85 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
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
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
3′UTR Deletion of NONO Leads to Corpus Callosum Anomaly, Left Ventricular Non-Compaction and Ebstein’s Anomaly in a Male Fetus
NONO (Non-Pou Domain-Containing Octamer-Binding Protein) gene maps on chromosome Xq13.1 and hemizygous loss-of-function nucleotide variants are associated with an emerging syndromic form of intellectual developmental disorder (MRXS34; MIM #300967), characterized by developmental delay, intellectual disability, poor language, dysmorphic facial features, and microcephaly. Structural brain malformation, such as corpus callosum and cerebellar abnormalities, and heart defects, in particular left ventricular non-compaction (LVNC), represent the most recurrent congenital malformations, recorded both in about 80% of patients, and can be considered the distinctive imaging findings of this disorder. We present on a further case of NONO-related disease; prenatally diagnosed in a fetus with complete corpus callosum agenesis; absence of septum pellucidum; pericallosal artery; LVNC and Ebstein’s anomaly. A high-resolution microarray analysis demonstrated the presence of a deletion affecting the NONO 3′UTR; leading to a marked hypoexpression of the gene and the complete absence of the protein in cultured amniocytes. This case expands the mutational spectrum of MRXS34, advises to evaluate NONO variants in pre- and postnatal diagnosis of subjects affected by LVNC and other heart defects, especially if associated with corpus callosum anomalies and confirm that CNVs (Copy Number Variants) represent a non-negligible cause of Mendelian disorders
Study of Percolative Transitions with First-Order Characteristics in the Context of CMR Manganites
The unusual magneto-transport properties of manganites are widely believed to
be caused by mixed-phase tendencies and concomitant percolative processes.
However, dramatic deviations from "standard" percolation have been unveiled
experimentally. Here, a semi-phenomenological description of Mn oxides is
proposed based on coexisting clusters with smooth surfaces, as suggested by
Monte Carlo simulations of realistic models for manganites, also briefly
discussed here. The present approach produces fairly abrupt percolative
transitions and even first-order discontinuities, in agreement with
experiments. These transitions may describe the percolation that occurs after
magnetic fields align the randomly oriented ferromagnetic clusters believed to
exist above the Curie temperature in Mn oxides. In this respect, part of the
manganite phenomenology could belong to a new class of percolative processes
triggered by phase competition and correlations.Comment: 4 pages, 4 eps figure
Phase Separation and the Low-Field Bulk Magnetic Properties of Pr0.7Ca0.3MnO3
We present a detailed magnetic study of the perovskite manganite
Pr0.7Ca0.3MnO3 at low temperatures including magnetization and a.c.
susceptibility measurements. The data appear to exclude a conventional spin
glass phase at low fields, suggesting instead the presence of correlated
ferromagnetic clusters embedded in a charge-ordered matrix. We examine the
growth of the ferromagnetic clusters with increasing magnetic field as they
expand to occupy almost the entire sample at H ~ 0.5 T. Since this is well
below the field required to induce a metallic state, our results point to the
existence of a field-induced ferromagnetic insulating state in this material.Comment: 15 pages with figures, submitted to Physical Review
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