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 Zn$_{.70}$Cd$_{.22}$Mn$_{.08}$Se 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