Lorentz transmission electron microscopy studies on topological magnetic domains

Lorentz transmission electron microscopy(TEM) is a powerful tool to study the crystal structures and magnetic domain structures in correlation with novel physical properties. Nanometric topological magnetic configurations such as vortices, bubbles, and skyrmions have received enormous attention from the viewpoint of both fundamental science and potential applications in magnetic logic and memory devices, in which understanding the physical properties of magnetic nanodomains is essential. In this review article, several magnetic imaging methods in Lorentz TEM including the Fresnel and Foucault modes, electron holography, and differential phase contrast(DPC) techniques are discussed, where the novel properties of topological magnetic domains are well addressed. In addition, in situ Lorentz TEM demonstrates that the topological domains can be efficiently manipulated by electric currents, magnetic fields, and temperatures, exhibiting novel phenomena under external fields, which advances the development of topological nanodomain-based spintronics.     

Electric current control of creation and annihilation of sub-100 nm magnetic bubbles examined by full-field transmission soft X-ray microscopy

The effect of electric current pulses on a sub-100 nm magnetic bubble state in a symmetric Pt/Co multilayer was directly observed using a full-field transmission soft X-ray microscope (MTXM). Field-induced evolution of the magnetic stripe domains into isolated bubbles with their sizes down to 100 nm was imaged under varying external magnetic fields. Electric current pulses were then applied to the created magnetic bubbles, and it was observed that the bubbles could be either created or annihilated by the current pulse depending on the strength of applied magnetic field. The results suggest that the Joule heating plays a critical role in the formation and/or elimination of the bubbles and skyrmions. Finally, the schematic phase diagram for the creation and annihilation of bubbles is presented, suggesting an optimized scheme with the combination of magnetic field and electric current necessary to utilize skyrmions in the practical devices.     

Multi-bubble motion behavior of uniform magnetic field based on phase field model

Aiming at the interaction and coalescence of bubbles in gas–liquid two-phase flow, a multi-field coupling model was established to simulate deformation and dynamics of multi-bubble in gas–liquid two-phase flow by coupling magnetic field, phase field, continuity equation, and momentum equation. Using the phase field method to capture the interface of two phases, the geometric deformation and dynamics of a pair of coaxial vertical rising bubbles under the applied uniform magnetic field in the vertical direction were investigated. The correctness of results is verified by mass conservation method and the comparison of the existing results. The results show that the applied uniform magnetic field can effectively shorten the distance between the leading bubble and the trailing bubble, the time of bubbles coalescence, and increase the velocity of bubbles coalescence. Within a certain range, as the intensity of the applied uniform magnetic field increases, the velocity of bubbles coalescence is proportional to the intensity of the magnetic field, and the time of bubbles coalescence is inversely proportional to the intensity of the magnetic field.     

Time Travel and Warp Drives: A Scientific Guide to Shortcuts through Time and Space,by Allen Everett and Thomas Roman

In favourable circumstances magnetic domains can be made to move very rapidly through a ferromagnetic material in response to an applied magnetic field. Recently this phenomenon has been exploited in a novel situation where cylindrical magnetic domains or magnetic ‘bubbles’ are used to store and manipulate information in a binary form.

Because of their small size and high mobility bubbles may be arranged to form a memory which has a high storage density and which can be quickly accessed. In recent years much effort has gone into developing suitable materials to support these bubbles and into the design of optimum device configurations. Obviously there have been many accompanying investigations into the physics of bubble domains and methods of observing them.

One of the important observational techniques, particularly for small bubbles, is electron microscopy which gives information on bubble structure which is unobtainable by any other method. This review seeks first to explain what magnetic bubbles are, secondly to discus briefly suitable materials and the workings of bubble devices and finally to outline the theory of observing bubbles in the electron microscope and the results of some experimental observations.     

Influence of magnetic fields on the behavior of bubbles in liquid metals

The paper reviews numerical and experimental investigations concerned with the physics of rising bubbles in conducting liquid metals under the action of a magnetic field. Different situations, characterized by different void fractions ranging from single bubbles to bubble swarms, are considered. The impact of the geometrical arrangement is addressed covering large containers with bubbles far from the walls and narrow containers with bubbles interacting with the walls. It is demonstrated that magnetic fields offer a convenient means to influence bubble dynamics, which makes them interesting for technological applications.     

导电流体中气泡径向振动行为的磁场控制

液态金属中气泡行为是磁流体力学的重要方面。为对磁场条件下导电流体中气泡动力学行为作全面理解,基于磁流体动力学方法建立了磁场条件下导电流体中气泡径向振动的无量纲化动力学方程,数值研究了磁场对导电流体中气泡径向非线性振动稳定性、泡内温度、泡内气压及液体空化阈值的影响。结果显示:磁场增强了气泡非线性振动的稳定性,随着磁场增强且当作用在泡上的电磁力与惯性力数量级可比时,气泡运动为稳定的周期性振动;同时,磁场引起泡内温度、泡内压力及液体空化阈值变化。研究表明,可用磁场调节和控制液态金属中气泡的运动使其满足工程应用需求。      

双等离子体团相互作用的磁流体力学模拟

利用商用磁流体力学模拟程序USIM对双等离子体团相互作用过程进行了数值模拟,分别考察和比较了双对流等离子体团在外加磁场和无外加磁场情况下,相互作用的物理过程.发现在外加磁场情况下等离子体团相互作用会伴随着磁重联(反向磁场)、磁排斥(同向磁场)以及一些不稳定过程.针对激光产生等离子体团错位相互作用实验,进行了标度模拟,发现外加磁场起着重要作用,进一步表明激光等离子体的磁化特征.研究结果为下一步在神光Ⅱ激光装置进行强磁环境下等离子体实验提供理论指导.     

甘油介质在同轴形成线中的击穿特性研究

针对甘油介质在形成线中的应用,总结了甘油作为储能介质已开展的相关研究工作,并在缩比的同轴电极试件中进行了电极表面、磁场、耐压极性、含气量等条件对甘油介质击穿特性影响的实验研究。搭建了基于晶闸管控制的空心脉冲变压器升压实验平台,设计了浸没于脉冲磁场中的同轴电极击穿试件,实验平台最大输出电压500 kV,上升时间26 s,最大磁场1 T,可通过控制晶闸管的先后触发使击穿过程发生于准稳衡磁场中,并制作了外形一致、表面不同的砂纸打磨、羊毛抛光、金属电镀和非金属电镀四种电极。实验结果表明:甘油的击穿是没有极性的;1 T量级磁场对甘油介质的击穿特性无影响;不同电极表面微观形貌差异较大,使甘油介质具有不同的击穿特性,说明甘油击穿在电极表面的过程具有较大影响;充分的排气能减少甘油中直径较大的气泡,减少概率性的低击穿场强,击穿后产生的大量微小气泡会整体降低甘油的击穿阈值,使甘油的平均击穿场强降低。     

Oscillations of a bubble separated from an air cavity under compression caused by magnetic field in a magnetic fluid

Based on the concept of mapping the free surface geometry of a weakly magnetic medium by the topography of isolines of the magnetic field strength magnitude, the shape of the free surface of a magnetic fluid is studied in the static state at two stages: the initial stage, where an annular magnet is moving toward the surface of the magnetic fluid column in a tube, and at the stage where the air cavity is pressed to the bottom. It is shown that the separation of bubbles from the air cavity occurs at the magnet axis in the immediate vicinity of the magnet symmetry plane. A method and an experimental setup are proposed to investigate possible electromagnetic indication of the size of air bubbles formed in the magnetic fluid. The results of an experimental study of bubble separation from an air cavity held in the magnetic fluid and compressed by the pondermotive forces of the magnetic field are discussed. The results may be of importance for designing a new technique for metering small gas shots to a reactor.     

A current density conservative scheme for incompressible MHD flows at a low magnetic Reynolds number. Part II: On an arbitrary collocated mesh

A conservative formulation of the Lorentz force is given here for magnetohydrodynamic (MHD) flows at a low magnetic Reynolds number with the current density calculated based on Ohm’s law and the electrical potential formula. This conservative formula shows that the total momentum contributed from the Lorentz force is conservative when the applied magnetic field is constant. For the case with a non-constant applied magnetic field, the Lorentz force has been divided into two parts: a strong globally conservative part and a weak locally conservative part.The conservative formula has been employed to develop a conservative scheme for the calculation of the Lorentz force on an unstructured collocated mesh. Only the current density fluxes on the cell faces, which are calculated using a consistent scheme with good conservation, are needed for the calculation of the Lorentz force. Meanwhile, a conservative interpolation technique is designed to get the current density at the cell center from the current density fluxes on the cell faces. This conservative interpolation can keep the current density at the cell center conservative, which can be used to calculate the Lorentz force at the cell center with good accuracy. The Lorentz force calculated from the conservative current at the cell center is equivalent to the Lorentz force from the conservative formula when the applied magnetic field is constant, which can conserve the total momentum. We will further prove that the simple interpolation scheme used in the Part I [M.-J. Ni, R. Munipalli, N.B. Morley, P.Y. Huang, M. Abdou, A current density conservative scheme for MHD flows at a low magnetic Reynolds number. Part I. On a rectangular collocated grid system, Journal of Computational Physics, in press, doi:10.1016/j.jcp.2007.07.025] of this series of papers is conservative on a rectangular grid and can keep the total momentum conservative in a rectangular grid.     

Simulation of a bubble chain in a container of high aspect ratio exposed to a magnetic field

The influence of a homogeneous magnetic field acting in the direction of gravity on a bubble chain is studied with phase-resolving numerical simulations. The bubbles rise in a narrow container filled with liquid metal. Individual bubbles are represented by an immersed boundary method with the bubble shape being described by spherical harmonics and deformed by the surrounding liquid metal. A Gaussian bubble size distribution is realized as suggested by corresponding experiments. Bubble-bubble and bubble-wall interactions are modelled based on a repelling potential. With a magnetic field, the averaged trajectory of the bubble chain becomes more rectilinear, and the transverse dispersion is reduced. The average rise velocity decreases under the impact of the field.     

面内磁场中垂直布洛赫线的消失

当面内磁场大于某一阈值H_in′时,硬磁泡和硬畴段将变为正常泡和段。我们测量了不同磁泡材料的H_in′。随垂直于膜面直流偏磁场的增大,H_in′减小。观察到这种转变是一种弛豫过程,在适当条件下可以持续数秒或数十秒。采用Slonczewski和Malozemoff提出的面内场布洛赫点形核的概念对得到的实验结果做了定性的解释。 关键词：     

Formation rate of vapor bubbles in magnetic fluid boiling at a single vaporization center: Measuring technique and experimental setup

A technique for measuring the bubbling rate during magnetic fluid boiling at a single vaporization center in an external uniform magnetic field is suggested and proved, and an experimental setup is designed based on this technique. It is shown that the magnetic field substantially influences the rate of detachment of the bubbles in a wide range of temperatures below the saturation temperature. The diameter of the vapor bubble at which it separates in the magnetic field (separation diameter) is estimated.     

Gradientless propulsion of magnetic bubble domains

We find that some types of magnetic bubbles can be propelled by homogeneous fields: a modulated bias field superimposed on a steady in-plane field. Characteristic velocity vectors are observed which can discriminate among wall configurations having a common winding number s. Two particular complementary bubbles having s = 1 move in opposite directions nearly orthogonal to the in-plane field because the translational coercive force couples to motions of the wall radius and two Bloch lines.     

Pinning of domain walls in two-layer ferromagnetic nanowire with scattering fields of nanoparticles

The results of a micromagnetic simulation of the pinning-depinning processes of a domain wall (DW) in a rectangular ferromagnetic nanowire (NW) consisting of two magnetic layers with scattering fields of two rectangular two-layer nanoparticles (NPs) located on NW opposite sides and oriented perpendicular to its axis are presented. The features of magnetization reversal of this system in the external magnetic field are studied depending on direction of the magnetic moments of the nanoparticle layers. The value of the depinning field in such a system depends essentially on mutual orientation of NP magnetic moments and NW magnetization. The possibility to realize a magnetic logic cell performing the “conjunction” operation of ternary logic is discussed.     

Nonlinear magneto-optical diffraction by two-dimensional magnetic superstructures

Nonlinear (at the second-harmonic frequency of the incident light) optical reflection by two-dimensional magnetic superstructures is theoretically studied. A square lattice of magnetic dots and a hexagonal lattice of magnetic bubbles (cylindrical magnetic domains) are considered. Because the periods of these structures are comparable with the wavelengths of the fundamental and the second-harmonic radiation, it would be possible to observe diffraction at the second-harmonic frequency. A polarization analysis of nonlinearly diffracted radiation is performed and the numbers of observable diffraction orders for the above structures are estimated. Received: 10 January 2002 / Published online: 11 June 2002     

Single Qubit and Its Universal Logic Gate Made of a Heisenberg Spin Cluster with Five Particles

We investigate an anisotropic Heisenberg spin cluster with five particles controlled by a time-dependent magnetic field. With the algebraic dynamical method, we obtain the exact analytical solution to the time dependent Schr6dinger equation. Based on the analytical solution, it is shown that the system can be used as a universal single qubit logic gate controlled by the strength and frequency of the magnetic field, and the six special single qubit logic gates can be realized physically. We also discuss the anti-decoherence property of the qubit and its logic gates resulted from particle coupling effect and collectivity of the cluster.     

Jump probability and bubble dynamics

The existence of a jump probability is introduced from the interpretation of transport measurements performed on magnetic bubbles. As a consequence the shape of the experimental curve giving the mobility and the extrapolated coercivity is shown to be dependent on the duration of the applied field gradient.     

Satellite observations of electric fields in the innermagnetosphere and their effects in the mid-to-low latitude ionosphere

During geomagnetic disturbances, momentum and energy are transferred in significant quantities from interplanetary space to the magnetosphere-ionosphere system through the mediation of charged particles and electric fields. The most dramatic manifestations occur in the plasma sheet and the conjugate auroral ionosphere. However, electric fields observed during magnetic storms also penetrate the inner magnetosphere that maps to subauroral latitudes in the ionosphere. For example, a sudden commencement shock wave initiating the March 1991 magnetic storm created a new radiation belt within minutes. Particle and field measurements by Combined Release and Radiation Effects Satellite (CRRES) near the equatorial plane of the magnetosphere and by Defense Meteorological Satellite Program (DMSP) satellites in the topside ionosphere during the magnetic storm of June 1991 indicate that penetration electric fields energized the stormtime ring current and rapidly transported plasma within subauroral ion drift (SAID) structures at midlatitudes and in upward drafting plasma bubbles at low latitudes. Through enhanced transport or chemical reactions, the SAIDs dug deep plasma troughs at topside altitudes. Equatorial plasma bubbles developed while the ring current was unable to shield the electric field from the innermost magnetosphere     

Study on magnetic fluid optical fiber devices for optical logic operations by characteristics of superparamagnetic nanoparticles and magnetic fluids

We propose two optical fiber-based schemes using two magnetic fluid optical fiber modulators in series or in parallel for optical logic signal processing and operation. Here, each magnetic fluid optical fiber modulator consists of a bare multimode fiber surrounded by magnetic fluid in which the refractive index is adjustable by applying external magnetic fields amplifying the input electrical signal to vary the transmission intensity of the optical fiber-based scheme. The physical mechanisms for the performances of the magnetic fluid optical fiber devices, such as the transmission loss related to Boolean number of the logic operation as well as the dynamic response, are studied by the characteristics of superparamagnetic nanoparticles and magnetic fluids. For example, in the dynamic response composed of the retarding and response sub-procedures except the response times of the actuation coil, the theoretical evaluation of the retarding time variation with cladding magnetic fluids length has good agreement with the experimental results.