Dynamics of magnetic vortex core switching in nanometer-scale permalloy disk, having a single vortex ground state, was investigated by micromagnetic modeling. When an in-plane magnetic field pulse with an appropriate strength and duration is applied to the vortex structure, additional two vortices, i.e., a circular-and an anti-vortex, are created near the original vortex core. Sequentially, the vortex-antivortex pair annihilates. A spin wave is created at the annihilation point and propagated through the entire element; the relaxed state for the system is the single vortex state with a switched vortex core.
A subaperture autofocus algorithm for synthetic aperture radar (SAR) partitions range-compressed phase-history data collected over a full aperture into equal-width subapertures.Application of a one-dimensional Fourier transform to each range bin converts each subaperture data set into a full-scene image (map). Any linear phase difference, or phase ramp, between a pair of subapertures expresses itself as cross-range drift in their maps.A traditional autofocus algorithm fits a polynomial to inferred equal-width phase ramps. If the true phase error function contains significant high-order components, then polynomial regression generates a poor estimate of the phase error functioa Instead of fitting a polynomial, we fit a sinusoidal function through the inferred phase ramps. An example with a degraded SAR image shows how a sinusoidal correction improves image quality.We compare lower bounds on mean squared error (MSE)for polynomial and sinusoidal parameterizations. Sinusoidal parameterization reduces MSE signiticantly for model orders greater than Eve.
This paper describes a family of adaptive change detectors for detecting and segmenting moving objects in video scenes. These change detectors employ several techniques to allow them to adjust to changing image conditions. One of these is a dynamic reference frame which allows the system to adapt to changing backgrounds and produces an unambiguous target silhouette for classification. Another is a scheme to estimate system noise and adjust detection thresholds accordingly to satisfy a chosen performance criterion. Finally, adaptive normalization is used to maintain constant image contrast.
We investigated the influence of the magnetic field pulse parameters and the size of the Fe element to the vortex core switching by micromagnetic modeling. When the magnetic field pulse with an appropriate strength and duration is applied to 30nm thick Fe circular disks with diameters between 100nm and 1μm, the vortex configuration is perturbed away from the equilibrium state, and the circular symmetric distribution of the in-plane magnetization around the vortex core deforms. This leads to the creation of a new vortex core with the opposite polarity and an antivortex. With increasing time, the vortex-antivortex pair annihilates. As a result of the annihilation, a single vortex core with opposite polarity remains and a vortex core switch is realized. The process of core switching, however, strongly depends on the amplitude and duration of the magnetic pulse.
The characteristics of a magnetic field pulse, its magnitude, direction, and rise and fall time on the mechanism of ultrafast magnetization reversal have been studied by micromagnetic simulations. An elliptically shaped, Permalloy thin film, having dimensions of 400nm long axis, 112nm short axis, and 3.2nm, thickness, was considered. A plot of the magnetic field pulse components describes three types of reversal behaviors, quasicoherent, incoherent, and nonreversal. The optimum magnetic field pulse for the suppression of magnetization ringing is found in the incoherent precession area by finding the lowest remanent total energy at the moment the pulse is cut off. The rise and fall time of the pulse has a significant effect on the reversal behavior only in the incoherent precession region. In this region, whether a reversal can occur depends on the rise time of the pulse. Extending the rise and fall time of the pulse, in the noncoherent rotation region, does not lead to a clear change of the remanent total energy but a rise time longer than 50ps does result in a distinct increase in the remanent exchange energy, which enhances the magnetization ringing.
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