Exchange switching of a magnetic junction by a current pulse

Results of numerical simulation of switching of a magnetic transition induced by a spin-polarized current pulse in the presence of an external magnetic field for a current density and a field strength that are lower than the corresponding threshold values are presented. The magnetic field regenerates the system and drives it to the threshold of an orientation phase transition, which simplifies exchange switching induced by a current pulse. This switching is characterized by the same locality as the exchange interaction, i.e., it is local at atomic lengths. It is demonstrated that there exists the possibility of switching with a controllable delay with respect to the current pulse.     

Gas-sensitive layers based on fractal-percolation structures

A model of sensitive layers with built-in percolation-cluster structure near the percolation threshold is developed. It is shown that, in the case of the interaction of such gas-sensitive layers with reducing gases, the value of the gas sensitivity can exceed the typical values by several orders of magnitude. The specific features of variation in the impedance response of the percolation structures in the vicinity of the percolation threshold in air and in the case of the influence of a reducing gas are considered.     

Spin-Polarized Transport in II–VI Magnetic Resonant-Tunneling Devices

This paper investigates electronic transport through II-VI semiconductor resonant-tunneling structures containing diluted magnetic impurities. Due to the exchange interaction between the conduction electrons and the impurities, there arises a giant Zeeman splitting in the presence of a moderately low magnetic field. As a consequence, when the quantum well is magnetically doped, the current-voltage characteristics show two peaks corresponding to transport for each spin channel. This behavior is experimentally observed and can be reproduced with a simple tunneling model. The model thus allows to analyze other configurations. First, the magnetic field was further increased, which leads to a spin polarization of the electronic current injected from the leads, thus giving rise to a relative change in the current amplitude. The authors demonstrate that the spin polarization in the emitter can be determined from such a change. Furthermore, in the case of an injector with magnetic impurities, the model shows a large increase in peak amplitude accompanied by a shift of the resonance to higher voltages with increasing fields. It was found that this effect arises from a combination of 3D incident distribution, giant Zeeman splitting, and broad resonance linewidth     

Size effect in a percolation model of interconnect failure

The problem of interconnect failure is studied in the framework of percolation theory. The probability failure of a long interconnect due to statistical fluctuations of defects is found. It is demonstrated that two possible diapasons of length to width ratios exist. For extremely long wires, the percolation threshold is determined by rare accumulation of defects, breaking down the wire. For moderately long wires, the threshold concentration of defects is near the macroscopic percolation threshold. The lowering of threshold due to finite width of wire is found. The percolation threshold is found in a two-dimensional system, containing the individual vacancies and voids, caused by coalescence of vacancies.     

磁共振图像阀值去噪方法研究

磁共振成像已成为脑功能病理和解剖研究的主要手段，是医学影像学领域中最活跃的技术。由于在成像过程中复杂的电磁场环境容易受到人体热噪声干扰，使得磁共振图像去噪成为很重要的研究热点。小波分析具有多尺度分辨和去相关性等特点，在去除被白噪声污染的磁共振图像方面得到了广泛应用。但磁共振图像经传统的小波分析去噪后，细节信息部分丢失，图像的边缘变得模糊．针时这些问题，时经典的小波阀值去噪方法进行了改进，将关键参数取值与预估计联系起来，将阀值的选定与图像的局部特征结合起来，提出一种灵活的、自适应的去噪新方法。与经典方法相比，采用本方法处理的噪声图像去噪后图像的细节更丰富，边缘信息完善，视觉效果更好。     

Electron spin resonance of interacting spins in <Emphasis Type="Italic">n</Emphasis>-Ge: II. Change in the width and shape of lines

The effect of spin interaction on the width and shape of the electron spin resonance line in compensated and uncompensated n-Ge:As has been studied. It is shown that, in the case of a magnetic field oriented along the [100] axis, the width of the resonance line decreases irrespective of the degree of compensation as the critical concentration of the insulator-metal transition is approached, owing to enhancement of the exchange interaction of spins and to an increase in the spin relaxation time. When the magnetic field is directed along other axes, an additional line broadening appears in compensated samples. This broadening is determined by the influence exerted on the g factor by fluctuations of the internal electrostatic field via the stresses generated by these fluctuations. For well-conducting samples, in which the thickness of the skin layer becomes smaller than that of the sample, the line takes on an asymmetric (Dysonian) shape. In this case, the ratio between the wings of the derivative, characteristic of this line shape, is determined by the ratio between the rates of spin diffusion and spin relaxation.     

Percolative charge transport in a co-evaporated organic molecular mixture

Understanding the charge transport in molecular semiconductor mixtures remains challenging, largely due to the lack of a universal dependence of carrier mobility upon doping concentration. Here we demonstrate that it is feasible to use the percolation theory to explain the change of charge mobility in a model system of 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) and tris-(8-hydroxyquinoline) aluminum (Alq₃) with various doping concentrations. As the fraction of CBP within the mixtures increases, the charge mobility firstly shows a reduction at low CBP fraction due to the scattering effect, and then increases well following a percolation model. Electron microscopy and atomic force microscopy analysis suggest that CBP and Alq₃ are homogeneously mixed in their co-evaporated amorphous films, which meets the precondition for using percolation theory. We describe the possible microcosmic percolating mechanism with a model combining bond percolation with charge transfer integral calculation. Based on this model, the percolation threshold in molecular semiconductor mixtures can be predicated. For the hole and electron transport in our system, the predicated percolation thresholds are very close to the experimental values.     

Evolution of exciton states near the percolation threshold in two-phase systems with II–VI semiconductor quantum dots

From studies of two-phase systems (borosilicate matrices containing ZnSe or CdS quantum dots), it was found that the systems exhibit a specific feature associated with the percolation phase transition of charge carriers (excitons). The transition manifests itself as radical changes in the optical spectra of both ZnSe and CdS quantum dot systems and by fluctuations of the emission band intensities near the percolation threshold. These effects are due to microscopic fluctuations of the density of quantum dots. The average spacing between quantum dots is calculated taking into account their finite dimensions and the volume fraction occupied by the quantum dots at the percolation threshold. It is shown that clustering of quantum dots occurs via tunneling of charge carriers between the dots. A physical mechanism responsible for the percolation threshold for charge carriers is suggested. In the mechanism, the permittivity mismatch of the materials of the matrix and quantum dots plays an important role in delocalization of charge carriers (excitons): due to the mismatch, “a dielectric trap” is formed at the external surface of the interface between the matrix and a quantum dot and, thus, surface exciton states are formed there. The critical concentrations of quantum dots are determined, such that the spatial overlapping of such surface states provides the percolation transition in both systems.     

Effect of p-d exchange with an itinerant carrier in a GaMnAs quantum dot

Hydrogenic acceptor binding energy as a function of dot radius in a GaMnAs/Ga_0.6Al_0.4As quantum dot is calculated including the exchange interaction of Mn alloy content with an itinerant carrier.Calculations are performed by varying its dot radius,for various Mn alloy contents in GaMnAs quantum dot within a single band effective mass approximation using variational method.The spin polaronic energy of the acceptor impurity for different Mn²⁺ is evaluated for different dot radii using a mean field theory in the presence of magnetic field strength.The magnetization is computed in the influence of magnetic field and the Mn ions.The effective g-factor of the valence band electron with the inclusion of effects of Mn ion impurities is found in the influence of the magnetic field.The exchange coupling constant is calculated for various magnetic field strengths.The results show that the p-d exchange interaction in the GaMnAs/Ga_0.6Al_0.4As quantum dot has a strong dependence on spatial confinement,effect of magnetic field strength and the Mn alloy content.Our results are in good agreement with the other investigators.     

Matrix-type effect on the magnetotransport properties of Ni–AlO and Ni–NbO composite systems

The effect of the insulating-matrix material on the electronic and magnetic properties of nanocomposites is investigated in the Ni_x(Al₂O₃)_100–x metal–insulator system and the Ni_x(Nb₂O₅)_100–x metal–semiconductor system. It is established that the characteristics of composites determined by electron transport through the matrix (the electrical resistivity, the position of the electrical percolation threshold, the magnetoresistance effect) depend on the material type. Replacement of the matrix from Al₂O₃ to Nb₂O₅ results in a decrease in the electrical resistivity by two–three orders of magnitude, a decrease in the magnetic resistivity by more than an order of magnitude, and in displacement of the percolation threshold from 40 to 30 at % of Ni. In this case, the magnetic properties of the composites are independent of the type of matrix: the concentration of the magnetic percolation threshold is identical in the two systems (~45 at % of Ni), and the coercive force of the samples occurring beyond the percolation threshold is close in magnitude (5–8 and 12–18 Oe) in the Ni_x(Nb₂O₅)_100–x and Ni_x(Al₂O₃)_100–x composites, respectively.