303 research outputs found
The Influence of Magnetic Domain Walls on Longitudinal and Transverse Magnetoresistance in Tensile Strained (Ga,Mn)As Epilayers
We present a theoretical analysis of recent experimental measurements of
magnetoresistance in (Ga,Mn)As epilayers with perpendicular magnetic
anisotropy. The model reproduces the field-antisymmetric anomalies observed in
the longitudinal magnetoresistance in the planar geometry (magnetic field in
the epilayer plane and parallel to the current density), as well as the unusual
shape of the accompanying transverse magnetoresistance. The magnetoresistance
characteristics are attributed to circulating currents created by the presence
of magnetic domain walls
Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures
We demonstrate the manipulation of the Curie temperature of buried layers of
the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the
effect of annealing. Patterning the GaAs-capped ferromagnetic layers into
nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As
layers and thus allows the removal of Mn interstitials using annealing. This
leads to an enhanced Curie temperature and reduced resistivity compared to
unpatterned samples. For a fixed annealing time, the enhancement of the Curie
temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections
Structural and Magnetic Characteristics of MnAs Nanoclusters Embedded in Be-doped GaAs
We describe a systematic study of the synthesis, microstructure and
magnetization of hybrid ferromagnet-semiconductor nanomaterials comprised of
MnAs nanoclusters embedded in a p-doped GaAs matrix. These samples are created
during the in situ annealing of Be-doped (Ga,Mn)As heterostructures grown by
molecular beam epitaxy. Transmission electron microscopy and magnetometry
studies reveal two distinct classes of nanoclustered samples whose structural
and magnetic properties depend on the Mn content of the initial (Ga,Mn)As
layer. For Mn content in the range 5% - 7.5%, annealing creates a
superparamagnetic material with a uniform distribution of small clusters
(diameter around 6 nm) and with a low blocking temperature (T_B approximately
10 K). While transmission electron microscopy cannot definitively identify the
composition and crystalline phase of these small clusters, our experimental
data suggest that they may be comprised of either zinc-blende MnAs or Mn-rich
regions of (Ga,Mn)As. At higher Mn content (> 8 %), we find that annealing
results in an inhomogeneous distribution of both small clusters as well as much
larger NiAs-phase MnAs clusters (diameter around 25 nm). These samples also
exhibit supermagnetism, albeit with substantially larger magnetic moments and
coercive fields, and blocking temperatures well above room temperature
Tuning alloy disorder in diluted magnetic semiconductors in high fields to 89 T
Alloy disorder in II-VI diluted magnetic semiconductors (DMS) is typically
reduced when the local magnetic spins align in an applied magnetic field. An
important and untested expectation of current models of alloy disorder,
however, is that alloy fluctuations in many DMS compounds should increase again
in very large magnetic fields of order 100 tesla. Here we measure the disorder
potential in a ZnCdMnSe quantum well via the low
temperature photoluminescence linewidth, using a new magnet system to 89 T.
Above 70 T, the linewidth is observed to increase again, in accord with a
simple model of alloy disorder.Comment: 4 pages, 3 figure
Giant Anisotropic Magnetoresistance in a Quantum Anomalous Hall Insulator
When a three-dimensional (3D) ferromagnetic topological insulator thin film
is magnetized out-of-plane, conduction ideally occurs through dissipationless,
one-dimensional (1D) chiral states that are characterized by a quantized,
zero-field Hall conductance. The recent realization of this phenomenon - the
quantum anomalous Hall effect - provides a conceptually new platform for
studies of edge-state transport, distinct from the more extensively studied
integer and fractional quantum Hall effects that arise from Landau level
formation. An important question arises in this context: how do these 1D edge
states evolve as the magnetization is changed from out-of-plane to in-plane? We
examine this question by studying the field-tilt driven crossover from
predominantly edge state transport to diffusive transport in Cr-doped
(Bi,Sb)2Te3 thin films, as the system transitions from a quantum anomalous Hall
insulator to a gapless, ferromagnetic topological insulator. The crossover
manifests itself in a giant, electrically tunable anisotropic magnetoresistance
that we explain using the Landauer-Buttiker formalism. Our methodology provides
a powerful means of quantifying edge state contributions to transport in
temperature and chemical potential regimes far from perfect quantization
Magnetic circular dichroism from the impurity band in III-V diluted magnetic semiconductors
The magnetic circular dichroism of III-V diluted magnetic semiconductors,
calculated within a theoretical framework suitable for highly disordered
materials, is shown to be dominated by optical transitions between the bulk
bands and an impurity band formed from magnetic dopant states. The theoretical
framework incorporates real-space Green's functions to properly incorporate
spatial correlations in the disordered conduction band and valence band
electronic structure, and includes extended and localized electronic states on
an equal basis. Our findings reconcile unusual trends in the experimental
magnetic circular dichroism in III-V DMSs with the antiferromagnetic p-d
exchange interaction between a magnetic dopant spin and its host.Comment: 5 pages, 4 figure
Internal magnetic fields in thin ZnSe epilayers
Strain induced spin-splitting is observed and characterized using pump-probe
Kerr rotation spectroscopy in n-ZnSe epilayers grown on GaAs substrates. The
spin-splitting energies are mapped out as a function of pump-probe separation,
applied voltage, and temperature in a series of samples of varying epilayer
thicknesses and compressive strain arising from epilayer-substrate lattice
mismatch. The strain is independently quantified using photoluminescence and
x-ray diffraction measurements. We observe that the magnitude of the spin
splitting increases with applied voltage and temperature, and is highly crystal
direction dependent, vanishing along [1 1-bar 0].Comment: 9 pages, 3 figure
Experimental exploration of compact convolutional neural network architectures for non-temporal real-time fire detection.
In this work we explore different Convolutional Neural Network (CNN) architectures and their variants for non-temporal binary fire detection and localization in video or still imagery. We consider the performance of experimentally defined, reduced complexity deep CNN architectures for this task and evaluate the effects of different optimization and normalization techniques applied to different CNN architectures (spanning the Inception, ResNet and EfficientNet architectural concepts). Contrary to contemporary trends in the field, our work illustrates a maximum overall accuracy of 0.96 for full frame binary fire detection and 0.94 for superpixel localization using an experimentally defined reduced CNN architecture based on the concept of InceptionV4. We notably achieve a lower false positive rate of 0.06 compared to prior work in the field presenting an efficient, robust and real-time solution for fire region detection
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