浅谈频域和时域

1频域和时域概念 我们经常讲的信号时域分析就是分析信号随时间的变化,比如24小时体温监测图即体温随时间变化,如图1.1;24小时动态血压监测图即血压随时间的变化,如图1.2;心电图即心电信号随收缩期,舒张期的时间变化,如图1.3;超声多普勒频谱图即血流速度随收缩期、舒张期的时间变化,如图1.4.      

Micromagnetic simulation of domain wall pinning and domain wall motion

Domain wall pinning is the coercivity mechanism of permanent magnets used in high temperature applications. In SmCo based magnets domain walls get trapped at the cellular precipitation structure causing a high coercive field. The motion of domain walls and their propagation velocity are important in soft magnets as used in sensor applications. A finite element micromagnetic algorithm was developed to study the motion of domain walls in complex microstructures. The cellular microstructure of SmCo magnets or the cylindrical soft wires can be easily built using tetrahedral finite elements. The pinning of the domain walls has been studied for different material compositions. Attractive and repulsive domain wall pinning are observed and their behaviour for increasing thickness of the precipitation structure is explained. The motion of domains in magnetic nanowires was calculated using adaptive mesh refinement. The wall velocity strongly depends on the domain wall structure. Transverse and vortex walls have been observed and their velocity in wires of different thickness has been studied.     

铁电陶瓷的电畴及畴变观测研究进展

铁电陶瓷材料，特别是锆钛酸铅(PZT)在众多领域具有广泛的应用前景，影响其推广应用的主要因素是使用过程中外电、力场引起的材料性能的退化。观测铁电电畴及畴变的方法对研究其在外场下性能破坏机理、提高其使用的可靠性和预防其失效具有重要的理论和实际意义。本文比较了不同实验方法和测试技术的优缺点，对铁电陶瓷的电畴观测进行了综述，并简要总结了铁电陶瓷的畴变观测技术研究现状，指出了目前该领域研究中存在的问题。     

植物微弱电波信号的时、频域分析

在时域和频域上对植物电波信号进行了分析.植物电波信号的幅值处于μV级量级,是一种微弱信号;随时间变化剧烈,在不同时刻的取值关系松散;植物电波信号属于低频信号,其功率谱主要分布在小于5 Hz这一频段.     

Time domain approximations in the solution of fields by time domain diakoptics

The numerical solution of field substructures in time domain diakoptics requires an interation time consistent with the numerical accuracy required. When storing and reconnecting the substructures, the information required can be obtained from considerably fewer samples. This paper shows how time domain approximations can further reduce the storage and computing time in space approximated time domain diakoptics.     

Bubble domain memory materials

The selection and preparation of bubble domain materials is discussed in the light of materials requirements for bubble domain memories. The history of bubble domain materials development is outlined with emphasis on the discovery and exploitation of two types of materials, magnetic garnets and amorphous magnetic alloys. It is shown how the great compositional flexibility of the garnet system has led to many garnet choices for device use without imposing undue hardship on the materials developer and supplier, and how liquid phase epitaxy of garnet films is a simple way of providing reproducibly films that meet device requirements. Amorphous magnetic alloys are discussed with emphasis on Gd-Co-Mo compositions. In the search for small bubble materials, it is pointed out that both garnets and amorphous alloys have limitations. In the former case,qwill usually be too low and in the latter case, the drift of properties with temperature may be unacceptable.     

Curvature of domain walls

An experiment was performed whereby domain walls were forced to bulge under the influence of an applied easy-axis fieldH. The region where the domain wall is bent appears to be approximately circular. To relate the radius of such a curved domain wall to theory, a model was chosen that takes into account magnetostatic, anisotropy, and exchange energy. The assumption was made that anisotropy and exchange energy, as well as the magnetostatic contribution due to the local magnetization distribution of a domain wall, may be summarized in a wall energy term. The energy of the entire system has been calculated under the constraint that the domain wall should be circular and that it is inscribed tangentially into a triangle. Minimizing the total energy with respect to the radius of curvatureA, the result can be expressed approximately byA propto (H - H_{c})^{-1}where Hcis the coercive force. This result fits the model of a membrane under pressure.     

Contiguous-disk bubble domain devices

Contiguous-disk bubble devices are an approach to higher bit density through the use of coarse overlay patterns in manipulating small bubbles to relax device lithography requirements. As a first step towards such an objective, a fully processed chip using ion-implanted devices has been tested, showing the feasibility of all required memory functions with 5-μm bubbles and 25-μm period overlay patterns. A critique of permalloy versus implanted contiguous-disk devices is made, pointing out their basic difference in magnetization reversal processes and explaining the superiority of the latter over the former in achieving a good edge affinity of bubbles. The requirements for a good implanted device are reviewed, including the selection of garnet material parameters (K1, λ111), of implantation parameters (ion energy and dosage) and of device pattern geometry (thickness and shape of implanted layer). An understanding of these requirements has made it possible to demonstrate 1-μm bubble propagation in several contiguous-disk type circuits with 4.5-μm periods, yielding an areal density of over 3 × 10⁷bit/in²made by conventional photolithography.     

Time domain Born approximation

The time domain Born approximation for ultrasonic scattering from volume flaws in an elastic medium is described. Results are given both for the direct and the inverse problem. The time domain picture leads to simple intuitive formulas, which we illustrate by means of several simple examples. Particular emphasis is given to the front surface echo and its use in reconstructing the properties of the flaw.     

空间域和频域结合的图像增强技术及实现

图像增强是数字图像的预处理,对图像整体或局部特征能有效地改善.为了实现对数字图像的增强处理,采用时域直方图均衡和频域高频加强滤波相结合的方法对图像进行了增强处理.利用图像中变化剧烈的信息只与高频成分有关这一原理,结合MATLAB设计实现了高频加强滤波器并对图像进行了增强处理,在此基础上使用时域直方图均衡技术再对图像进行处理.试验结果表明,两种技术的结合可以使图像的细部特征更加明显,图像更加锐化,其图像增强效果要好于单独采用其中任意一种技术的处理结果.     

Nanopore domain growth behavior by nanopore changes near domain boundaries in porous anodic alumina

We investigate the nanopore changes near domain boundaries during the nanopore growth in porous anodic alumina (PAA) to understand the domain growth behavior with the anodization time. In order to observe the pore changes with the time, we analyze cleavage planes of PAA according to the nanopore length using a field emission scanning electron microscopy. The domain growth can be explained with three kinds of nanopore changes observed near domain boundaries: a change of pore diameter, a pore-branching, and a pore-movement.     

Current-induced domain nucleation in ferromagnet

A key mechanism of the current-induced magnetization dynamics is the spin torque from a spin polarized current (spin current), which couples to spatial gradient of magnetization. Recently, it was pointed out that a large spin current applied to a uniform ferromagnet leads to a spin-wave instability. In this paper, we show that such instability is absent in a state containing a domain wall. This may indicate that nucleation of magnetic domains occurs above a certain critical spin current. This scenario is supported by an explicit energy comparison between the uniformly magnetized state and the domain-wall state under spin current.     

Arlequin based PGD domain decomposition

Problems defined in fully or partially separable domains can be solved by considering a space separated representation of the unknown fields. Thus three-dimensional problems can be solved from the solution of some one-dimensional problems in the case of fully separated representations involving the three space coordinates or as a sequence of 2D and 1D problems in the case of partially separated representations (plates, shells or extruded geometries). When the domains become more complex, sometimes they can be simplified by using appropriate mappings. When it is not possible or such a transformation becomes too complex, the use of domain decomposition could facilitate the use of separated representations. However, domain coupling in the context of space separated representations have never been analyzed. In this paper we propose a domain decomposition strategy based on the use of space separated representations and the Arlequin coupling strategy. First we consider separated representations of the physical space that will be then extended to address parametric solutions.     

Thermally activated domain wall motion

The slow motion of magnetic domain walls was investigated over the range77 leq T leq 423K. It is concluded that this slow motion is dominated by thermally activated unpinning of walls from crystal defects.     

Editorial domain specific aspect languages

Most research in the AOSD community focuses on generalpurpose aspect languages. Unfortunately, the trend towards ever more expressive languages has a major drawback: general-purpose aspect languages (GPALs) are losing their purposefullness. In the extreme, pointcut expressions, for instance, become Turing-complete queries.     

Thermally activated domain wall motion

A theory of thermally activated domain wall motion is presented in which attention is focused on the functional form of the domain wall energy variation in the direction of motion. It is shown that in general the activation volume is magnetic field dependent, and not necessarily constant, as is often assumed. A simple power law form of the wall energy variation, due to sample inhomogeneities, is found to exhibit a ln (wall veiocity) against field slope which decreases with decreasing temperature as is observed in thin films. The usual assumption of a constant activation volume predicts a slope which increases as the temperature falls.     

Wall clusters and domain structure conversions

The significance of wall clusters, which is a new concept in the theory of soft magnetic materials, is experimentally demonstrated in thin Permalloy configurations. The wall cluster is a collection of domain walls that have one intersection line in common. The transformation of the domain structures takes place through a coherent movement of the domain walls. The correlation between the walls is especially dominant at the intersection line of the walls, called the cluster knot. Relations for the mutual positions of the walls in the wall clusters of great practical relevance are derived explicitly and verified experimentally. The domain structure is formed by the concatenation of wall clusters. The clock sense of the rotation over the walls in the clusters determine which walls of two clusters are linked during the formation of the domain structure. The creation of new clusters takes place through the unfolding of the walls of the clusters which originally coincide with the so-called creation line. As is demonstrated fully, the application of these ideas improves the insight into the complex process of domain structure transformations.     

Optical angular correlation in fourier domain

Abstract

The angular correlation of Fourier spectra is optically implemented by means of a single Fourier transformer. Fourier-domain-based angular correlation, which is a technique specific to periodic pattern recognition and characterization, is efficiently applied in real time to ordinary textile structures. By introducing scale corrections, either isomorphic or anamorphic, the optical system is capable of recognizing different structures of the same sort of fabric even when the fundamental frequencies—or thread densities—do not coincide. Two possible methods to introduce the information into the input image of the optical angular correlator are considered: an opto-mechanical rotator containing a transparency with the input sample image; and an electronic addressed spatial light modulator that displays the input sample image controlled by computer. Experimental results of both possibilities are presented and discussed.