor channel is not suppressed, as
previously argued, but genuinely enhanced by the additional spin current via
interface states. Spin detection with a ferromagnetic contact yields a weighted
average of the spin accumulation in the channel and in the localized states. In
the regime where the spin accumulation in the localized states is much larger
than that in the channel, the detected spin signal is insensitive to the spin
accumulation in the localized states and the ferromagnet probes the spin
accumulation in the semiconductor channel.Comment: 7 pages, 2 figures. Theory onl
Thermal spin current and magnetothermopower by Seebeck spin tunneling
The recently observed Seebeck spin tunneling, the thermoelectric analog of
spin-polarized tunneling, is described. The fundamental origin is the spin
dependence of the Seebeck coefficient of a tunnel junction with at least one
ferromagnetic electrode. Seebeck spin tunneling creates a thermal flow of
spin-angular momentum across a tunnel barrier without a charge tunnel current.
In ferromagnet/insulator/semiconductor tunnel junctions this can be used to
induce a spin accumulation (\Delta \mu) in the semiconductor in response to a
temperature difference (\Delta T) between the electrodes. A phenomenological
framework is presented to describe the thermal spin transport in terms of
parameters that can be obtained from experiment or theory. Key ingredients are
a spin-polarized thermoelectric tunnel conductance and a tunnel spin
polarization with non-zero energy derivative, resulting in different Seebeck
tunnel coefficients for majority and minority spin electrons. We evaluate the
thermal spin current, the induced spin accumulation and \Delta\mu/\Delta T,
discuss limiting regimes, and compare thermal and electrical flow of spin
across a tunnel barrier. A salient feature is that the thermally-induced spin
accumulation is maximal for smaller tunnel resistance, in contrast to the
electrically-induced spin accumulation that suffers from the impedance mismatch
between a ferromagnetic metal and a semiconductor. The thermally-induced spin
accumulation produces an additional thermovoltage proportional to \Delta\mu,
which can significantly enhance the conventional charge thermopower. Owing to
the Hanle effect, the thermopower can also be manipulated with a magnetic
field, producing a Hanle magnetothermopower.Comment: 10 pages, 3 figures, 1 tabl
Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars
We report experimental and analytical work on spin-transfer torque induced
vortex dynamics in metallic nanopillars with in-plane magnetized layers. We
study nanopillars with a diameter of 150 nm, containing two Fe layers with a
thickness of 15 nm and 30 nm respectively, separated by a 6 nm Ag spacer. The
sample geometry is such that it allows for the formation of magnetic vortices
in the Fe disks. As confirmed by micromagnetic simulations, we are able to
prepare states where one magnetic layer is homogeneously magnetized while the
other contains a vortex. We experimentally show that in this configuration
spin-transfer torque can excite vortex dynamics and analyze their dependence on
a magnetic field applied in the sample plane. The center of gyration is
continuously dislocated from the disk center, and the potential changes its
shape with field strength. The latter is reflected in the field dependence of
the excitation frequency. In the second part we propose a novel mechanism for
the excitation of the gyrotropic mode in nanopillars with a perfectly
homogeneously magnetized in-plane polarizing layer. We analytically show that
in this configuration the vortex can absorb energy from the spin-polarized
electric current if the angular spin-transfer efficiency function is
asymmetric. This effect is supported by micromagnetic simulations.Comment: The article has been sent to J. Phys. D. Submitted on August 9, 2010.
(7 pages and 4 figures.
Experimental determination of superconducting parameters for the intermetallic perovskite superconductor ${\text {MgCNi}}_3
We have measured upper-critical-field , specific heat C, and
tunneling spectra of the intermetallic perovskite superconductor MgCNi
with a superconducting transition temperature K. Based
on these measurements and relevant theoretical relations, we have evaluated
various superconducting parameters for this material, including the
thermodynamic critical field (0), coherence length (0),
penetration depth (0), lower-critical-field (0), and
Ginsberg-Landau parameter (0). From the specific heat, we obtain the
Debye temperature 280 K. We find a jump of
=2.3 at (where is the
normal state electronic specific coefficient), which is much larger than the
weak coupling BCS value of 1.43. Our tunneling measurements revealed a gap
feature in the tunneling spectra at with 4.6, again larger than the weak-coupling value
of 3.53. Both findings indicate that MgCNi is a strong-coupling
superconductor. In addition, we observed a pronounced zero-bias conductance
peak (ZBCP) in the tunneling spectra.
We discuss the possible physical origins of the observed ZBCP, especially in
the context of the pairing symmetry of the material.Comment: 5 pages, 4 figure
How does over-squashing affect the power of GNNs?
Graph Neural Networks (GNNs) are the state-of-the-art model for machine
learning on graph-structured data. The most popular class of GNNs operate by
exchanging information between adjacent nodes, and are known as Message Passing
Neural Networks (MPNNs). Given their widespread use, understanding the
expressive power of MPNNs is a key question. However, existing results
typically consider settings with uninformative node features. In this paper, we
provide a rigorous analysis to determine which function classes of node
features can be learned by an MPNN of a given capacity. We do so by measuring
the level of pairwise interactions between nodes that MPNNs allow for. This
measure provides a novel quantitative characterization of the so-called
over-squashing effect, which is observed to occur when a large volume of
messages is aggregated into fixed-size vectors. Using our measure, we prove
that, to guarantee sufficient communication between pairs of nodes, the
capacity of the MPNN must be large enough, depending on properties of the input
graph structure, such as commute times. For many relevant scenarios, our
analysis results in impossibility statements in practice, showing that
over-squashing hinders the expressive power of MPNNs. We validate our
theoretical findings through extensive controlled experiments and ablation
studies
Quenched Slonczewski-Windmill in Spin-Torque Vortex-Oscillators
We present a combined analytical and numerical study on double-vortex
spin-torque nano-oscillators and describe a mechanism that suppresses the
windmill modes. The magnetization dynamics is dominated by the gyrotropic
precession of the vortex in one of the ferromagnetic layers. In the other layer
the vortex gyration is strongly damped. The dominating layer for the
magnetization dynamics is determined by the current polarity. Measurements on
Fe/Ag/Fe nano-pillars support these findings. The results open up a new
perspective for building high quality-factor spin-torque oscillators operating
at selectable, well-separated frequency bands
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